County-Scale Distribution of Ixodes scapularis and Ixodes pacificus (Acari: Ixodidae) in the Continental United States

Abstract

The blacklegged tick, Ixodes scapularis Say, is the primary vector to humans in the eastern United States of the Lyme disease spirochete Borrelia burgdorferi, as well as causative agents of anaplasmosis and babesiosis. Its close relative in the far western United States, the western blacklegged tick Ixodes pacificus Cooley and Kohls, is the primary vector to humans in that region of the Lyme disease and anaplasmosis agents. Since 1991, when standardized surveillance and reporting began, Lyme disease case counts have increased steadily in number and in geographical distribution in the eastern United States. Similar trends have been observed for anaplasmosis and babesiosis. To better understand the changing landscape of risk of human exposure to disease agents transmitted by I. scapularis and I. pacificus, and to document changes in their recorded distribution over the past two decades, we updated the distribution of these species from a map published in 1998. The presence of I. scapularis has now been documented from 1,420 (45.7%) of the 3,110 continental United States counties, as compared with 111 (3.6%) counties for I. pacificus. Combined, these vectors of B. burgdorferi and other disease agents now have been identified in a total of 1,531 (49.2%) counties spread across 43 states. This marks a 44.7% increase in the number of counties that have recorded the presence of these ticks since the previous map was presented in 1998, when 1,058 counties in 41 states reported the ticks to be present. Notably, the number of counties in which I. scapularis is considered established (six or more individuals or one or more life stages identified in a single year) has more than doubled since the previous national distribution map was published nearly two decades ago. The majority of county status changes occurred in the North-Central and Northeastern states, whereas the distribution in the South remained fairly stable. Two previously distinct foci for I. scapularis in the Northeast and North-Central states appear to be merging in the Ohio River Valley to form a single contiguous focus. Here we document a shifting landscape of risk for human exposure to medically important ticks and point to areas of re-emergence where enhanced vector surveillance and control may be warranted.

The blacklegged tick, Ixodes scapularis Say, is the primary vector to humans in the eastern United States of the Lyme disease spirochete Borrelia burgdorferi, as well as the relapsing fever spirochete, Borrelia miyamotoi; causative agents of anaplasmosis (Anaplasma phagocytophilum) and babesiosis (Babesia microti); and Powassan encephalitis virus (Piesman and Eisen 2008, Ebel 2010, Krause et al. 2015). Its close relative in the far western United States, the western blacklegged tick Ixodes pacificus Cooley and Kohls, is the primary vector to humans in that region of Lyme disease and anaplasmosis agents, as well as relapsing fever spirochetes (B. miyamotoi; Lane et al. 1994, Teglas and Foley 2006, Krause et al. 2015). Lyme disease is the most commonly reported vector-borne disease in the United States. It is a geographically focal illness, with the majority of cases reported from the Northeastern and North-Central states and discrete areas of risk in the Pacific Coast states (Mead 2015). Since 1991, when standardized surveillance and reporting began, Lyme disease case counts have increased steadily from roughly 10,000 cases in 1991 to more than 30,000 cases in 2008 and subsequent years (Bacon et al. 2008, Mead 2015); the true burden of disease is estimated to be roughly 10-fold greater (Hinckley et al. 2014, Nelson et al. 2015). In addition to the increase in case counts over time, the geographical foci of high-incidence counties have expanded both in the North-Central and in the Northeastern United States (Kugeler et al. 2015).

Rising case counts and geographical expansion of Lyme disease endemic areas have been attributed to range expansion of I. scapularis in the eastern United States (Bacon et al. 2008, Rydzewski et al. 2012, Lee et al. 2013, Brinkerhoff et al. 2014, Robinson et al. 2014, Wang et al. 2014, Khatchikian et al. 2015, Kugeler et al. 2015, Stone et al. 2015). However, because of a lack of systematic surveillance of I. scapularis and I. pacificus, national trends in the geographic distribution of these medically important ticks are difficult to document. To better understand the changing landscape of risk of human exposure to I. scapularis and I. pacificus in the United States, and to document changes in their distribution over the past two decades, we updated the reported distribution of these species from the map previously published by Dennis et al. (1998).

Materials and Methods

County Status Definitions

The definitions used to classify I. scapularis or I. pacificus as “established” or “reported” in a county follow Dennis et al. (1998). Counties were classified as established if at least six individual ticks or at least two of the three host-seeking life stages had been identified in a single collection period. Here, a single collection period is defined as a single year. Counties were classified as reported if they failed to meet the criteria for established but if at least one tick of any life stage had been identified at any time in that county, or if county records did not specify the number of ticks or life stages collected. Lack of tick records from a county—“no records”—does not imply that ticks are absent from that county, only that records of ticks having been collected in the county are lacking.

The county status (i.e., established, reported, or no records) given by Dennis et al. (1998) was used as the basis for our updated county status. If a county was classified as established by Dennis et al. (1998), it remained established in the updated classification regardless of whether more recent tick records were available. A county classified as reported by Dennis et al. (1998) retained this status in the updated classification, unless more recent collection records changed the county’s classification from reported to established. Herein, the term county refers to counties and county equivalents corresponding with five-digit Federal Information Processing Standard (FIPS) coding.

Data Sources

Six independent literature searches were conducted using Scopus and PubMed databases with the following key words “Ixodes scapularis,” “Ixodes pacificus,” and “tick” to identify relevant articles and abstracts published from 1996 through 25 August 2015. We also conducted a search to ensure that papers using the junior synonym “Ixodes dammini” rather than Ixodes scapularis were included. All reports that explicitly presented county-specific tick data were included in our database. In addition, we visited individual state health department Web sites to identify county-level tick surveillance data, and contacted public health officials, acarologists, and Lyme disease investigators throughout the United States to assess county-level tick collection data.

GIS Mapping

Our final database containing state, county, county FIPS code, county status as per Dennis et al. (1998), and the updated county status was joined based on FIPS codes to a continental United States county map using ArcMap 10.3 (ESRI, Redlands, CA).

Results

Counties With Recorded Presence of I. scapularis or I. pacificus

Our updated county status records show that I. scapularis now has been collected from 37 states, from the eastern seaboard to the eastern edge of the Great Plains, and I. pacificus from six western states (Tables 1–3; Fig. 1). No single state has records of both tick species, and five states in the Rocky Mountain region lack records for either I. scapularis or I. pacificus: Colorado, Idaho, Montana, New Mexico, and Wyoming. The presence of I. scapularis has now been documented from 1,420 (45.7%) of the 3,110 continental United States counties, as compared with 111 (3.6%) counties for I. pacificus. Combined, these primary vectors of B. burgdorferi and other tick-borne disease agents now have been identified in a total of 1,531 (49.2%) counties spread across 43 states. This marks a 44.7% increase in the number of counties that have recorded the presence of these ticks since the survey conducted by Dennis et al. (1998), when 1,058 counties in 41 states reported the ticks to be present. Nebraska and North Dakota are the two states where I. scapularis was recorded only after the Dennis et al. (1998) survey.

Table 1.

Number (%) of continental United States counties in which I. scapularis or I. pacificus were classified as reported or established, by December 1996 (from Dennis et al. 1998) and August 2015

Species/State	No. (%) counties with reported status		No. (%) counties with established status
	By 1996	By 2015	By 1996	By 2015
I. scapularis	556 (17.8)	578 (18.6)	396 (12.7)	842 (27.1)
Alabama	20 (29.9)	21 (31.3)	25 (37.3)	25 (37.3)
Arkansas	37 (49.3)	25 (33.3)	9 (12.0)	27 (36.0)
Connecticut	0 (0)	0 (0)	8 (100)	8 (100)
Delaware	0 (0)	0 (0)	3 (100)	3 (100)
Florida	22 (32.8)	15 (22.4)	35 (52.2)	52 (77.6)
Georgia	31 (19.5)	35 (22.0)	23 (14.5)	35 (22.0)
Illinois	47 (46.1)	29 (28.4)	4 (3.9)	35 (34.3)
Indiana	25 (27.2)	37 (40.2)	8 (8.7)	29 (31.5)
Iowa	17 (17.2)	25 (25.3)	7 (7.1)	14 (14.1)
Kansas	14 (13.3)	14 (13.3)	1 (1.0)	1 (1.0)
Kentucky	2 (1.7)	4 (3.3)	0 (0)	14 (11.7)
Louisiana	12 (18.8)	23 (36.0)	12 (18.8)	12 (18.8)
Maine	3 (18.8)	0 (0)	13 (81.3)	16 (100)
Maryland	2 (8.3)	2 (8.7)	21 (87.5)	21 (91.3)
Massachusetts	3 (21.4)	0 (0)	9 (64.3)	14 (100)
Michigan	22 (26.5)	16 (19.3)	5 (6.0)	24 (28.9)
Minnesota	12 (13.8)	3 (3.5)	9 (10.3)	45 (51.7)
Mississippi	72 (87.8)	71 (86.6)	10 (12.2)	11 (13.4)
Missouri	8 (7.0)	8 (7.0)	21 (18.3)	23 (20.0)
Nebraska	0 (0)	3 (3.2)	0 (0)	0 (0)
New Hampshire	5 (50.0)	1 (10.0)	5 (50.0)	9 (90.0)
New Jersey	0 (0)	0 (0)	21 (100)	21 (100)
New York	20 (32.3)	1 (1.6)	31 (50.0)	61 (98.4)
North Carolina	23 (23.0)	16 (16.0)	7 (7)	43 (43.0)
North Dakota	0 (0)	3 (5.7)	0 (0)	5 (9.4)
Ohio	5 (5.7)	31 (35.6)	0 (0)	33 (37.5)
Oklahoma	36 (46.8)	36 (46.8)	3 (4.0)	3 (4.0)
Pennsylvania	26 (38.8)	0 (0)	23 (34.3)	67 (100)
Rhode Island	0 (0)	0 (0)	5 (100)	5 (100)
South Carolina	14 (30.4)	19 (41.3)	13 (28.3)	14 (30.4)
South Dakota	2 (3.0)	2 (3.0)	2 (3.0)	0 (0)
Tennessee	6 (6.3)	27 (28.4)	1 (1.1)	16 (16.8)
Texas	39 (15.4)	45 (17.7)	24 (9.5)	26 (10.2)
Vermont	6 (43.0)	2 (14.3)	1 (7.1)	11 (78.6)
Virginia	4 (3.0)	29 (21.6)	8 (6.0)	43 (32.1)
West Virginia	2 (3.6)	20 (36.4)	2 (3.6)	23 (41.8)
Wisconsin	16 (22.2)	15 (20.8)	29 (40.3)	51 (70.8)
I. pacificus	16 (0.5)	16 (0.5)	90 (2.9)	95 (3.1)
Arizona	1 (6.7)	1 (6.7)	0 (0)	0 (0)
California	1 (1.72)	1 (1.72)	55 (94.8)	55 (94.8)
Nevada	2 (11.8)	2 (11.8)	0 (0)	0 (0)
Oregon	4 (11.1)	4 (11.1)	18 (50.0)	18 (50.0)
Utah	4 (13.8)	3 (10.3)	4 (13.8)	4 (13.8)
Washington	5 (12.8)	6 (15.3)	12 (30.8)	16 (41.3)

Table 3.

Records of I. pacificus by state and county. Fields left blank indicate status was inherited from Dennis et al. (1998)

State and county	Status by Aug. 2015a	Status change from Dennis et al. (1998)b	Source for change of status from Dennis et al. (1998) survey
Arizona
Mohave	Established
California
Alameda	Established
Amador	Established
Butte	Established
Calaveras	Established
Colusa	Established
Contra Costa	Established
Del Norte	Established
El Dorado	Established
Fresno	Established
Glenn	Established
Humboldt	Established
Imperial	Established
Inyo	Established
Kern	Established
Kings	Established
Lake	Established
Lassen	Established
Los Angeles	Established
Madera	Established
Marin	Established
Mariposa	Established
Mendocino	Established
Merced	Established
Mono	Reported
Monterey	Established
Napa	Established
Nevada	Established
Orange	Established
Placer	Established
Plumas	Established
Riverside	Established
Sacramento	Established
San Benito	Established
San Bernardino	Established
San Diego	Established
San Francisco	Established
San Joaquin	Established
San Luis Obispo	Established
San Mateo	Established
Santa Barbara	Established
Santa Clara	Established
Santa Cruz	Established
Shasta	Established
Sierra	Established
Siskiyou	Established
Solano	Established
Sonoma	Established
Stanislaus	Established
Sutter	Established
Tehama	Established
Trinity	Established
Tulare	Established
Tuolumne	Established
Ventura	Established
Yolo	Established
Yuba	Established
Nevada
Clark	Reported
Lincoln	Reported
Oregon
Benton	Established
Clackamas	Established
Clatsop	Established
Columbia	Reported
Coos	Established
Curry	Established
Douglas	Established
Hood River	Established
Jackson	Established
Jefferson	Reported
Josephine	Established
Lane	Established
Lincoln	Established
Linn	Established
Marion	Established
Multnomah	Established
Polk	Reported
Sherman	Established
Tillamook	Established
Umatilla	Reported
Wasco	Established
Washington	Established
Utah
Beaver	Reported
Juab	Established
Millard	Reported
Piute	Reported
Salt Lake	Established
Tooele	Established	R-E	(Davis et al. 2015)
Utah	Established
Washington	Established
Washington
Chelan	Established
Clallam	Established	N-E	E. Dykstra, unpublished
Clark	Established
Cowlitz	Established	R-E	E. Dykstra, unpublished
Island	Reported
Jefferson	Established
King	Established	R-E	E. Dykstra, unpublished
Kitsap	Established	R-E	E. Dykstra, unpublished
Kittitas	Reported	N-R	E. Dykstra, unpublished
Klickitat	Established
Lewis	Established
Mason	Established
Okanogan	Reported	N-R	E. Dykstra, unpublished
Pacific	Reported	N-R	E. Dykstra, unpublished
Pierce	Established
San Juan	Established
Skagit	Established
Skamania	Established
Snohomish	Reported
Thurston	Established
Whatcom	Established
Yakima	Reported	N-R	E. Dykstra, unpublished

Fields left blank indicate status was inherited from Dennis et al. (1998)

^aEstablished: Six or more ticks or two or more tick life stages; Reported: Fewer than six ticks and one tick life stage only.

