Reported County-Level Distribution of the American Dog Tick (Acari: Ixodidae) in the Contiguous United States

Abstract

In the United States, tick-borne diseases are increasing in incidence and cases are reported over an expanding geographical area. Avoiding tick bites is a key strategy in tick-borne disease prevention, and this requires current and accurate information on where humans are at risk for exposure to ticks. Based on a review of published literature and records in the U.S. National Tick Collection and National Ecological Observatory Network databases, we compiled an updated county-level map showing the reported distribution of the American dog tick, Dermacentor variabilis (Say). We show that this vector of the bacterial agents causing Rocky Mountain spotted fever and tularemia is widely distributed, with records derived from 45 states across the contiguous United States. However, within these states, county-level records of established tick populations are limited. Relative to the range of suitable habitat for this tick, our data imply that D. variabilis is currently underreported in the peer-reviewed literature, highlighting a need for improved surveillance and documentation of existing tick records.

In recent decades in the United States, tick-borne diseases have increased in incidence and cases reported over an expanding geographical area (Eisen et al. 2017, Rosenberg et al. 2018). These trends can be explained, in part, by expanding geographical ranges of medically important hard ticks (Acari: Ixodidae) resulting in an increase in the numbers of persons exposed to potentially infectious tick bites (Eisen and Eisen 2018). Tick surveillance is intended to monitor changes in the distribution and abundance of ticks to aid in assessing and mitigating risk of human exposure to ticks and tick-borne pathogens (CDC 2018a). However, tick surveillance is not standardized or routinely conducted across the United States, which poses challenges in accurately representing the current distribution of medically important ticks.

Among the more than 80 species of ticks described in the United States, fewer than a dozen are commonly found to infest humans (Merten and Durden 2000). Arguably, based on their demonstrated ability to serve as vectors of human pathogens and their propensity to feed on human blood, the most medically important hard ticks in the United States include: the lone star tick (Amblyomma americanum [L.]), the Gulf Coast tick (Amblyomma maculatum [Koch]), the Rocky Mountain wood tick (Dermacentor andersoni [Stiles]), the Pacific Coast tick (Dermacentor occidentalis [Marx]), the American dog tick (Dermacentor variabilis [Say]), the western blacklegged tick (Ixodes pacificus [Cooley and Kohls]), the blacklegged tick or the deer tick (Ixodes scapularis [Say]), and the brown dog tick (Rhipicephalis sanguineus sensu lato) (Eisen et al. 2017). Distributions of these ticks were compiled in a seminal publication by Hooker (1909) followed by distribution maps by Bishopp and Trembley (1945). However, the distributions of these ticks, and consequently human risk of exposure to tick bites, have changed over time. Therefore, following criteria originally described by Dennis et al. (1998), recent studies updated the reported distributions of D. andersoni (James et al. 2006), A. americanum (Springer et al. 2014), and I. scapularis and I. pacificus (Eisen et al. 2016). Others have updated the distributions of A. maculatum (Teel et al. 2010) and D. variabilis (James et al. 2015), and other medically important ticks (CDC 2018b), but did not define distributions using the same county-scale criteria as Dennis et al. (1998).

Dermacentor variabilis is an important pest species and significant as a vector of both human and animal pathogens, including the bacterial agents causing Rocky Mountain spotted fever (Rickettsia rickettsii) and tularemia (Francisella tularensis) (Burgdorfer 1975, Reese et al. 2011, James et al. 2015). A number of publications have provided limited state, county, or local level data and broad scale maps of D. variabilis, but have not been updated for many years (Banks 1908, Hooker 1909, Bishopp and Trembley 1945).

In this study, we aimed to use the Dennis et al. (1998) county classification criteria to generate an updated map showing the reported distribution of D. variabilis. Our findings suggest that relative to the range in suitable habitat for this tick (James et al. 2015), D. variabilis is currently underreported in the peer- reviewed literature, highlighting a need for improved surveillance and reporting.

Materials and Methods

To identify records of the American dog tick within specific counties in the contiguous United States, we conducted a literature review using the Scopus database with search terms ‘Dermacentor’ and ‘variabilis’ to identify articles published from 1913 through 13 November 2018. To expand our county records of D. variabilis, we obtained collection records from the U.S. National Tick Collection (USNTC), spanning 1907–2018, and from the National Ecological Observatory Network (NEON), spanning 2014–2018. Additional records were obtained from specific monographs, state publications on the ticks of a state, and from studies conducted at localities where the county level was clearly evident. One additional source of unpublished data was provided to us opportunistically from the California Department of Health (CDPH).

Using records obtained from these data sources, we classified counties into those with ‘established’, ‘reported’, or ‘no records’ of D. variabilis. To do this, we used two different sets of inclusion criteria termed ‘strict’ and ‘expanded’. The strict inclusion criteria follows Dennis et al. (1998) and Eisen et al. (2016) definitions. Specifically, we categorized counties as ‘established’ if at least six individual D. variabilis ticks or at least two of the three host-seeking life stages were collected within a 1-yr time period. We categorized counties as ‘reported’ if they failed to meet the criteria for established and if at least one D. variabilis tick of any life stage was identified at any time in the county or if the county had records of the tick that failed to detail the number or life stage collected. Although these criteria were originally developed in reference to I. scapularis and I. pacificus, given the similarities in life cycles of these three-host non-nidicolous ticks, the same criteria were applied in this and a previous survey of the distribution of A. americanum (Springer et al. 2014). Using our strict criteria, we included in our database articles including county-level data on ticks collected from vegetation or from hosts with limited home ranges (e.g., <5–10 km²) as these hosts’ limited ranges presumably do not extend beyond a single county. Unless travel histories were accounted for, we excluded ticks collected from mobile hosts such as humans, companion animals, livestock, or zoo animals owing to the frequency of their transport outside of a single county. That is, because ticks can remain attached to a host for 7–10 d, if the host traveled outside of the county of record in the previous 10 d the county of exposure cannot be clearly ascertained (CDC 2018a).

In an effort to recognize counties for which there was compelling evidence that D. variabilis is present, but that did not meet the strict inclusion criteria, our expanded criteria included county records from long-term programs that collected ticks found on humans. We included articles reporting results of such programs if the majority (>50%) of counties in the state of interest reported records of D. variabilis; this is assuming that if the majority of counties in the state have records of D. variabilis from humans, then human exposure to the tick within the reporting county is plausible. In addition to adhering to the strict criteria for classifying counties as reported or established, using arbitrary but reasonable criteria, we classified counties as ‘established’ if greater than 50 records of D. variabilis obtained from these programs were documented in a 5-yr or greater time span and ‘reported’ if they did not meet the established criteria or records failed to detail the number of ticks collected. In short, the county classifications using the expanded criteria duplicate the classifications based on the strict criteria, but amount to a more extensive (cumulative) distribution of counties with reported or established records because long-term programs cataloging ticks collected from humans were included in the former. We coded all remaining counties as having ‘no records’. Notably, a lack of tick records in a county does not necessarily indicate absence of the tick, only that we did not identify any tick collection records for the county.

We used the five-digit Federal Information Processing Standard (FIPS) codes to designate counties or county equivalents. The final database included state, county, FIPS code, and county status data. We joined these data with a contiguous U.S. county map based on FIPS code using ArcMap 10.5 (ERSI, Redlands, CA). Counties were shaded based on their recorded status: established, reported, or no records.

Results

In total, 765 research articles met our Scopus search criteria, of which 111 contained adequate information for us to classify counties as having reported or established D. variabilis populations. Additionally, seven relevant monographs and state publications were included, as well as one unpublished list of California counties that met the criteria defined herein (California Department of Health, unpublished data). The USNTC database contained 2,040 records, of which 156 were included; the remainder were excluded primarily because they were collected from a mobile host. Of the 463 records from the NEON data set 20 records were retained, with the remainder largely representing repeat sampling in counties.