^bN-R, change from No Records to Reported; N-E, change from No Records to Established; R-E, change from Reported to Established.

Fig. 1.

Distribution by county of recorded presence of I. scapularis and I. pacificus in the continental United States (a) 1907–1996 (from Dennis et al. 1998), (b) 1907–2015. Counties classified as established (red or green) for a given tick species had at least six ticks or two life stages recorded within a single calendar year. Counties with fewer ticks of a single life stage were classified as reported (blue or yellow) for the tick species. Counties shown in white indicate“no records.”

Counties Where I. scapularis Is Classified as Established or Reported

Ixodes scapularis now is classified as established in 842 counties (27.1% of counties in the continental United States) distributed across 35 states (Tables 1–3; Fig. 1). This more than doubles the number of counties in which the tick is classified as established since the previous survey by Dennis et al. (1998), when it was considered established in 396 counties (12.7% of counties in the continental United States) spanning 32 states (Tables 1–2; Fig. 1–2). In total, 446 counties were updated from either no records (n = 262) or reported (n = 184) to established, and 208 counties were updated from no records to reported (Table 2; Fig. 2). Counties with I. scapularis classified as established were added for three states: Kentucky, North Dakota, and Ohio.

Table 2.

Status for I. scapularis by continental United States county. Fields left blank indicate that status was inherited from Dennis et al. (1998)