Based on the strict inclusion criteria, D. variabilis has been recorded in a total of 516 counties (16.6% of 3,109 FIPS codes in the contiguous United States) across 44 states (Tables 1 and 2; Fig. 1A). In total, 249 (8.0%) counties distributed across 41 states were classified as established and 267 counties (8.6%) across 37 states were classified as reported.

Table 1.

Number (%) of contiguous U.S. counties in which D. variabilis is classified as reported or established as of November 2018 using the strict and expanded criteria.

State	Strict criteria		Expanded criteria
	No. (%) counties with reported status	No. (%) counties with established status	No. (%) counties with reported status	No. (%) counties with established status
Alabama	4 (6.0)	1 (1.5)	4 (6.0)	1 (1.5)
Arkansas	6 (8.0)	4 (5.3)	6 (8.0)	4 (5.3)
California	13 (22.4)	26 (44.8)	13 (22.4)	26 (44.8)
Colorado	1 (1.6)	1 (1.6)	1 (1.6)	1 (1.6)
Connecticut	2 (25.0)	3 (37.5)	2 (25.0)	3 (37.5)
Delaware	0 (0)	3 (100.0)	0 (0)	3 (100.0)
Florida	17 (25.4)	15 (22.4)	17 (25.4)	15 (22.4)
Georgiaa	19 (11.9)	21 (13.2)	54 (34.0)	21 (13.2)
Idaho	1 (2.3)	2 (4.5)	1 (2.3)	2 (4.5)
Illinois	11 (10.8)	8 (7.8)	11 (10.8)	8 (7.8)
Indiana	5 (5.4)	6 (6.5)	5 (5.4)	6 (6.5)
Iowaa	1 (1.0)	1 (1.0)	75 (75.8)	3 (3.0)
Kansas	5 (4.8)	6 (5.7)	5 (4.8)	6 (5.7)
Kentucky	3 (2.5)	3 (2.5)	3 (2.5)	3 (2.5)
Louisiana	0 (0)	1 (1.6)	0 (0)	1 (1.6)
Mainea	0 (0)	1 (6.3)	11 (68.8)	4 (25.0)
Maryland	3 (12.5)	9 (37.5)	3 (12.5)	9 (37.5)
Massachusetts	1 (7.1)	6 (42.9)	1 (7.1)	6 (42.9)
Michigana	0 (0)	1 (1.2)	61 (73.5)	13 (15.7)
Minnesota	9 (10.3)	1 (1.1)	9 (10.3)	1 (1.1)
Mississippi	4 (4.9)	2 (2.4)	4 (4.9)	2 (2.4)
Missouri	9 (7.8)	14 (12.2)	9 (7.8)	14 (12.2)
Montana	3 (5.4)	2 (3.6)	3 (5.4)	2 (3.6)
Nebraskaa	8 (8.6)	1 (1.1)	73 (78.5)	1 (1.1)
New Hampshire	1 (10.0)	1 (10.0)	1 (10.0)	1 (10.0)
New Jersey	2 (9.5)	1 (4.8)	2 (9.5)	1 (4.8)
New Mexico	1 (3.0)	0 (0)	1 (3.0)	0 (0)
New York	1 (1.6)	4 (6.5)	1 (1.6)	4 (6.5)
North Carolina	31 (31.0)	3 (3.0)	31 (31.0)	3 (3.0)
North Dakota	2 (3.8)	10 (18.9)	2 (3.8)	10 (18.9)
Ohioa	0 (0)	2 (2.3)	50 (56.8)	38 (43.2)
Oklahoma	25 (32.5)	15 (19.5)	25 (32.5)	15 (19.5)
Oregon	3 (8.3)	2 (5.6)	3 (8.3)	2 (5.6)
Pennsylvaniaa	—	—	49 (73.1)	14 (20.9)
Rhode Island	0 (0)	1 (20.0)	0 (0)	1 (20.0)
South Carolina	1 (2.2)	4 (8.7)	1 (2.2)	4 (8.7)
South Dakota	21 (31.8)	9 (13.6)	21 (31.8)	9 (13.6)
Tennessee	15 (15.8)	28 (29.5)	15 (15.8)	28 (29.5)
Texas	20 (7.9)	13 (5.1)	20 (7.9)	13 (5.1)
Vermont	3 (21.4)	0 (0)	3 (21.4)	0 (0)
Virginia	4 (3.0)	9 (6.7)	4 (3.0)	9 (6.7)
Washington	1 (2.6)	2 (5.1)	1 (2.6)	2 (5.1)
West Virginia	0 (0)	1 (1.8)	0 (0)	1 (1.8)
Wisconsin	9 (12.5)	6 (8.3)	9 (12.5)	6 (8.3)
Wyoming	2 (8.7)	0 (0)	2 (8.7)	0 (0)
Total	267 (8.6)	249 (8.0)	612 (19.7)	316 (10.2)

^aStates with additional county records when inclusion criteria expanded.

Table 2.

Status of D. variabilis by county in the contiguous United States

State and county		Status by November 2018	Source for status
Alabama
	Bibb	Reported	NEON
	Choctaw	Reported	NEON
	Greene	Reported	NEON
	Hale	Reported	NEON
	Lee	Established	(Luckhart et al. 1991)
Arkansas
	Franklin	Established	USNTC
	Jefferson	Established	USNTC
	Madison	Reported	(Trout and Steelman 2010)
	Marion	Reported	(McAllister et al. 2013)
	Polk	Reported	USNTC; (Trout and Steelman 2010)
	Pulaski	Established	(Blanton et al. 2014)
	Searcy	Reported	USNTC
	Stone	Established	USNTC
	Union	Reported	(McAllister et al. 2013)
	Van Buren	Reported	USNTC
California
	Alameda	Established	CDPH; (Kramer et al. 1999)
	Butte	Reported	CDPH
	Calaveras	Established	CDPH
	Colusa	Reported	CDPH
	Contra Costa	Reported	(Lane and Voie 1988, Dennis et al. 1998)
	El Dorado	Reported	CDPH
	Fresno	Established	USNTC
	Humboldt	Established	(Clemons et al. 2000, Gabriel et al. 2009, Stephenson et al. 2017)
	Inyo	Reported	CDPH
	Lake	Established	CDPH; (Kaufman et al. 2018)
	Los Angeles	Established	USNTC; CDPH
	Marin	Established	CDPH
	Mariposa	Established	CDPH
	Mendocino	Established	CDPH
	Merced	Reported	CDPH
	Monterey	Established	CDPH; (Coultrip et al. 1973, Philip et al. 1981)
	Nevada	Established	CDPH
	Orange	Established	CDPH; (Barlough et al. 1997, Chang et al. 2002)
	Placer	Established	CDPH
	Riverside	Established	CDPH
	Sacramento	Established	CDPH
	San Bernardino	Established	CDPH
	San Diego	Established	CDPH (Lang 1999)
	San Francisco	Reported	CDPH
	San Joaquin	Established	CDPH
	San Diego	Established	CDPH; (Lang 1999)
	San Francisco	Reported	CDPH
	San Joaquin	Established	CDPH
	San Luis Obispo	Reported	CDPH; USNTC
	San Mateo	Established	CDPH
	Santa Barbara	Established	CDPH; (Wikswo et al. 2014, MacDonald 2018)
	Santa Clara	Established	CDPH
	Santa Cruz	Established	CDPH; (Kramer et al. 1999, Holden et al. 2003)
	Shasta	Reported	CDPH
	Solano	Established	CDPH
	Sonoma	Established	CDPH
	Sutter	Reported	CDPH
	Tehama	Reported	CDPH
	Trinity	Reported	CDPH
	Tulare	Reported	CDPH; (Furman and Loomis 1984)
	Ventura	Established	CDPH
	Yolo	Reported	CDPH; (Kaufman et al. 2018)
Colorado
	Boulder	Established	USNTC
	Conejos	Reported	USNTC
Connecticut
	Fairfield	Established	(Magnarelli et al. 1983)
	Middlesex	Established	(Magnarelli et al. 1991)
	New Haven	Reported	USNTC; (Anderson and Magnarelli 1980)
	New London	Established	(Magnarelli and Anderson 1988, Magnarelli et al. 1991)