State and county	Status by August 2015a	Status change from Dennis et al. (1998)b	Source for change of status from Dennis et al. (1998) survey
Alabama
Autauga	Established
Baldwin	Reported
Barbour	Established
Bibb	Established
Bullock	Established
Butler	Established
Chambers	Established
Chilton	Reported
Choctaw	Established
Clarke	Established
Clay	Reported
Cleburne	Reported
Coffee	Reported	N-R	http://vectormap.nhm.ku.edu/vectormap/
Colbert	Established
Conecuh	Established
Coosa	Established
Covington	Established
Crenshaw	Reported
Dale	Reported
Elmore	Established
Escambia	Established
Franklin	Established
Geneva	Reported
Hale	Established
Henry	Established
Houston	Reported
Jackson	Established
Jefferson	Reported
Lawrence	Reported
Lee	Established
Lowndes	Reported
Madison	Reported
Marengo	Reported
Mobile	Established
Monroe	Established
Montgomery	Reported
Pickens	Established
Randolph	Reported
Russell	Established
Sumter	Reported
Talladega	Reported
Tallapoosa	Established
Tuscaloosa	Reported
Washington	Reported
Wilcox	Reported
Winston	Established
Arkansas
Ashley	Established
Baxter	Established
Benton	Established	R-E	(Trout and Steelman 2010)
Boone	Established	R-E	(Trout and Steelman 2010)
Calhoun	Reported
Carroll	Reported
Clark	Established	N-E	(Trout and Steelman 2010)
Cleburne	Established	N-E	(Trout and Steelman 2010)
Columbia	Reported
Conway	Reported
Crawford	Established	R-E	(Trout and Steelman 2010)
Dallas	Reported
Drew	Reported
Faulkner	Established	N-E	(Trout and Steelman 2010)
Franklin	Established
Fulton	Established	R-E	(Trout and Steelman 2010)
Garland	Established	R-E	(Trout and Steelman 2010)
Grant	Established	R-E	(Trout and Steelman 2010)
Hempstead	Established	N-E	(Trout and Steelman 2010)
Howard	Reported
Izard	Reported
Jefferson	Reported
Johnson	Reported
Lafayette	Reported
Lawrence	Established	R-E	(Trout and Steelman 2010)
Lincoln	Reported
Logan	Reported
Madison	Established
Marion	Established	R-E	(McAllister et al. 2013)
Montgomery	Reported
Nevada	Reported
Newton	Reported
Ouachita	Reported
Perry	Reported
Pike	Reported
Poinsett	Established	N-E	(Trout and Steelman 2010)
Polk	Established
Pope	Established	R-E	(Trout and Steelman 2010)
Prairie	Established	R-E	(Trout and Steelman 2010)
Pulaski	Reported
Randolph	Reported
Saline	Established	N-E	(Trout and Steelman 2010)
Scott	Reported
Searcy	Established
Sebastian	Established
Sevier	Established	R-E	(Trout and Steelman 2010)
Sharp	Reported
Stone	Established
Union	Established	R-E	(Trout and Steelman 2010, McAllister et al. 2013)
Van Buren	Reported
Washington	Established
Yell	Reported
Connecticut
Fairfield	Established
Hartford	Established
Litchfield	Established
Middlesex	Established
New Haven	Established
New London	Established
Tolland	Established
Windham	Established
Delaware
Kent	Established
New Castle	Established
Sussex	Established
District of Columbia	Established	N-E	T. L. Johnson, unpublished
Florida
Alachua	Established
Baker	Established
Bay	Established
Bradford	Reported
Brevard	Established	N-E	J. Corn/SCWDS, unpublished; J. Mertins/NVSL, unpublished
Brevard	Reported
Broward	Reported
Calhoun	Established
Charlotte	Reported
Citrus	Reported
Clay	Established
Collier	Established
Columbia	Established
DeSoto	Established	N-E	J. Corn/SCWDS, unpublished; J. Mertins/NVSL, unpublished
Dixie	Established
Duval	Reported
Escambia	Established
Flagler	Established
Franklin	Established
Gadsden	Established
Gilchrist	Reported	N-R	K. Sayler, unpublished
Glades	Established
Gulf	Established
Hamilton	Established	R-E	J. Corn/SCWDS, unpublished; J. Mertins/NVSL, unpublished
Hardee	Reported
Hendry	Established	R-E	J. Corn/SCWDS, unpublished; J. Mertins/NVSL, unpublished
Hernando	Established
Highlands	Established	R-E	J. Corn/SCWDS, unpublished; J. Mertins/NVSL, unpublished
Hillsborough	Established	R-E	J. Corn/SCWDS, unpublished; J. Mertins/NVSL, unpublished
Holmes	Reported	N-R	J. Corn/SCWDS, unpublished; J. Mertins/NVSL, unpublished
Indian River	Established	N-E	C. Lord, unpublished
Jackson	Established
Jefferson	Established
Lafayette	Established
Lake	Established	R-E	J. Corn/SCWDS, unpublished; J. Mertins/NVSL, unpublished
Lee	Established	R-E	J. Corn/SCWDS, unpublished; J. Mertins/NVSL, unpublished
Leon	Established
Levy	Established	R-E	K. Sayler, unpublished
Liberty	Established
Madison	Established	N-E	K. Sayler, unpublished
Manatee	Reported
Marion	Established
Martin	Established
Miami-Dade	Established
Monroe	Established	R-E	J. Corn/SCWDS, unpublished; J. Mertins/NVSL, unpublished
Nassau	Established
Okaloosa	Established
Okeechobee	Established	N-E	Wisely, Cleveland, Satterlee, and Lord, unpublished
Orange	Established
Osceola	Established
Palm Beach	Established
Pasco	Reported
Pinellas	Established	N-E	J. Corn/SCWDS, unpublished; J. Mertins/NVSL, unpublished
Polk	Established
Putnam	Established	R-E	J. Corn/SCWDS, unpublished; J. Mertins/NVSL, unpublished
Santa Rosa	Established
Sarasota	Established	R-E	J. Corn/SCWDS, unpublished; J. Mertins/NVSL, unpublished
Seminole	Established	R-E	J. Corn/SCWDS, unpublished; J. Mertins/NVSL, unpublished
St. Johns	Reported
St. Lucie	Reported	N-R	L. Durden, unpublished
Sumter	Established
Suwannee	Reported	N-R	L. Durden, unpublished
Taylor	Established
Union	Reported
Volusia	Established
Wakulla	Established
Walton	Established
Washington	Reported	N-R	J. Corn/SCWDS, unpublished; J. Mertins/NVSL, unpublished
Georgia
Appling	Reported	N-R	L. Durden, unpublished
Atkinson	Reported
Bacon	Reported	N-R	L. Durden, unpublished
Baker	Reported
Baldwin	Reported
Bibb	Reported
Bleckley	Established
Brantley	Established
Brooks	Reported
Bryan	Reported
Bulloch	Established
Burke	Established	R-E	L. Durden, unpublished
Calhoun	Reported
Camden	Reported
Candler	Established	N-E	L. Durden, unpublished
Charlton	Established
Chatham	Established
Chattahoochee	Reported
Clarke	Reported	N-R	L. Durden, unpublished
Clinch	Established
Coffee	Reported
Columbia	Reported
Cook	Reported
Crisp	Reported
Decatur	Reported
Dougherty	Established
Echols	Reported
Effingham	Established
Emanuel	Established	N-E	L. Durden, unpublished
Evans	Established	R-E	L. Durden, unpublished
Forsyth	Reported	N-R	L. Durden, unpublished
Glynn	Established
Grady	Established
Hancock	Reported
Houston	Established
Irwin	Established
Jasper	Established
Jeff Davis	Reported
Jefferson	Established	N-E	L. Durden, unpublished
Jenkins	Established	N-E	L. Durden, unpublished
Johnson	Established	N-E	L. Durden, unpublished
Jones	Reported
Laurens	Established	N-E	L. Durden, unpublished
Liberty	Established
Long	Reported
Lowndes	Established
McDuffie	Established
McIntosh	Established
Monroe	Established
Montgomery	Established	N-E	L. Durden, unpublished
Morgan	Reported
Muscogee	Reported
Pierce	Reported
Pulaski	Reported
Putnam	Established
Talbot	Established
Taliaferro	Reported
Tattnall	Reported	N-R	L. Durden, unpublished
Telfair	Established
Terrell	Reported
Thomas	Established
Tift	Reported	N-R	L. Durden, unpublished
Toombs	Established	N-E	L. Durden, unpublished
Treutlen	Established	N-E	L. Durden, unpublished
Twiggs	Reported
Ware	Established
Washington	Established	N-E	L. Durden, unpublished
Wayne	Reported
Wilkes	Reported
Wilkinson	Reported
Illinois
Boone	Reported
Brown	Reported
Bureau	Established	R-E	(Cortinas and Kitron 2006)
Carroll	Established
Cass	Reported	N-R	(Cortinas and Kitron 2006)
Champaign	Established	R-E	http://www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
Clark	Established	R-E	http://www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
Coles	Established	R-E	http://www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
Cook	Established	R-E	(Rydzewski et al. 2012); http://www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
Crawford	Reported
Cumberland	Reported
De Witt	Established	N-E	http://www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
DuPage	Established	R-E	(Rydzewski et al. 2012); http://www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
Edgar	Reported
Fayette	Reported
Franklin	Reported
Fulton	Established	N-E	(Cortinas et al. 2002)); (Cortinas and Kitron 2006); http://www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
Gallatin	Reported
Grundy	Established	R-E	(Cortinas and Kitron 2006); http://www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
Henry	Established	R-E	http://www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
Iroquois	Reported
Jackson	Reported
Jo Daviess	Established	R-E	http://www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
Kankakee	Established	R-E	(Cortinas and Kitron 2006); http://www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
Knox	Reported
Lake	Established	N-E	(Rydzewski et al. 2012); http://www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
La Salle	Established	R-E	(Cortinas and Kitron 2006); http://www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
Lawrence	Reported
Lee	Established	R-E	http://www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
Macoupin	Established	N-E	http://www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
Madison	Reported
Marshall	Established	R-E	(Cortinas and Kitron 2006); http://www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
Mason	Established	N-E	http://www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
McDonough	Established	N-E	http://www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
McHenry	Reported
McLean	Established	R-E	http://www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
Menard	Reported
Mercer	Reported
Monroe	Established
Montgomery	Reported	N-R	http://www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
Morgan	Reported	N-R	(Cortinas and Kitron 2006)
Ogle	Established
Peoria	Established	R-E	(Cortinas and Kitron 2006); http://www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
Perry	Reported
Piatt	Established	R-E	(Rydzewski et al. 2011); http://www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
Pope	Reported
Putnam	Established	R-E	(Cortinas and Kitron 2006); http://www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
Randolph	Reported
Rock Island	Established
Saline	Reported
Sangamon	Reported
Schuyler	Established	R-E	(Cortinas and Kitron 2006)
Scott	Reported
Shelby	Established	N-E	http://www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
St. Clair	Reported
Stephenson	Reported
Tazewell	Established	R-E	(Cortinas et al. 2002, Cortinas and Kitron 2006); http://www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
Union	Reported
Vermilion	Established	N-E	http://www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
Wabash	Established	N-E	http://www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
Will	Established	R-E	(Cortinas and Kitron 2006); http://www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
Williamson	Reported
Winnebago	Established	R-E	http://www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
Woodford	Established	N-E	(Cortinas and Kitron 2006); http://www.idph.state.il.us/envhealth/pdf/Deer_Tick_Range.pdf
Indiana
Adams	Reported	N-R	R. Pinger, unpublished
Bartholomew	Established	R-E	R. Pinger, unpublished
Benton	Reported	N-R	R. Pinger, unpublished
Boone	Reported	N-R	R. Pinger, unpublished
Brown	Reported
Carroll	Reported	N-R	R. Pinger, unpublished
Cass	Established	R-E	(Raizman et al. 2012); E. Raizman, unpublished
Clark	Reported	N-R	R. Pinger, unpublished
Clay	Established	R-E	(Raizman et al. 2012); E. Raizman, unpublished
Daviess	Reported	N-R	R. Pinger, unpublished
Dearborn	Reported	N-R	R. Pinger, unpublished
Dubois	Established	R-E	(Raizman et al. 2012); E. Raizman, unpublished
Elkhart	Reported	N-R	R. Pinger, unpublished
Fountain	Reported
Franklin	Established	R-E	(Raizman et al. 2012); E. Raizman, unpublished
Fulton	Established	R-E	(Raizman et al. 2012); E. Raizman, unpublished
Grant	Reported
Greene	Reported	N-R	R. Pinger, unpublished
Hamilton	Reported	N-R	(Raizman et al. 2012); E. Raizman, unpublished
Hendricks	Reported	N-R	R. Pinger, unpublished
Howard	Reported	N-R	R. Pinger, unpublished
Huntington	Reported
Jasper	Established
Jefferson	Established	N-E	(Raizman et al. 2012); E. Raizman, unpublished
Jennings	Established	N-E	(Raizman et al. 2012); E. Raizman, unpublished
Johnson	Reported	N-R	R. Pinger, unpublished
Knox	Reported	N-R	R. Pinger, unpublished
Kosciusko	Established	N-E	(Raizman et al. 2012); E. Raizman, unpublished
LaGrange	Reported
Lake	Established	R-E	(Raizman et al. 2012); E. Raizman, unpublished
LaPorte	Established
Lawrence	Reported	N-R
Madison	Reported
Marshall	Reported
Martin	Established	N-E	(Raizman et al. 2012); E. Raizman, unpublished
Monroe	Established	N-E	(Raizman et al. 2012); E. Raizman, unpublished
Montgomery	Established	N-E	(Raizman et al. 2012); E. Raizman, unpublished d
Morgan	Established	R-E	(Raizman et al. 2012); E. Raizman, unpublished
Newton	Established
Orange	Reported
Owen	Established	N-E	(Raizman et al. 2012); E. Raizman, unpublished
Parke	Established	R-E	(Raizman et al. 2012); E. Raizman, unpublished
Pike	Reported
Porter	Established
Posey	Reported
Pulaski	Established
Putnam	Established	N-E	(Raizman et al. 2012); E. Raizman, unpublished
Randolph	Reported	N-R	R. Pinger, unpublished
Ripley	Reported	N-R	R. Pinger, unpublished
Shelby	Reported	N-R	(Raizman et al. 2012); E. Raizman, unpublished
St. Joseph	Established	R-E	(Raizman et al. 2012); E. Raizman, unpublished
Starke	Established
Steuben	Reported	N-R	R. Pinger, unpublished
Sullivan	Established	N-E	(Raizman et al. 2012); E. Raizman, unpublished
Switzerland	Reported	N-R	R. Pinger, unpublished
Tippecanoe	Established	R-E	(Raizman et al. 2012); E. Raizman, unpublished
Union	Reported	N-R	(Raizman et al. 2012); E. Raizman, unpublished
Vanderburgh	Reported	N-R	R. Pinger, unpublished
Vermillion	Reported
Vigo	Established
Wabash	Reported
Warren	Established	N-E	(Raizman et al. 2012); E. Raizman, unpublished
Warrick	Reported
Washington	Reported
White	Established
Whitley	Reported	N-R	(Raizman et al. 2012); E. Raizman, unpublished
Iowa
Benton	Reported	N-R	(Lingren et al. 2005)
Bremer	Established
Buena Vista	Reported
Calhoun	Reported	N-R	(Lingren et al. 2005)
Cedar	Reported	N-R	(Lingren et al. 2005)
Cerro Gordo	Established
Clayton	Reported
Clinton	Reported	N-R	(Lingren et al. 2005)
Crawford	Reported	N-R	(Lingren et al. 2005)
Dallas	Reported
Delaware	Reported
DesMoines	Established
Dubuque	Reported
Fayette	Reported
Floyd	Reported
Greene	Reported	N-R	(Lingren et al. 2005)
Guthrie	Reported	N-R	(Lingren et al. 2005)
Hancock	Reported	N-R	(Lingren et al. 2005)
Iowa	Reported
Jackson	Established
Jasper	Reported	N-R	(Lingren et al. 2005)
Johnson	Established	R-E	(Lingren et al. 2005)
Keokuk	Established	N-E	(Lingren et al. 2005)
Kossuth	Reported	N-R	(Lingren et al. 2005)
Linn	Established
Louisa	Reported
Marshall	Reported
Muscatine	Established
Palo Alto	Reported	N-R	(Lingren et al. 2005)
Polk	Established	R-E	(Lingren et al. 2005)
Scott	Established	R-E	(Lingren et al. 2005)
Story	Established	R-E	(Lingren et al. 2005)
Tama	Reported	N-R	(Lingren et al. 2005)
Washington	Reported	N-R	(Lingren et al. 2005)
Webster	Established	R-E	(Lingren et al. 2005)
Winnebago	Reported
Winneshiek	Established	R-E	(Lingren et al. 2005)
Woodbury	Reported	N-R	(Lingren et al. 2005)
Kansas
Bourbon	Reported
Chautauqua	Reported
Cherokee	Reported
Coffey	Reported
Cowley	Reported
Crawford	Reported
Douglas	Established
Jefferson	Reported
Johnson	Reported
Labette	Reported
Linn	Reported
Miami	Reported
Montgomery	Reported
Riley	Reported
Shawnee	Reported
Kentucky
Ballard	Reported
Boone	Established	N-E	L. Townsend, unpublished
Carroll	Established	N-E	L. Townsend, unpublished
Christian	Reported
Clay	Established	N-E	L. Townsend, unpublished
Estill	Established	N-E	L. Townsend, unpublished
Fayette	Established	N-E	L. Townsend, unpublished
Hancock	Reported	N-R	L. Townsend, unpublished
Hardin	Established	N-E	L. Townsend, unpublished
Jackson	Established	N-E	L. Townsend, unpublished
Knox	Established	N-E	L. Townsend, unpublished
Lee	Established	N-E	L. Townsend, unpublished
McCreary	Established	N-E	L. Townsend, unpublished
Meade	Established	N-E	B. Pagac and M. Miller, unpublished
Owsley	Established	N-E	L. Townsend, unpublished
Pulaski	Established	N-E	L. Townsend, unpublished
Trimble	Established	N-E	L. Townsend, unpublished
Warren	Reported	N-R	(Onwu 2012)
Louisiana
Allen	Established
Avoyelles	Established
Bienville	Established
Bossier	Reported
Caldwell	Established
Catahoula	Reported	N-R	(Mackay and Foil 2005)
Claiborne	Reported
Concordia	Established
DeSoto	Reported	N-R	(Mackay and Foil 2005)
East Baton Rouge	Reported
East Carroll	Reported	N-R	(Mackay and Foil 2005)
Grant	Established
Jackson	Established
Jefferson Davis	Reported	N-R	(Mackay and Foil 2005)
Lafayette	Reported	N-R	(Mackay and Foil 2005)
Lincoln	Established
Madison	Reported
Morehouse	Established
Natchitoches	Established
Ouachita	Reported
Pointe Coupee	Reported	N-R	(Mackay and Foil 2005)
Rapides	Reported
Sabine	Reported
St. John the Baptist	Reported	N-R	(Mackay and Foil 2005)
St. Landry	Reported
St. Martin	Reported	N-R	(Mackay and Foil 2005)
St. Mary	Reported	N-R	(Mackay and Foil 2005)
St. Tammany	Reported
Tangipahoa	Reported
Tensas	Reported
Union	Established
Vermilion	Reported	N-R	(Mackay and Foil 2005)
Vernon	Established
Washington	Reported	N-R	(Mackay and Foil 2005)
Winn	Reported
Maine
Androscoggin	Established
Aroostook	Established	R-E	http://www.maine.gov/dhhs/mecdc/infectious-disease/epi/vector-borne/lyme/documents/2014-lyme-legislature.pdf
Cumberland	Established
Franklin	Established	R-E	http://www.maine.gov/dhhs/mecdc/infectious-disease/epi/vector-borne/lyme/documents/2014-lyme-legislature.pdf
Hancock	Established
Kennebec	Established
Knox	Established
Lincoln	Established
Oxford	Established
Penobscot	Established
Piscataquis	Established	R-E	http://www.maine.gov/dhhs/mecdc/infectious-disease/epi/vector-borne/lyme/documents/2014-lyme-legislature.pdf
Sagadahoc	Established
Somerset	Established
Waldo	Established
Washington	Established
York	Established
Maryland
Allegany	Reported
Anne Arundel	Established
Baltimore City
Baltimore	Established
Calvert	Established
Caroline	Established
Carroll	Established
Cecil	Established
Charles	Established
Dorchester	Established
Frederick	Established
Garrett	Reported
Harford	Established
Howard	Established
Kent	Established
Montgomery	Established
Prince George’s	Established
Queen Anne’s	Established
Somerset	Established
St. Mary’s	Established
Talbot	Established
Washington	Established
Wicomico	Established
Worcester	Established
Massachusetts
Barnstable	Established
Berkshire	Established	R-E	http://stats.tickdiseases.org/
Bristol	Established
Dukes	Established
Essex	Established
Franklin	Established
Hampden	Established
Hampshire	Established
Middlesex	Established	R-E	http://stats.tickdiseases.org/
Nantucket	Established
Norfolk	Established	N-E	http://stats.tickdiseases.org/
Plymouth	Established
Suffolk	Established	N-E	http://stats.tickdiseases.org/
Worcester	Established	R-E	http://stats.tickdiseases.org/
Michigan
Alger	Reported	N-R	(Schaar 2012)
Allegan	Established	R-E	(Foster 2004, Hamer et al. 2010)
Baraga	Reported
Barry	Established	N-E	J. Tsao, S. Hamer, I. Arsnoe, and G. Hickling, unpublished
Benzie	Established	N-E	(Hamer et al. 2010); J. Sidge unpublished
Berrien	Established
Cass	Established	N-E	(Hamer et al. 2009); E. Foster, unpublished
Charlevoix	Established	N-E	E. Foster and J. Sidge, unpublished
Chippewa	Established
Clinton	Reported
Delta	Established
Dickinson	Established	R-E	I. Arsnoe, unpublished
Emmet	Reported	N-R	J. Sidge, unpublished
Genesee	Reported
Gogebic	Established
Hillsdale	Reported	N-R	E. Foster, and J. Tsao, unpublished
Houghton	Reported
Ingham	Established	R-E	E. Foster, and J. Tsao, unpublished
Ionia	Established	N-E	(Hamer et al. 2010)
Iron	Reported	N-R	(Walker et al. 1998)
Jackson	Reported
Kalamazoo	Established	N-E	J. Tsao and G. Hickling, unpublished
Kent	Reported
Lapeer	Reported
Leelanau	Established	R-E	E. Foster, J. Tsao and J. Sidge, unpublished
Livingston	Reported
Mackinac	Reported
Manistee	Established	R-E	(Hamer et al. 2010)
Marquette	Reported
Mason	Established	N-E	J. Tsao and J. Sidge, unpublished
Menominee	Established
Midland	Reported
Muskegon	Established	R-E	(Hamer et al. 2009, Hamer et al. 2010)
Oakland	Reported
Oceana	Established	R-E	J. Tsao, and J. Sidge, unpublished
Ontonagon	Established	R-E	E. Foster, unpublished
Ottawa	Established	R-E	(Hamer et al. 2009); E. Foster, unpublished
Schoolcraft	Established	R-E	I. Arsnoe, unpublished
St. Joseph	Established	N-E	J. Tsao and E. Foster, unpublished
Van Buren	Established	N-E	(Foster 2004, Hamer et al. 2009, Hamer et al. 2014)
Minnesota
Aitkin	Established	R-E	D. Neitzel, unpublished
Anoka	Established
Becker	Established	N-E	(Sanders and Guilfoile 2000), D. Neitzel, unpublished
Beltrami	Established	R-E	(Sanders and Guilfoile 2000), D. Neitzel, unpublished
Benton	Established	N-E	T. Johnson and D. Neitzel, unpublished
Brown	Reported	N-R	D. Neitzel, unpublished
Carlton	Established	R-E	(Sanders and Guilfoile 2000), D. Neitzel, unpublished
Carver	Established
Cass	Established	N-E	(Sanders and Guilfoile 2000)
Chisago	Established
Clearwater	Established	N-E	(Sanders and Guilfoile 2000), D. Neitzel, unpublished
Cook	Established	N-E	D. Neitzel, unpublished
Crow Wing	Established	R-E
Dakota	Established
Douglas	Established	R-E	D. Neitzel, unpublished
Fillmore	Established	N-E	D. Neitzel, unpublished
Goodhue	Established	N-E	D. Neitzel, unpublished
Hennepin	Established	N-E	D. Neitzel, unpublished
Houston	Established	R-E	D. Neitzel, unpublished
Hubbard	Established	N-E	(Sanders and Guilfoile 2000), D. Neitzel, unpublished
Isanti	Established	R-E	D. Neitzel, unpublished
Itasca	Established	N-E	(Sanders and Guilfoile 2000)
Kanabec	Established	R-E	D. Neitzel, unpublished
Kandiyohi	Established	N-E	(Diuk-Wasser et al. 2006); D. Neitzel, unpublished
Koochiching	Established	N-E	(Sanders and Guilfoile 2000)
Lake	Established	N-E	D. Neitzel, unpublished
Lake of the Woods	Established	N-E	(Sanders and Guilfoile 2000), D. Neitzel, unpublished
Mahnomen	Established	N-E	D. Neitzel, unpublished
Mille Lacs	Established	R-E	(Sanders and Guilfoile 2000)
Morrison	Established
Nicollet	Reported	N-R	T. Johnson and D. Neitzel, unpublished
Olmsted	Established	R-E	D. Neitzel, unpublished
Otter Tail	Established	N-E	D. Neitzel, unpublished
Pine	Established
Pope	Established	N-E	D. Neitzel, unpublished
Ramsey	Established
Rice	Reported	N-R	T. Johnson and D. Neitzel, unpublished
Scott	Established
Sherburne	Established	N-E	(Diuk-Wasser et al. 2006), D. Neitzel, unpublished
Sibley	Established	N-E	T. Johnson and D. Neitzel, unpublished
St. Louis	Established	N-E	(Sanders and Guilfoile 2000), D. Neitzel, unpublished
Stearns	Established	N-E	D. Neitzel, unpublished
Todd	Established	R-E	D. Neitzel, unpublished
Wabasha	Established	N-E	D. Neitzel, unpublished
Wadena	Established	N-E	(Sanders and Guilfoile 2000), D. Neitzel, unpublished
Washington	Established
Winona	Established	R-E	D. Neitzel, unpublished
Wright	Established	N-E	D. Neitzel, unpublished
Mississippi
Adams	Reported
Alcorn	Reported
Amite	Reported
Attala	Reported
Benton	Reported
Bolivar	Established
Calhoun	Reported
Carroll	Reported
Chickasaw	Reported
Choctaw	Reported
Claiborne	Reported
Clarke	Reported
Clay	Reported
Coahoma	Reported
Copiah	Established
Covington	Reported
DeSoto	Reported
Forrest	Reported
Franklin	Reported
George	Reported
Greene	Reported
Grenada	Reported
Hancock	Reported
Harrison	Reported
Hinds	Established
Holmes	Reported
Humphreys	Reported
Issaquena	Reported
Itawamba	Reported
Jackson	Established
Jasper	Reported
Jefferson	Reported
Jefferson Davis	Reported
Jones	Reported
Kemper	Reported
Lafayette	Reported
Lamar	Reported
Lauderdale	Reported
Lawrence	Reported
Leake	Reported
Lee	Reported
Leflore	Reported
Lincoln	Reported
Lowndes	Reported
Madison	Reported
Marion	Reported
Marshall	Established	R-E	(Goltz and Goddard 2013, Goltz et al. 2013)
Monroe	Reported
Montgomery	Reported
Neshoba	Reported
Newton	Reported
Noxubee	Established
Oktibbeha	Established
Panola	Reported
Pearl River	Reported
Perry	Established
Pike	Reported
Pontotoc	Reported
Prentiss	Reported
Quitman	Reported
Rankin	Established
Scott	Established
Sharkey	Reported
Simpson	Reported
Smith	Reported
Stone	Reported
Sunflower	Reported
Tallahatchie	Reported
Tate	Reported
Tippah	Reported
Tishomingo	Reported
Tunica	Reported
Union	Reported
Walthall	Reported
Warren	Reported
Washington	Reported
Wayne	Reported
Webster	Reported
Wilkinson	Reported
Winston	Established
Yalobusha	Reported
Yazoo	Reported
Missouri
Adair	Established	N-E	S. Fore and H.-J. Kim, unpublished
Benton	Established
Bollinger	Established
Boone	Reported
Callaway	Established
Cape Girardeau	Established	R-E	(Kollars et al. 1997, Kollars et al. 1999)
Clark	Reported
Dallas	Established
Dent	Established
Douglas	Established
Gasconade	Established
Greene	Established
Howell	Established
Jasper	Established
Johnson	Established
Laclede	Established
Montgomery	Reported
New Madrid	Reported
Newton	Reported	N-R	http://vectormap.nhm.ku.edu/vectormap/
Osage	Established
Pulaski	Established
Scott	Reported
St. Charles	Established
St. Clair	Reported
St. Louis	Reported
Stoddard	Established
Stone	Established
Taney	Established
Texas	Established
Wayne	Established
Webster	Established
Nebraska
Cass	Reported	N-R	(Cortinas and Spomer 2014)
Lancaster	Reported	N-R	(Cortinas and Spomer 2014)
Pawnee	Reported	N-R	(Cortinas and Spomer 2014)
New Hampshire
Belknap	Established	R-E	http://www.dhhs.nh.gov/dphs/cdcs/lyme/documents/tick-borne-bulletin-2014.pdf
Carroll	Established	R-E	http://www.dhhs.nh.gov/dphs/cdcs/lyme/documents/tick-borne-bulletin-2014.pdf
Cheshire	Established	R-E	http://www.dhhs.nh.gov/dphs/cdcs/lyme/documents/tick-borne-bulletin-2014.pdf
Coos	Reported
Grafton	Established
Hillsborough	Established
Merrimack	Established
Rockingham	Established
Strafford	Established
Sullivan	Established	R-E	http://www.dhhs.nh.gov/dphs/cdcs/lyme/documents/tick-borne-bulletin-2014.pdf
New Jersey
Atlantic	Established
Bergen	Established
Burlington	Established
Camden	Established
Cape May	Established
Cumberland	Established
Essex	Established
Gloucester	Established
Hudson	Established
Hunterdon	Established
Mercer	Established
Middlesex	Established
Monmouth	Established
Morris	Established
Ocean	Established
Passaic	Established
Salem	Established
Somerset	Established
Sussex	Established
Union	Established
Warren	Established
New York
Albany	Established
Allegany	Established	R-E	M. Prusinski/New York Department of Health, unpublished
Bronx	Established
Broome	Established
Cattaraugus	Established
Cayuga	Established	N-E	M. Prusinski/New York Department of Health, unpublished
Chautauqua	Established	R-E	M. Prusinski/New York Department of Health, unpublished
Chemung	Established	N-E	M. Prusinski/New York Department of Health, unpublished
Chenango	Established	R-E	M. Prusinski/New York Department of Health, unpublished
Clinton	Established
Columbia	Established
Cortland	Established	N-E	M. Prusinski/New York Department of Health, unpublished
Delaware	Established
Dutchess	Established
Erie	Established
Essex	Established
Franklin	Established	R-E	M. Prusinski/New York Department of Health, unpublished
Fulton	Established
Genesee	Established	R-E	M. Prusinski/New York Department of Health, unpublished
Greene	Established
Hamilton	Established	N-E	M. Prusinski/New York Department of Health, unpublished
Herkimer	Established	R-E	M. Prusinski/New York Department of Health, unpublished
Jefferson	Established
Kings	Established
Lewis	Established	R-E	M. Prusinski/New York Department of Health, unpublished
Livingston	Established	R-E	M. Prusinski/New York Department of Health, unpublished
Madison	Established	R-E	M. Prusinski/New York Department of Health, unpublished
Monroe	Established	R-E	M. Prusinski/New York Department of Health, unpublished
Montgomery	Established
Nassau	Established
New York	Established	N-E	M. Prusinski/New York Department of Health, unpublished
Niagara	Established	N-E	M. Prusinski/New York Department of Health, unpublished
Oneida	Established