	Windham	Reported	(Anderson and Magnarelli 1980)
Delaware
	Kent	Established	(MacCreary 1945)
	New Castle	Established	(MacCreary 1945)
	Sussex	Established	(MacCreary 1945)
Florida
	Alachua	Established	(Sayler et al. 2017)
	Bay	Established	(Cilek and Olson 2000)
	Bradford	Reported	USNTC; (Hertz et al. 2017)
	Brevard	Established	(Durden et al. 1993)
	Clay	Reported	(Hertz et al. 2017)
	Collier	Reported	(Forrester et al. 1996, Hertz et al. 2017)
	Duval	Reported	(Wilson et al. 1991)
	Escambia	Reported	(Hertz et al. 2017)
	Flagler	Established	USNTC
	Franklin	Established	USNTC
	Gadsden	Established	USNTC
	Glades	Established	(Greiner et al. 1984)
	Hardee	Reported	USNTC
	Hendry	Established	USNTC
	Highlands	Established	(Durden et al. 2000)
	Hillsborough	Established	USNTC
	Indian River	Established	(Wilson and Kale 1972)
	Leon	Established	(Durden et al. 2000)
	Levy	Reported	USNTC
	Marion	Reported	(Hertz et al. 2017)
	Miami-Dade	Reported	(Wehinger et al. 1995)
	Monroe	Established	USNTC
	Nassau	Reported	USNTC
	Orange	Established	USNTC
	Osceola	Reported	NEON
	Palm Beach	Reported	USNTC
	Pasco	Reported	(Hertz et al. 2017)
	Pinellas	Reported	USNTC
	Polk	Reported	NEON; (Hertz et al. 2017)
	Putnam	Established	USNTC
	Santa Rosa	Reported	(Hertz et al. 2017)
	Taylor	Reported	(Hertz et al. 2017)
Georgia
	Barrow	Reporteda	(Gleim et al. 2016)
	Bartow	Reporteda	(Gleim et al. 2016)
	Berrien	Established	USNTC
	Bibb	Reporteda	(Gleim et al. 2016)
	Brantley	Established	(Wilson and Baker 1972)
	Brooks	Established	(Wilson and Baker 1972)
	Bryan	Established	USNTC
	Bulloch	Established	(Lavender and Oliver 1996)
	Burke	Established	(Wilson and Baker 1972)
	Camden	Reporteda	(Gleim et al. 2016)
	Candler	Established	(Nims et al. 2008)
	Carroll	Reported	(Wilson and Baker 1972)
	Catoosa	Reporteda	(Gleim et al. 2016)
	Chatham	Reporteda	(Gleim et al. 2016)
	Chattooga	Reporteda	(Gleim et al. 2016)
	Cherokee	Reporteda	(Gleim et al. 2016)
	Clarke	Established	(Wilson and Baker 1972)
	Clay	Reporteda	(Gleim et al. 2016)
	Clayton	Reporteda	(Gleim et al. 2016)
	Clinch	Reported	USNTC
	Cobb	Reporteda	(Gleim et al. 2016)
	Coffee	Reporteda	(Gleim et al. 2016)
	Crisp	Established	(Wilson and Baker 1972)
	Dade	Reporteda	(Gleim et al. 2016)
	Dawson	Reported	(Nims and Durden 2011, Gleim et al. 2016)
	DeKalb	Reporteda	(Gleim et al. 2016)
	Dougherty	Reported	(Wilson and Baker 1972)
	Effingham	Reporteda	(Gleim et al. 2016)
	Emanuel	Reported	USNTC
	Fannin	Reporteda	(Gleim et al. 2016)