Onondaga	Established	R-E	M. Prusinski/New York Department of Health, unpublished
Ontario	Established	R-E	M. Prusinski/New York Department of Health, unpublished
Orange	Established
Orleans	Reported	N-R	M. Prusinski/New York Department of Health, unpublished
Oswego	Established	R-E	M. Prusinski/New York Department of Health, unpublished
Otsego	Established
Putnam	Established
Queens	Established	R-E	M. Prusinski/New York Department of Health, unpublished
Rensselaer
Richmond	Established	R-E	M. Prusinski/New York Department of Health, unpublished
Rockland	Established
Saratoga	Established
Schenectady	Established
Schoharie	Established
Schuyler	Established	N-E	M. Prusinski/New York Department of Health, unpublished
Seneca	Established	N-E	M. Prusinski/New York Department of Health, unpublished
St. Lawrence	Established	R-E	M. Prusinski/New York Department of Health, unpublished
Steuben	Established	N-E	M. Prusinski/New York Department of Health, unpublished
Suffolk	Established
Sullivan	Established
Tioga	Established	R-E	M. Prusinski/New York Department of Health, unpublished
Tompkins	Established	R-E	M. Prusinski/New York Department of Health, unpublished
Ulster	Established
Warren	Established
Washington	Established
Wayne	Established	N-E	M. Prusinski/New York Department of Health, unpublished
Westchester	Established
Wyoming	Established	R-E	M. Prusinski/New York Department of Health, unpublished
Yates	Established	R-E	M. Prusinski/New York Department of Health, unpublished
North Carolina
Alamance	Established	N-E	B. Harrison/NCPHPMc, unpublished
Anson	Reported
Beaufort	Established
Bertie	Established	N-E	B. Harrison/NCPHPMc, unpublished
Bladen	Established
Brunswick	Established
Camden	Established	R-E	B. Harrison/NCPHPMc, unpublished
Carteret	Established	R-E	B. Harrison/NCPHPMc, unpublished
Catawba	Reported
Chatham	Established	R-E	(Smith et al. 2010)
Chowan	Established	R-E	B. Harrison/NCPHPMc, unpublished
Columbus	Established	R-E	B. Harrison/NCPHPMc, unpublished
Craven	Established	R-E	B. Harrison/NCPHPMc, unpublished
Cumberland	Established
Currituck	Reported
Dare	Established
Duplin	Established	N-E	B. Harrison/NCPHPMc, unpublished
Durham	Established	N-E	B. Harrison/NCPHPMc, unpublished
Edgecombe	Established	N-E	B. Harrison/NCPHPMc, unpublished
Forsyth	Reported	N-R	B. Harrison/NCPHPMc, unpublished
Gates	Established	R-E	B. Harrison/NCPHPMc, unpublished
Granville	Reported	N-R	B. Harrison/NCPHPMc, unpublished; http://vectormap.nhm.ku.edu/vectormap/
Greene	Established	N-E	B. Harrison/NCPHPMc, unpublished
Guilford	Reported	N-R	B. Harrison/NCPHPMc, unpublished
Halifax	Established	N-E	B. Harrison/NCPHPMc, unpublished
Harnett	Established	N-E	B. Harrison/NCPHPMc, unpublished
Haywood	Reported	N-R	B. Harrison/NCPHPMc, unpublished
Hertford	Established	R-E	B. Harrison/NCPHPMc, unpublished
Hoke	Established
Hyde	Established	R-E	B. Harrison/NCPHPMc, unpublished
Johnston	Established	N-E	B. Harrison/NCPHPMc, unpublished
Jones	Established	R-E	B. Harrison/NCPHPMc, unpublished
Lenoir	Reported
Martin	Established	R-E	B. Harrison/NCPHPMc, unpublished
Mecklenburg	Reported	N-R	B. Harrison/NCPHPMc, unpublished
Moore	Reported
Nash	Established	N-E	B. Harrison/NCPHPMc, unpublished
New Hanover	Established	R-E	B. Harrison/NCPHPMc, unpublished
Onslow	Established
Orange	Established	N-E	B. Harrison/NCPHPMc, unpublished
Pamlico	Established	R-E	B. Harrison/NCPHPMc, unpublished
Pasquotank	Reported	N-R	B. Harrison/NCPHPMc, unpublished
Pender	Established	R-E	B. Harrison/NCPHPMc, unpublished
Perquimans	Established	N-E	B. Harrison/NCPHPMc, unpublished
Pitt	Reported	N-R	B. Harrison/NCPHPMc, unpublished
Randolph	Reported	N-R	http://vectormap.nhm.ku.edu/vectormap/
Robeson	Established	N-E	B. Harrison/NCPHPMc, unpublished
Rowan	Established	N-E	B. Harrison/NCPHPMc, unpublished
Rutherford	Reported
Sampson	Established	R-E	B. Harrison/NCPHPMc, unpublished
Scotland	Established	N-E	B. Harrison/NCPHPMc, unpublished
Stokes	Established	N-E	(Sakamoto et al. 2014); B. Harrison/NCPHPMc, unpublished
Surry	Reported	N-R	B. Harrison/NCPHPMc, unpublished
Tyrrell	Established	N-E	B. Harrison/NCPHPMc, unpublished
Vance	Reported	N-R	B. Harrison/NCPHPMc, unpublished
Wake	Established	R-E	B. Harrison/NCPHPMc, unpublished
Warren	Established	N-E	B. Harrison/NCPHPMc, unpublished
Washington	Established	R-E	B. Harrison/NCPHPMc, unpublished
Wayne	Established	N-E	H. Gaff, unpublished; B. Harrison/NCPHPMc, unpublished
North Dakota
Bottineau	Established	N-E	M. Feist and J. Vaughan, unpublished
Eddy	Established	N-E	(Russart et al. 2014)
GrandForks	Established	N-E	(Russart et al. 2014, Stone et al. 2015)
Pembina	Established	N-E	(Russart et al. 2014)
Ramsey	Established	N-E	(Russart et al. 2014)
Ransom	Reported	N-R	M. Feist and J. Vaughan, unpublished
Rolette	Reported	N-R	(Russart et al. 2014)
Steele	Reported	N-R	(Russart et al. 2014)
Ohio
Adams	Established	N-E	R. Gary/Ohio Department of Health, unpublished
Ashland	Reported	N-R	http://www.odh.ohio.gov/lyme
Ashtabula	Established	N-E	(Wang et al. 2014), R. Gary/Ohio Department of Health, unpublished
Athens	Reported	N-R	(Wang et al. 2014), http://www.odh.ohio.gov/lyme
Auglaize	Reported	N-R	http://www.odh.ohio.gov/lyme
Belmont	Established	N-E	R. Gary/Ohio Department of Health, unpublished
Brown	Reported	N-R	http://www.odh.ohio.gov/lyme
Butler	Reported
Carroll	Established	N-E	(Wang et al. 2014), R. Gary/Ohio Department of Health, unpublished
Clermont	Reported
Columbiana	Established	N-E	(Wang et al. 2014), R. Gary/Ohio Department of Health, unpublished
Coshocton	Established	N-E	(Wang et al. 2014), R. Gary/Ohio Department of Health, unpublished
Cuyahoga	Established	N-E	R. Gary/Ohio Department of Health, unpublished
Delaware	Reported	N-R	http://www.odh.ohio.gov/lyme
Erie	Reported	N-R	http://www.odh.ohio.gov/lyme
Fairfield	Reported	N-R	http://www.odh.ohio.gov/lyme
Fayette	Reported	N-R	http://www.odh.ohio.gov/lyme
Franklin	Reported	N-R	R. Gary, unpublished; http://vectormap.nhm.ku.edu/vectormap/
Gallia	Established	N-E	(Wang et al. 2014), R. Gary/Ohio Department of Health, unpublished
Geauga	Reported	N-R	http://www.odh.ohio.gov/lyme
Greene	Reported	N-R	http://www.odh.ohio.gov/lyme
Guernsey	Established	N-E	(Wang et al. 2014), R. Gary/Ohio Department of Health, unpublished
Hamilton	Established	R-E	R. Gary/Ohio Department of Health, unpublished
Harrison	Established	N-E	(Wang et al. 2014), R. Gary/Ohio Department of Health, unpublished
Highland	Reported	N-R	(Wang et al. 2014), http://www.odh.ohio.gov/lyme
Hocking	Established	N-E	(Wang et al. 2014), R. Gary/Ohio Department of Health, unpublished
Holmes	Established	N-E	(Wang et al. 2014), R. Gary/Ohio Department of Health, unpublished
Jackson	Established	N-E	(Wang et al. 2014), R. Gary/Ohio Department of Health, unpublished
Jefferson	Established	N-E	(Wang et al. 2014), R. Gary/Ohio Department of Health, unpublished
Knox	Established	N-E	(Wang et al. 2014), R. Gary/Ohio Department of Health, unpublished
Lake	Established	N-E	R. Gary/Ohio Department of Health, unpublished
Lawrence	Established	N-E	(Wang et al. 2014), R. Gary/Ohio Department of Health, unpublished
Licking	Established	N-E	(Wang et al. 2014), R. Gary/Ohio Department of Health, unpublished
Lucas	Reported	N-R	http://www.odh.ohio.gov/lyme
Madison	Reported	N-R	http://www.odh.ohio.gov/lyme
Mahoning	Reported	N-R	http://www.odh.ohio.gov/lyme
Medina	Established	N-E	R. Gary/Ohio Department of Health, unpublished
Meigs	Reported	N-R	http://www.odh.ohio.gov/lyme
Monroe	Established	N-E	(Wang et al. 2014), R. Gary/Ohio Department of Health, unpublished
Montgomery	Reported	N-R	http://www.odh.ohio.gov/lyme
Morgan	Established	N-E	(Wang et al. 2014), R. Gary/Ohio Department of Health, unpublished
Morrow	Reported	N-R	http://www.odh.ohio.gov/lyme
Muskingum	Established	N-E	(Wang et al. 2014), R. Gary/Ohio Department of Health, unpublished
Noble	Established	N-E	(Wang et al. 2014), R. Gary/Ohio Department of Health, unpublished
Ottawa	Reported	N-R	http://www.odh.ohio.gov/lyme
Paulding	Reported	N-R
Perry	Established	N-E	R. Gary/Ohio Department of Health, unpublished
Pickaway	Reported	N-R	http://www.odh.ohio.gov/lyme
Pike	Established	N-E	(Wang et al. 2014), R. Gary/Ohio Department of Health, unpublished
Portage	Established	N-E	R. Gary/Ohio Department of Health, unpublished
Richland	Established	N-E	(Wang et al. 2014), R. Gary/Ohio Department of Health, unpublished
Ross	Established	N-E	(Wang et al. 2014), R. Gary/Ohio Department of Health, unpublished
Scioto	Reported	N-R	http://www.odh.ohio.gov/lyme
Stark	Established	N-E	R. Gary/Ohio Department of Health, unpublished
Summit	Established	N-E	R. Gary/Ohio Department of Health, unpublished
Trumbull	Established	N-E	R. Gary/Ohio Department of Health, unpublished
Tuscarawas	Reported	N-R	(Wang et al. 2014),http://www.odh.ohio.gov/lyme
Union	Reported	N-R	http://www.odh.ohio.gov/lyme
Vinton	Reported	N-R	(Wang et al. 2014), http://www.odh.ohio.gov/lyme
Warren	Reported	N-R	http://www.odh.ohio.gov/lyme
Washington	Reported	N-R	http://www.odh.ohio.gov/lyme
Wayne	Established	R-E	R. Gary/Ohio Department of Health, unpublished
Williams	Reported
Wood	Reported	N-R	http://www.odh.ohio.gov/lyme
Oklahoma
Adair	Reported
Atoka	Reported
Caddo	Reported
Carter	Reported
Cherokee	Established
Coal	Reported
Comanche	Reported
Creek	Reported
Delaware	Reported
Garfield	Reported
Garvin	Reported
Haskell	Reported
Hughes	Reported
Johnston	Reported
Latimer	Reported
Le Flore	Established
Lincoln	Reported
Love	Reported
Mayes	Reported
McClain	Established
McCurtain	Reported
McIntosh	Reported
Murray	Reported
Muskogee	Reported
Oklahoma	Reported
Okmulgee	Reported
Ottawa	Reported
Pawnee	Reported
Payne	Reported
Pittsburg	Reported
Pontotoc	Reported
Pottawatomie	Reported
Pushmataha	Reported
Rogers	Reported
Seminole	Reported
Sequoyah	Reported
Tulsa	Reported
Wagoner	Reported
Washington	Reported
Pennsylvania
Adams	Established	R-E	(Han et al. 2014, Hutchinson et al. 2015)
Allegheny	Established	R-E	(Hutchinson et al. 2015)
Armstrong	Established	R-E	(Hutchinson et al. 2015)
Beaver	Established	N-E	(Hutchinson et al. 2015)
Bedford	Established	N-E	(Hutchinson et al. 2015)
Berks	Established
Blair	Established
Bradford	Established	N-E	(Hutchinson et al. 2015)
Bucks	Established
Butler	Established
Cambria	Established	R-E	(Hutchinson et al. 2015)
Cameron	Established
Carbon	Established	N-E	(Hutchinson et al. 2015)
Centre	Established
Chester	Established
Clarion	Established
Clearfield	Established
Clinton	Established	R-E	(Hutchinson et al. 2015)
Columbia	Established	R-E	(Hutchinson et al. 2015)
Crawford	Established	R-E	(Hutchinson et al. 2015)
Cumberland	Established	R-E	(Hutchinson et al. 2015)
Dauphin	Established	R-E	(Hutchinson et al. 2015)
Delaware	Established
Elk	Established
Erie	Established
Fayette	Established	N-E	(Hutchinson et al. 2015)
Forest	Established	N-E	(Hutchinson et al. 2015)
Franklin	Established	N-E	(Hutchinson et al. 2015)
Fulton	Established	R-E	(Hutchinson et al. 2015)
Greene	Established	N-E	(Hutchinson et al. 2015)
Huntingdon	Established	R-E	(Hutchinson et al. 2015)
Indiana	Established	N-E	(Hutchinson et al. 2015)
Jefferson	Established	R-E	(Hutchinson et al. 2015)
Juniata	Established	N-E	(Hutchinson et al. 2015)
Lackawanna	Established	R-E	(Hutchinson et al. 2015)
Lancaster	Established
Lawrence	Established	R-E	(Hutchinson et al. 2015)
Lebanon	Established	R-E	(Hutchinson et al. 2015)
Lehigh	Established
Luzerne	Established	R-E	(Hutchinson et al. 2015)
Lycoming	Established	R-E	(Hutchinson et al. 2015)
McKean	Established
Mercer	Established
Mifflin	Established	R-E	(Hutchinson et al. 2015)
Monroe	Established
Montgomery	Established
Montour	Established	N-E	(Hutchinson et al. 2015)
Northampton	Established
Northumberland	Established	R-E	(Hutchinson et al. 2015)
Perry	Established	R-E	(Hutchinson et al. 2015)
Philadelphia	Established
Pike	Established
Potter	Established	R-E	(Hutchinson et al. 2015)
Schuylkill	Established	R-E	(Hutchinson et al. 2015)
Snyder	Established	N-E	(Hutchinson et al. 2015)
Somerset	Established	N-E	(Hutchinson et al. 2015)
Sullivan	Established	N-E	(Hutchinson et al. 2015)
Susquehanna	Established	N-E	(Hutchinson et al. 2015)
Tioga	Established	N-E	(Hutchinson et al. 2015)
Union	Established	R-E	(Hutchinson et al. 2015)
Venango	Established	N-E	(Hutchinson et al. 2015)
Warren	Established	R-E	(Hutchinson et al. 2015)
Washington	Established	R-E	(Hutchinson et al. 2015)
Wayne	Established
Westmoreland	Established	R-E	(Hutchinson et al. 2015)
Wyoming	Established	N-E	(Hutchinson et al. 2015)
York	Established
Rhode Island
Bristol	Established
Kent	Established
Newport	Established
Providence	Established
Washington	Established
South Carolina
Abbeville	Reported
Aiken	Established
Allendale	Reported
Anderson	Reported
Barnwell	Established
Beaufort	Established
Berkeley	Established
Calhoun	Reported	N-R	(Williams et al. 1999)
Charleston	Established
Chester	Established
Chesterfield	Reported	N-R	(Williams et al. 1999)
Colleton	Reported
Darlington	Reported
Dorchester	Reported
Edgefield	Established
Fairfield	Reported
Florence	Established
Georgetown	Established
Greenville	Reported	N-R	(Williams et al. 1999)
Greenwood	Reported
Hampton	Established
Horry	Reported
Jasper	Established	R-E	L. Durden, unpublished
Laurens	Reported
McCormick	Established
Newberry	Established
Orangeburg	Reported
Pickens	Reported
Richland	Reported	N-R	(Williams et al. 1999)
Saluda	Reported	N-R	(Williams et al. 1999)
Spartanburg	Reported
Sumter	Reported	N-R	http://vectormap.nhm.ku.edu/vectormap/
Union	Established
South Dakota
Brookings	Reported
Codington	Reported
Tennessee
Anderson	Established	N-E	(Rosen et al. 2012)
Bedford	Established	R-E	(Rosen et al. 2012)
Bledsoe	Reported	N-R	(Rosen et al. 2012)
Campbell	Established	N-E	(Rosen et al. 2012)
Claiborne	Reported	N-R	(Rosen et al. 2012)
Clay	Reported	N-R	(Rosen et al. 2012)
Coffee	Established	N-E	G. Hickling, unpublished
Cumberland	Reported	N-R	(Harmon et al. 2011, Rosen et al. 2012)
Davidson	Reported	N-R	(Rosen et al. 2012)
DeKalb	Established	N-E	(Rosen et al. 2012)
Fayette	Established	N-E	(Rosen et al. 2012, Mays et al. 2014)
Fentress	Reported
Franklin	Established	N-E	G. Hickling, unpublished
Giles	Reported	N-R	(Rosen et al. 2012)
Grainger	Reported	N-R	(Rosen et al. 2012)
Hamilton	Established	N-E	(Rosen et al. 2012)
Henry	Reported	N-R	(Rosen et al. 2012)
Humphreys	Reported	N-R	(Rosen et al. 2012)
Jackson	Established	N-E	(Rosen et al. 2012)
Knox	Established	N-E	G. Hickling, unpublished
Lake	Reported
Lauderdale	Reported	N-R	(Rosen et al. 2012)
Lawrence	Established	N-E	(Rosen et al. 2012)
Loudon	Established	N-E	(Rosen et al. 2012)
Marion	Established	R-E	(Rosen et al. 2012)
Marshall	Established	N-E	(Rosen et al. 2012)
Maury	Reported	N-R	(Rosen et al. 2012)
Montgomery	Reported	N-R	http://vectormap.nhm.ku.edu/vectormap/
Obion	Reported	N-R	(Rosen et al. 2012)
Pickett	Reported	N-R	(Rosen et al. 2012)
Rhea	Established	N-E	(Rosen et al. 2012)
Roane	Reported	N-R	(Rosen et al. 2012)
Rutherford	Reported
Scott	Reported
Sequatchie	Reported	N-R	(Rosen et al. 2012)
Shelby	Established
Stewart	Reported	N-R	(Rosen et al. 2012)
Sumner	Reported	N-R	(Rosen et al. 2012)
Tipton	Reported	N-R	(Rosen et al. 2012)
Warren	Reported	N-R	(Rosen et al. 2012)
Wayne	Reported	N-R	(Rosen et al. 2012)
White	Reported	N-R	(Rosen et al. 2012)
Williamson	Reported	N-R	(Rosen et al. 2012)
Texas
Anderson	Established
Angelina	Established
Aransas	Established
Austin	Reported
Bandera	Reported
Bastrop	Reported
Bell	Established
Bexar	Established
Blanco	Reported	N-R	S. Hamer, unpublished
Bowie	Reported
Brazoria	Established
Brazos	Established	R-E	(Sanders et al. 2013, Rodriguez et al. 2015)
Cameron	Reported	N-R	(Feria-Arroyo et al. 2014)
Cass	Established
Cherokee	Established
Colorado	Reported
Coryell	Reported	N-R	(Sanders et al. 2013)
Dallas	Reported
Edwards	Reported
FortBend	Reported
Franklin	Reported
Freestone	Established	N-E	S. Hamer, unpublished
Grayson	Reported
Gregg	Reported
Hamilton	Reported
Harris	Established
Harrison	Reported
Hays	Reported
Henderson	Established
Hidalgo	Reported	N-R	(Feria-Arroyo et al. 2014)
Houston	Reported
Hunt	Reported
Jasper	Established
Jefferson	Reported
Kerr	Reported
Kleberg	Reported
Lamar	Reported
Lampasas	Reported
Lavaca	Established
Leon	Established
Liberty	Reported
Llano	Reported
Marion	Established
Mason	Reported	N-R	(Feria-Arroyo et al. 2014)
Matagorda	Reported
Montgomery	Established
Nacogdoches	Established
Newton	Reported
Palo Pinto	Reported
Parker	Established
Polk	Established
Real	Established
Robertson	Reported
Rusk	Reported
Sabine	Reported
San Augustine	Reported
Shelby	Reported
Smith	Established
Sutton	Reported
Tarrant	Reported	N-R	(Feria-Arroyo et al. 2014)
Taylor	Reported
Trinity	Established
Tyler	Established
Upshur	Reported
Uvalde	Reported
Victoria	Reported
Walker	Reported
Waller	Established
Washington	Reported	N-R	S. Hamer, unpublished
Wood	Established
Zavala	Reported
Vermont
Addison	Established	N-E	https://apps.health.vermont.gov/gis/vttracking/ticktracker/2014Summary/
Bennington	Established	R-E	https://apps.health.vermont.gov/gis/vttracking/ticktracker/2014Summary/
Caledonia	Established	R-E	https://apps.health.vermont.gov/gis/vttracking/ticktracker/2014Summary/
Chittenden	Established	R-E	https://apps.health.vermont.gov/gis/vttracking/ticktracker/2014Summary/
Franklin	Established	N-E	https://apps.health.vermont.gov/gis/vttracking/ticktracker/2014Summary/
GrandIsle	Established	N-E	https://apps.health.vermont.gov/gis/vttracking/ticktracker/2014Summary/
Lamoille	Reported
Orange	Established	N-E	(Serra et al. 2013)
Orleans	Reported	N-R	https://apps.health.vermont.gov/gis/vttracking/ticktracker/2014Summary/
Rutland	Established	R-E	https://apps.health.vermont.gov/gis/vttracking/ticktracker/2014Summary/
Washington	Established	N-E	https://apps.health.vermont.gov/gis/vttracking/ticktracker/2014Summary/
Windham	Established	R-E	https://apps.health.vermont.gov/gis/vttracking/ticktracker/2014Summary/
Windsor	Established
Virginia
Accomack	Established
Albemarle	Established	N-E	D. Gaines, unpublished
Alleghany	Reported	N-R	D. Gaines, unpublished
Appomattox	Established	N-E	R.J. Brinkerhoff, unpublished
Arlington	Reported	N-R	http://vectormap.nhm.ku.edu/vectormap/
Augusta	Established	N-E	D. Gaines, unpublished
Bath	Reported	N-R	D. Gaines, unpublished
Bedford City	Reported	N-R	http://vectormap.nhm.ku.edu/vectormap/
Bedford	Established	N-E	M. Shanks, D. Gaines, unpublished
Bland	Established	N-E	M. Shanks, D. Gaines, unpublished
Brunswick	Reported	N-R	D. Gaines, unpublished
Buckingham	Established	N-E	D. Gaines, unpublished; R.J. Brinkerhoff, unpublished
Caroline	Established
Charles City	Reported	N-R	D. Gaines, unpublished; R.J. Brinkerhoff, unpublished
Chesapeake	Established
Chesterfield	Established	N-E	(Kelly et al. 2014)
Craig	Established	N-E	M. Shanks, D. Gaines, unpublished
Culpeper	Reported	N-R	H. Gaff, unpublished; http://vectormap.nhm.ku.edu/vectormap/
Cumberland	Reported	N-R	R.J. Brinkerhoff, unpublished
Dinwiddie	Reported	N-R	http://vectormap.nhm.ku.edu/vectormap/
Essex	Reported	N-R	H. Gaff, unpublished
Fairfax	Established
Fauquier	Established	R-E	R.J. Brinkerhoff, unpublished
Floyd	Established	N-E	D. Gaines, unpublished
Fluvanna	Established	N-E	R.J. Brinkerhoff, unpublished
Franklin	Established	N-E	M. Shanks, D. Gaines, unpublished
Frederick	Established	N-E	D. Gaines, unpublished
Giles	Established	N-E	(Herrin et al. 2014)
Gloucester	Reported	N-R	H. Gaff, unpublished; http://vectormap.nhm.ku.edu/vectormap/
Goochland	Established	N-E	(Kelly et al. 2014)
Greensville	Reported	N-R	H. Gaff, unpublished
Hampton	Established	N-E	H. Gaff, unpublished
Hanover	Reported	N-R	R.J. Brinkerhoff, unpublished
Henrico	Reported	N-R	D. Gaines, unpublished
Isle of Wight	Established	N-E	H. Gaff, unpublished
James City	Established	N-E	(Kelly et al. 2014); H. Gaff, unpublished
King and Queen	Reported	N-R	H. Gaff, unpublished
King George	Established	N-E	D. Gaines, unpublished; H. Gaff, unpublished
Lancaster	Reported	N-R	H. Gaff, unpublished
Loudoun	Established	N-E	H. Gaff, unpublished
Lynchburg	Established	N-E	D. Gaines, unpublished
Mecklenburg	Reported	N-R	D. Gaines, unpublished
Middlesex	Established	N-E	H. Gaff, unpublished
Montgomery	Established	N-E	D. Gaines, unpublished
Nelson	Established	N-E	Kelly et al. 2014
New Kent	Established	N-E	R.J. Brinkerhoff, unpublished
Newport News	Established
Norfolk	Reported	N-R	H. Gaff, unpublished
Northampton	Established	N-E	(Kelly et al. 2014); H. Gaff, unpublished
Northumberland	Reported	N-R	H. Gaff, unpublished; http://vectormap.nhm.ku.edu/vectormap/
Portsmouth	Established	N-E	H. Gaff, unpublished
Powhatan	Established	N-E	R.J. Brinkerhoff, unpublished
Prince Edward	Reported	N-R	R.J. Brinkerhoff, unpublished
Prince George	Reported
Prince William	Established	N-E	H. Gaff, unpublished; http://vectormap.nhm.ku.edu/vectormap/
Pulaski	Established	N-E	(Herrin et al. 2014)
Rappahannock	Reported	N-R	http://vectormap.nhm.ku.edu/vectormap/
Richmond	Reported	N-R	D. Gaines, unpublished; H. Gaff, unpublished
Roanoke	Established	N-E	D. Gaines, unpublished
Rockbridge	Established	N-E	D. Gaines, unpublished
Rockingham	Reported	N-R	D. Gaines, unpublished
Shenandoah	Reported	N-R	D. Gaines, unpublished
Southampton	Established	R-E	H. Gaff, unpublished
Spotsylvania	Reported	N-R	http://vectormap.nhm.ku.edu/vectormap/
Stafford	Established
Suffolk	Established	N-E	D. Gaines, unpublished; H. Gaff, unpublished
Sussex	Reported	N-R	H. Gaff, unpublished
Virginia Beach	Established
Warren	Established	N-E	D. Gaines, unpublished
Washington	Reported	N-R	http://vectormap.nhm.ku.edu/vectormap/
Westmoreland	Established	N-E	H. Gaff, unpublished
York	Established
West Virginia
Barbour	Established	N-E	http://www.dhhr.wv.gov/oeps/disease/Zoonosis/Mosquito/Documents/arbovirus/vectorborne-disease-report.pdf#page=3; E. Dotseth, unpublished
Berkeley	Established
Boone	Reported
Braxton	Established	N-E	E. Dotseth, unpublished
Brooke	Established	N-E	http://www.mamca.org/2014Meeting/0306_0840_SR_WV.pdf; E. Dotseth, unpublished
Cabell	Reported	N-R	http://www.dhhr.wv.gov/oeps/disease/zoonosis/documents/wv-zd-summary-2014.pdf
Doddridge	Reported	N-R	http://www.dhhr.wv.gov/oeps/disease/Zoonosis/Mosquito/Documents/arbovirus/vectorborne-disease-report.pdf#page=3
Fayette	Reported	N-R	E. Dotseth, unpublished
Gilmer	Established	N-E	E. Dotseth, unpublished
Grant	Reported	N-R	http://www.dhhr.wv.gov/oeps/disease/Zoonosis/Mosquito/Documents/arbovirus/vectorborne-disease-report.pdf#page=3
Greenbrier	Established	N-E	http://www.mamca.org/2014Meeting/0306_0840_SR_WV.pdf; E. Dotseth, unpublished
Hampshire	Reported	N-R	http://www.mamca.org/2014Meeting/0306_0840_SR_WV.pdf
Hancock	Established	N-E	http://www.dhhr.wv.gov/oeps/disease/Zoonosis/Tick/documents/tickborne-disease-summary-2013.pdf
Hardy	Established	N-E	http://www.dhhr.wv.gov/oeps/disease/zoonosis/tick/documents/tickborne-disease-summary-2013.pdf; E. Dotseth, unpublished
Harrison	Established	N-E	http://www.dhhr.wv.gov/oeps/disease/zoonosis/tick/documents/tickborne-disease-summary-2013.pdf; E. Dotseth, unpublished
Jackson	Reported	N-R	http://www.dhhr.wv.gov/oeps/disease/Zoonosis/Mosquito/Documents/arbovirus/vectorborne-disease-report.pdf#page=3
Jefferson	Established
Kanawha	Established	N-E	http://www.dhhr.wv.gov/oeps/disease/Zoonosis/Tick/documents/tickborne-summary-2012.pdf
Lewis	Established	N-E	E. Dotseth & M. Mark-Carew, unpublished
Lincoln	Reported	N-R	http://www.dhhr.wv.gov/oeps/disease/Zoonosis/Mosquito/Documents/arbovirus/vectorborne-disease-report.pdf#page=3
Logan	Reported	N-R	http://www.dhhr.wv.gov/oeps/disease/Zoonosis/Mosquito/Documents/arbovirus/vectorborne-disease-report.pdf#page=3
Marion	Established	N-E	http://www.dhhr.wv.gov/oeps/disease/zoonosis/tick/documents/tickborne-disease-summary-2013.pdf; E. Dotseth, unpublished
Marshall	Reported	N-R	http://www.dhhr.wv.gov/oeps/disease/zoonosis/tick/documents/tickborne-disease-summary-2013.pdf
Mercer	Established	N-E	http://www.dhhr.wv.gov/oeps/disease/Zoonosis/Mosquito/Documents/arbovirus/vectorborne-disease-report.pdf#page=3; E. Dotseth, unpublished
Mineral	Reported	N-R	http://www.dhhr.wv.gov/oeps/disease/Zoonosis/Mosquito/Documents/arbovirus/vectorborne-disease-report.pdf#page=3
Monongalia	Established	N-E	http://www.dhhr.wv.gov/oeps/disease/zoonosis/tick/documents/tickborne-disease-summary-2013.pdf; E. Dotseth, unpublished
Monroe	Established	N-E	E. Dotseth, unpublished
Morgan	Reported
Ohio	Reported	N-R	E. Dotseth, unpublished
Pocahontas	Reported	N-R	E. Dotseth, unpublished
Preston	Established	N-E	http://www.mamca.org/2014Meeting/0306_0840_SR_WV.pdf; E. Dotseth, unpublished
Putnam	Reported	N-R	http://www.dhhr.wv.gov/oeps/disease/zoonosis/documents/wv-zd-summary-2014.pdf
Raleigh	Established	N-E	E. Dotseth, unpublished
Randolph	Reported	N-R	http://www.mamca.org/2014Meeting/0306_0840_SR_WV.pdf
Ritchie	Reported	N-R	E. Dotseth, unpublished
Summers	Established	N-E	http://www.dhhr.wv.gov/oeps/disease/zoonosis/tick/documents/tickborne-disease-summary-2013.pdf; E. Dotseth, unpublished
Taylor	Established	N-E	http://www.dhhr.wv.gov/oeps/disease/zoonosis/tick/documents/tickborne-disease-summary-2013.pdf; E. Dotseth, unpublished
Tucker	Reported	N-R	http://www.dhhr.wv.gov/oeps/disease/Zoonosis/Tick/documents/tickborne-disease-summary-2013.pdf
Tyler	Reported	N-R	http://www.dhhr.wv.gov/oeps/disease/zoonosis/documents/wv-zd-summary-2014.pdf
Upshur	Established	N-E	E. Dotseth, unpublished
Wetzel	Established	N-E	E. Dotseth, unpublished
Wirt	Established	N-E	E. Dotseth, unpublished
Wood	Reported	N-R	E. Dotseth, unpublished
Wisconsin
Adams	Established
Ashland	Established	R-E	S. Paskewitz, unpublished
Barron	Established
Bayfield	Established	N-E	S. Paskewitz, unpublished
Brown	Established	N-E	S. Paskewitz, unpublished
Buffalo	Established	R-E	(Michalski et al. 2006)
Burnett	Established
Chippewa	Established
Clark	Established
Columbia	Established
Crawford	Established
Dane	Established
Dodge	Reported	N-R	(Lee et al. 2013)
Door	Established	R-E	S. Paskewitz, unpublished
Douglas	Established	N-E	S. Paskewitz, unpublished.
Dunn	Established	N-E	S. Paskewitz, unpublished
Eau Claire	Established
Florence	Reported	N-R	http://labs.russell.wisc.edu/wisconsin-ticks/presence-of-ixodes-scapularis-on-hunter-killed-deer-in-wisconsin-2008-09/
Fond du Lac	Reported	N-R	Lee et al. 2013; http://labs.russell.wisc.edu/wisconsin-ticks/presence-of-ixodes-scapularis-on-hunter-killed-deer-in-wisconsin-2008-09/
Forest	Established	N-E	S. Paskewitz, unpublished
Grant	Established
Green	Established
Green Lake	Established	N-E	(Michalski et al. 2006)
Iowa	Established
Jackson	Established
Jefferson	Established	R-E	S. Paskewitz, unpublished
Juneau	Established	N-E	S. Paskewitz, unpublished
Kenosha	Reported	N-R	(Lee et al. 2013)
La Crosse	Established
Lafayette	Reported
Langlade	Established	N-E	S. Paskewitz, unpublished
Lincoln	Established
Manitowoc	Reported
Marathon	Established
Marinette	Established
Menominee	Established	N-E	S. Paskewitz, unpublished
Milwaukee	Established	N-E	S. Paskewitz, unpublished
Monroe	Established
Oconto	Established	N-E	S. Paskewitz, unpublished
Oneida	Established	R-E	S. Paskewitz, unpublished
Outagamie	Reported
Ozaukee	Reported	N-R	http://labs.russell.wisc.edu/wisconsin-ticks/presence-of-ixodes-scapularis-on-hunter-killed-deerin-wisconsin-2008-09/
Pierce	Reported	N-R	S. Paskewitz, unpublished
Polk	Reported
Portage	Established
Price	Established
Racine	Established
Richland	Established
Rock	Established
Rusk	Established
Sauk	Established
Sawyer	Established
Shawano	Reported	N-R	(Lee et al. 2013)
Sheboygan	Established	R-E	S. Paskewitz, unpublished
St. Croix	Established	R-E	S. Paskewitz, unpublished
Taylor	Established
Trempealeau	Established
Vernon	Reported
Vilas	Established	N-E	S. Paskewitz, unpublished
Walworth	Established	R-E	(Caporale et al. 2005, Lee et al. 2013)
Washburn	Established
Waukesha	Established	R-E	S. Paskewitz, unpublished
Waupaca	Established	N-E	S. Paskewitz, unpublished
Waushara	Reported
Winnebago	Reported
Wood	Reported	N-R	S. Paskewitz, unpublished

^aEstablished: Six or more ticks or two or more tick life stages; Reported: Fewer than six ticks and one tick life stage only.

^bN-R, change from No Records to Reported; N-E, change from No Records to Established; R-E, change from Reported to Established.

^cNCPHPM: North Carolina Public Health Pest Management, terminated in 2011.

^dSCWDS: Southeastern Cooperative Wildlife Disease Study.

^eNational Veterinary Services Laboratories.

Fig. 2.

Changes in county status for I. scapularis and I. pacificus from December 1996 (Dennis et al. 1998) to August 2015 (our data). Black or gray color indicates that county status already was established (black) or reported (gray) for I. scapularis or I. pacificus by Dennis et al. (1998) and considered to be the same in this study. Red or orange color indicates that the status of a county changed from no records to established (red) or from reported to established (orange). Green color indicates that the status of a county changed from no records to reported.

The data presented here suggest that I. scapularis over the past two decades has expanded from its northeastern focus northward into upstate New York, Vermont, New Hampshire, and northern Maine; westward across Pennsylvania, eastern Ohio, and New York; and south- and southwestward into West Virginia, Virginia, and North Carolina (Fig. 2). A similar geographic expansion for I. scapularis appears to have occurred from the long-established focus in the North-Central states, with notable spread of counties where the tick is now classified as established in all four cardinal directions (Fig. 2). The two previously distinct foci in the Northeast and North-Central states appear to be merging in the Ohio River Valley to form a single contiguous focus. In striking contrast to the Northeast and North-Central states, in the far South and South- Central states, counties where I. scapularis is classified as established have remained relatively stable since the survey by Dennis et al. (1998) (Figs. 1–2).

Ixodes scapularis is now classified as reported in 578 counties (18.6% of counties in the continental United States) distributed across 30 states (Table 1; Fig. 1). Counties classified as reported for I. scapularis generally clustered around counties classified as established for this tick. The overall ratio of counties in which I. scapularis is classified as established versus reported (established:reported) was 1.41:1 in this study, as compared with 0.71:1 in the previous Dennis et al. (1998) survey.

Counties Where I. pacificus Is Classified as Established or Reported

Ixodes pacificus is now classified as established in 95 counties, and as reported in 16 additional counties, spanning 6 states (Tables 1, 3; Fig. 1). The majority of these counties are in the Pacific Coast states of California, Oregon, and Washington. Our new data mark a very modest increase in the number of western counties in which I. pacificus is classified as established since the survey by Dennis et al. (1998), when the tick was listed as established in 90 counties; the number of counties with reported status remained stable. In total, five counties were updated from either no records (n = 1) or reported (n = 4) to established and four counties were updated from no records to reported (Table 3; Fig. 2). All county status changes occurred in Washington, Oregon, or Utah (Tables 1, 3; Figs. 1–2). The overall ratio of counties in which I. pacificus is classified as established versus reported was 5.94:1 in this study, compared with 5.6:1 (Dennis et al. 1998).

Discussion

Data on the current geographic distributions of medically important tick vectors, such as I. scapularis and I. pacificus, provide information complementary to epidemiological data on geographic disease case occurrence to inform the medical community and the public of where risk for exposure to tick-borne disease agents may occur. The lack of routine systematic surveillance across the continental United States of ticks of public health importance hampers our ability to define their current geographic distributions and to monitor changes in their ranges and densities over time. Although we are able to report in this paper where I. scapularis and I. pacificus are now known to be present at the county level, our certainty in where the tick is absent is low, especially at the edges of their ranges and in regions where they can be assumed to occur only at low densities. Range contractions, if they occurred, were not quantified in this study because counties that were previously considered established maintained that status here. Nonetheless, using survey methods similar to those of Dennis et al. (1998), specifically, literature review and inclusion of unpublished data from individual researchers and state public health departments, we showed a substantial increase over the past nearly two decades in counties classified as having I. scapularis present. Moreover, the number of counties in which I. scapularis is considered established has more than doubled since the previous national distribution map was published (Dennis et al. 1998). The majority of county status changes occurred in the North, while the distribution in the South remained fairly stable. The North-Central focus for I. scapularis in Minnesota and Wisconsin appears to have expanded in all cardinal directions, and the Northeastern focus has spread inland from the Atlantic seaboard and expanded in both northerly and southerly directions. As a result, the two previously distinct foci in the North-Central and Northeastern United States have now converged in the Ohio River Valley to form a single larger focus. In striking contrast to I. scapularis, increases in counties reporting the presence of I. pacificus in the Far West were very modest.

Population genetic analyses provide support for the theory that I. scapularis was once established across the Northeastern and North-Central United States for thousands of years and likely colonized the region following the recession of the Pleistocene ice sheet (Humphrey et al. 2010). Thus, current trends may represent recolonization of the tick’s historical range. Rapid deforestation and suppression of white-tailed deer during the late 1800s and early 1990s may have restricted I. scapularis to focal refugia (Spielman et al. 1985, Lee et al. 2013). Reforestation and increasing abundance of white-tailed deer, the primary hosts of adult I. scapularis (Spielman et al. 1985), are considered to have contributed to the dramatic expansion of the tick’s range over the past half century (Spielman 1994).

Our updated species distribution map shows a continued range expansion for I. scapularis, particularly in northern states. Given the lack of systematic surveillance for I. scapularis, one might ask if the range expansion suggested by our data is real or merely an artifact of enhanced tick surveillance and research activities in some areas. A true range expansion of I. scapularis in northern states, as described in this report, is supported by the largely concordant changes in the distribution of human Lyme disease cases captured through mandatory reporting of the disease since 1991 (Rand et al. 2007, Raizman et al. 2012, Lee et al. 2013, Serra et al. 2013, Brinkerhoff et al. 2014, Robinson et al. 2014, Wang et al. 2014, Kugeler et al. 2015, Mead 2015). Moreover, as detailed later in the text, true range expansions of I. scapularis have been documented in some areas where tick surveillance was conducted routinely during the period of emergence, or where extensive surveys were conducted at discrete time-points spanning periods from when the tick was absent through invasion and establishment.

The North-Central States

Within the North-Central United States, I. scapularis was first described in a focal area of northwestern Wisconsin in the late 1960s (Jackson and DeFoliart 1970). State-wide surveys of adult I. scapularis collected from hunter-killed deer in Wisconsin from 1981–1989 (French et al. 1992) revealed that the tick had become established in western, southern, and focal parts of the north, but no evidence of the tick was found in other areas in the north or southeastern reaches of Wisconsin. Subsequent surveys of hunter-killed deer documented continued expansion into the north during the early 1990s (Riehle and Paskewitz 1996) and eventual invasion of eastern Wisconsin by 2008–2009 (Lee et al. 2013). Ixodes scapularis now appears to be present throughout most of the habitat that is predicted to be suitable for the tick in the state of Wisconsin (Guerra et al. 2002, Diuk-Wasser et al. 2010).

In neighboring Minnesota to the west, I. scapularis was classified as reported or established primarily in counties bordering Wisconsin in the east-central portion of Minnesota in the mid-1990s (Dennis et al. 1998). Opportunistic sampling during 1998–1999 revealed the presence of I. scapularis in additional northern and central Minnesota counties (Sanders and Guilfoile 2000), and new county records from the south-central portion of the state are presented in this report. Paralleling this expansion of the tick’s known range in Minnesota, Robinson et al. (2014) noted increases in both the numbers and geographical distributions of I. scapularis-borne diseases in Minnesota from 1996 through 2011. The north-westerly expansion appears to have continued into eastern North Dakota (Russart et al. 2014, Stone et al. 2015), beyond or near the limit of habitat previously predicted to be suitable for I. scapularis (Estrada-Pena 2002, Brownstein et al. 2003, Diuk-Wasser et al. 2010).

Wisconsin likely served as a primary source for a southerly invasion of I. scapularis into Illinois, specifically along the Rock River corridor (Cortinas and Kitron 2006). Surveys of hunter-killed deer from Illinois conducted from 1988–1996 showed that infested deer were restricted largely to northern counties (Cortinas et al. 2002). However, similar surveys conducted from 1998 to 2003 showed expansion of the tick’s range to more southern counties and noted that I. scapularis densities decreased along a northern to southern gradient, suggesting that Illinois was first colonized in the northwestern and north-central counties, where I. scapularis indeed was first discovered in the state in the late 1980s (Bouseman et al. 1990, Cortinas and Kitron 2006). Populations of I. scapularis in the extreme northeast along the Illinois River speculatively may have originated from established populations in northwestern Indiana (Cortinas and Kitron 2006). In Indiana, where I. scapularis was first collected from deer in northwestern counties in 1987 (Pinger and Glancy 1989), densities of I. scapularis are greatest along the western border and decrease eastward; expansion to eastern counties was observed between 2005 and 2007 (Pinger et al. 1996, Keefe 2008, Raizman et al. 2012).

Hamer et al. (2010) proposed that established I. scapularis populations in Indiana seeded colonization of lower Michigan, where the tick was first discovered in southwestern lower Michigan in 2002 (Foster 2004). Invasion of I. scapularis northward along the coast of Lake Michigan was documented from 2004–2008; in 2004, ticks were collected only from the southernmost of the sampled sites, whereas they were found in all sites by 2008. Tick densities decreased from south to north, supporting a view that densities would be higher in areas where the tick has been longer established. Notably, although inland transects were also surveyed, no evidence of I. scapularis invasion was observed in these transects (Hamer et al. 2010). Subsequently, I. scapularis has been reported also from inland counties in southern Michigan (Table 2). Colonization of the Upper Peninsula of Michigan most likely occurred via northern Wisconsin and preceded colonization of the Lower Peninsula by more than a decade, as I. scapularis was discovered already in the 1980s in Menominee County in the Upper Peninsula of Michigan (Strand et al. 1992, Walker et al. 1994).

The Northeast

Similar to I. scapularis expansion in the North-Central focus, the tick’s range in the northeastern focus appears to have expanded in all directions, except for eastward, where the Atlantic Ocean prevents further spread. Since the survey by Dennis et al. (1998), I. scapularis appears to have expanded northward in Maine, New Hampshire, and Vermont. In Maine, analysis of public submission of ticks from 1989 to 2006 showed a northerly expansion along the Atlantic coastline, followed by invasion inland along river corridors (Rand et al. 2007). Ixodes scapularis is now considered established in all Maine counties. In Vermont, drag sampling was conducted from June 2011 to June 2012 along a north-south transect following the Connecticut River: densities of I. scapularis generally decreased from south to north, with no ticks collected from the northernmost sites (Serra et al. 2013). Expansion of the tick’s range in these New England states likely contributed, together with increasing tick densities in already established areas, to a 5–10-fold increase in incidence of reported Lyme disease cases in those states during the past decade (Mead 2015).

Since the survey by Dennis et al. (1998), the number of New York counties where I. scapularis is considered established has nearly doubled from 50.0 to 98.4%. At the time of the previous report, the tick was established primarily in the southeastern and eastern portions of the state and appears to have expanded in northerly and westerly directions. In parallel with this observation, from 1990 to 2000, Lyme disease surveillance data revealed a northward and westward expansion in the disease focus from a central cluster in the southeastern portion of the state (i.e., Westchester County). Moreover, during the same timeframe, the primary epidemiological focus shifted northward along the Hudson River (Chen et al. 2005). Population genetic analysis of I. scapularis collected from a transect along the Hudson River from 2004 to 2009 indicated recent rapid expansion of the tick’s range, primarily in a northerly direction along the Hudson River (Khatchikian et al. 2015); expansion appears to be the result of local migration of the ticks, via movements of mammal hosts, but some long-distance migration, perhaps via infestation of birds, was detected. Importantly, the DNA sequence analyses provide evidence for recent range expansion, as opposed to recent detection of in situ populations.

Neighboring New York to the south, Pennsylvania also experienced a recent westward expansion of I. scapularis. In 2003, Lyme disease cases were reported primarily from eastern counties in Pennsylvania. By 2013, human Lyme disease case counts increased markedly in western counties, with cases reported throughout the state (Mead 2015). This mirrors data for range expansion of I. scapularis in Pennsylvania. No I. scapularis were collected during a statewide survey from 1963 to 1967 (Snetsinger 1968), whereas the tick was recorded from 49 of 67 counties by the late 1990s (Dennis et al. 1998). A statewide survey conducted during 2012–2014 (Hutchinson et al. 2015) revealed that the tick now is established in all 67 counties. It is likely that the east-to-west tick expansion across Pennsylvania continued into neighboring Ohio to the west, where active tick surveillance was conducted from 1983 to 2012. Surveillance data showed a dramatic increase in I. scapularis abundance beginning in 2009. Arguing against increasing tick surveillance as a primary source for the observed range expansion in Ohio, the spread of the tick was observed when Ohio’s tick surveillance programs were being considered for termination and their budgets were dwindling (Wang et al. 2014). Currently, the I. scapularis range in Ohio is largely consistent with the range of deciduous forest in the state. As a result of the westward expansion of the previous northeastern focus and the eastward expansion of the previous North-Central focus, the distribution of I. scapularis now appears to be continuous across northern states with convergence of the two previously distinct foci in the Ohio River Valley.

The West Virginia, Virginia, and North Carolina Area

Ixodes scapularis has also expanded its range in West Virginia, where only 4 counties reported the tick previously (Dennis et al. 1998) and now 43 counties are classified as either reported (n = 20) or established (n = 23). Review of Lyme disease and I. scapularis surveillance reports (see references in Table 2) suggests that the tick is expanding westward across the state, with highly Lyme disease endemic counties still focused in the eastern panhandle. Similarly, in Virginia, I. scapularis was considered established primarily in eastern coastal counties previously (Dennis et al. 1998), but the current survey shows the tick to now be established throughout most of Virginia, with the highest densities of openly host-seeking ticks occurring in higher elevation sites (Brinkerhoff et al. 2014, Kelly et al. 2014). The spread of openly host-seeking I. scapularis appears to have proceeded in a southwesterly direction in Virginia, concordant with the expanding geographic distribution of Lyme disease cases and increasing incidence in Virginia (Brinkerhoff et al. 2014, Lantos et al. 2015). Comparison of the previous and current distributions of I. scapularis in North Carolina also suggests an inland incursion of the tick (Fig. 1). One important caveat to these findings for West Virginia, Virginia, and North Carolina is that the observed spread of I. scapularis may have resulted from southerly spread of I. scapularis from states to the north where this tick is more prone, as compared with southern populations, to seek hosts openly from vegetation (Arsnoe et al. 2015) and therefore is more readily contacted by tick dragging or flagging (Diuk-Wasser et al. 2006), or by humans and their pets (Stromdahl and Hickling 2012). Stated differently, this may be an invasion of more easily surveyed northern populations of I. scapularis rather than invasion at the species level in areas where more cryptic southern populations already may have been present but had not been recognized. Regardless, the end result is range expansion of I. scapularis populations that commonly contact and bite humans in West Virginia, Virginia, and North Carolina.

The Allegheny Mountains to Mississippi Valley Area

The authors (Dennis et al. 1998) of the previous survey were intrigued with the lack of I. scapularis records, despite collection efforts, from the Allegheny Mountains to the Mississippi Valley (an area spanning western Pennsylvania southeastward across Kentucky and Tennessee), because of the tick’s large geographical coverage in the eastern United States spanning variable climates and forested habitat types. Habitat suitability modeling suggested that the area ranged from low (Brownstein et al. 2003, Diuk-Wasser et al. 2010) to moderately suitable (Estrada-Pena 2002). Our revised distribution map shows the tick to now be established in this region, attesting to its climate and habitat suitability for I. scapularis. Several studies have documented a trend in which the tick expands along riparian corridors (Cortinas et al. 2002, Cortinas and Kitron 2006, Rand et al. 2007, Hamer et al. 2010, Serra et al. 2013, Kelly et al. 2014, Khatchikian et al. 2015). This provides a hypothesis for how the area from the Allegheny Mountains to the Mississippi Valley could have been or is currently being invaded from areas to the north and east with already established tick populations, first along distinct dispersal corridors followed by more diffuse short-range tick dispersal to suitable habitats across the landscape.

The Southeast

In contrast to the observed concordance between the reported distributions of I. scapularis and human Lyme disease in the North-Central, Northeastern, and Mid-Atlantic States, a wide distribution of this tick vector in the Southeast is not similarly associated with widespread Lyme disease case occurrence (Mead 2015). Lack of concordance in the Southeast could arise for several reasons. First, our map displays a coarse, county-scale representation of where the tick is classified as reported or established. The advantage of the county-scale representation is that it matches the spatial scale at which epidemiological surveillance data are presented. However, in some instances, the vector tick may be established only in limited areas of a particular county, putting very few humans at risk for exposure to tick bites (Eisen and Eisen 2007, Eisen and Eisen 2008). Second, while the presence of at least one tick vector species is a prerequisite for sustaining enzootic transmission of B. burgdorferi, density of B. burgdorferi-infected host-seeking nymphs of a species that commonly bite humans is a better surrogate for human risk of exposure to Lyme disease spirochetes compared with tick presence data alone. At coarse spatial scales such as states or regions of the United States, density of infected I. scapularis nymphs is significantly and positively associated with Lyme disease incidence (Mather et al. 1996, Stafford et al. 1998, Pepin et al. 2012).

A recent systematic field survey of I. scapularis revealed that both the density of host-seeking I. scapularis nymphs and their rate of infection with B. burgdorferi generally were greater in the northern compared with southern states (Diuk-Wasser et al. 2012). This is in agreement with field studies indicating that I. scapularis larvae feed primarily on white-footed mice, Peromyscus leucopus Rafinesque, and other small, highly reservoir-competent mammals in the northeast (Spielman et al. 1985, Giardina et al. 2000), whereas they feed frequently on lizards of, at best, low reservoir-competence for B. burgdorferi in the southeast (Apperson et al. 1993). Moreover, the tick’s host-seeking behavior differs between northern and southern states, such that collection of I. scapularis nymphs by drag sampling (Diuk-Wasser et al. 2006) or from humans (Stromdahl and Hickling 2012) is rare in the south but commonplace in the north. Variable contact rates between humans and nymphal ticks resulting from regional differences in host-seeking behavior could, in large part, explain regional differences in Lyme disease incidence between the northern and southern parts of the eastern United States (Diuk-Wasser et al. 2012, Stromdahl and Hickling 2012, Kelly et al. 2014, Arsnoe et al. 2015). A recent experimental field study showed differences in host-seeking behavior between I. scapularis of northern versus southern origin, such that ticks of northern origin were more likely to ascend vegetation while questing for a host, regardless of whether field release arenas were located in the north or south, suggesting that host-seeking behavior is strongly determined by genetics and to a lesser extent by environmental conditions (Arsnoe et al. 2015). Indeed, population genetic studies show two distinct clades, with the southern clade restricted to the south and the so-called American clade predominant in the north (Norris et al. 1996, Qiu et al. 2002, Humphrey et al. 2010, Van Zee et al. 2013, Sakamoto et al. 2014).

The Far-Western States

The recorded county-level distribution of I. pacificus has changed very little since the previous survey (Dennis et al. 1998). The tick is established primarily in coastal states along the Pacific Ocean (Washington, Oregon, and California), but also can occur locally in especially cool or moist settings in more arid inland states (Arizona, Nevada, and Utah). In contrast to I. scapularis, few studies have sought to define the environmental variables that define the distribution of I. pacificus (Eisen et al. 2006b). Owing in part to sizeable western counties commonly encompassing vast ecological diversity, often with only a portion of a given county presenting risk for human exposure to I. pacificus, there is a lack of concordance between the vector’s range as defined at the county level and the incidence of Lyme disease. For example, in California, although the tick is established in all but three counties, Lyme disease incidence is highest in north-coastal counties (Eisen et al. 2006b). Although B. burgdorferi- infected host-seeking nymphs may be established in limited regions of counties, few humans may be exposed (Eisen et al. 2006b). In addition, densities of host-seeking I. pacificus appear to be much lower in southern (Lane et al. 2013) compared with northern California (Eisen et al. 2006a). Likewise, infection rates with B. burgdorferi also appear to be lower in host-seeking nymphs from southern compared with northern California (Eisen et al. 2010, Lane et al. 2013).

Future Research Needs

The data presented and discussed here provide strong support for systematic sampling to assess the density of host-seeking I. scapularis, and the density of nymphs infected with B. burgdorferi and other I. scapularis-borne human pathogens, in strategic areas where the tick can be expected to invade or increase dramatically in numbers in the near future. We also recognize needs for: 1) improved regional habitat suitability models to better define the likely extent for continued expansion of I. scapularis; 2) population genetic studies aimed at identifying changes in the geographic range of the American clade of I. scapularis, especially in areas previously dominated by the southern clade such as Virginia, and North Carolina where American clade invasion likely results in increased human tick bites; and 3) longitudinal studies aimed at identifying how the convergence of the North-Central and Northeastern tick foci may result in changes in B. burgdorferi genotypes (Pepin et al. 2012), particularly those most likely to cause disease in humans, in the convergence area itself as well as across the North-Central and Northeast states.