	Fayette	Reporteda	(Gleim et al. 2016)
	Floyd	Established	(Wilson and Baker 1972)
	Forsyth	Reporteda	(Gleim et al. 2016)
	Fulton	Reporteda	(Gleim et al. 2016)
	Gilmer	Reporteda	(Gleim et al. 2016)
	Glynn	Established	(Wilson and Baker 1972)
	Gordon	Reporteda	(Gleim et al. 2016)
	Greene	Reporteda	(Wilson and Baker 1972)
	Gwinnett	Established	(Nims and Durden 2011)
	Habersham	Reported	(Wilson and Baker 1972, Gleim et al. 2016)
	Hall	Reporteda	(Gleim et al. 2016)
	Haralson	Reporteda	(Gleim et al. 2016)
	Harris	Reported	(Wilson and Baker 1972)
	Houston	Reported	(Wilson and Baker 1972)
	Irwin	Reported	(Wilson and Baker 1972)
	Jackson	Reported	(Wilson and Baker 1972, Gleim et al. 2016)
	Jefferson	Reported	(Wilson and Baker 1972)
	Jenkins	Established	(Wilson and Baker 1972)
	Liberty	Reported	(Durden et al. 2001)
	Lincoln	Reporteda	(Gleim et al. 2016)
	Lowndes	Established	(Wilson and Baker 1972)
	Lumpkin	Reported	(Wilson and Baker 1972)
	Madison	Reporteda	(Gleim et al. 2016)
	Mclntosh	Established	(Wilson and Baker 1972)
	Murray	Reporteda	(Gleim et al. 2016)
	Oconee	Reporteda	(Gleim et al. 2016)
	Oglethorpe	Established	(Wilson and Baker 1972, Gleim et al. 2016)
	Paulding	Reporteda	(Gleim et al. 2016)
	Pierce	Established	(Wilson and Baker 1972)
	Polk	Reported	(Wilson and Baker 1972, Gleim et al. 2016)
	Rabun	Reporteda	(Gleim et al. 2016)
	Rockdale	Established	(Newhouse 1983)
	Screven	Established	(Wilson and Baker 1972)
	Stephens	Reporteda	(Gleim et al. 2016)
	Tattnall	Reporteda	(Gleim et al. 2016)
	Telfair	Reported	(Wilson and Baker 1972)
	Thomas	Established	(Wilson and Baker 1972)
	Tift	Reporteda	(Gleim et al. 2016)
	Towns	Reported	(Nims and Durden 2011)
	Turner	Established	(Wilson and Baker 1972)
	Upson	Reporteda	(Gleim et al. 2016)
	Walker	Reported	(Wilson and Baker 1972, Gleim et al. 2016)
	Walton	Reporteda	(Gleim et al. 2016)
	Whitfield	Reporteda	(Gleim et al. 2016)
	Wilkes	Reported	(Wilson and Baker 1972)
Idaho
	Clearwater	Established	(Stout et al. 1971)
	Latah	Reported	(Stout et al. 1971)
	Nez Perce	Established	(Stout et al. 1971, Scoles 2004)
Illinois
	Brown	Reported	USNTC
	Carroll	Reported	USNTC
	Cook	Established	(Rydzewski et al. 2012)
	DuPage	Established	(Rydzewski et al. 2012)
	Jackson	Established	(Zieman et al. 2017)
	Jo Daviess	Reported	USNTC
	Lake	Established	(Rydzewski et al. 2012)
	Massac	Reported	USNTC
	McHenry	Established	(Rydzewski et al. 2012)
	McLean	Reported	USNTC
	Monroe	Reported	USNTC
	Piatt	Reported	USNTC
	Pike	Established	USNTC
	Pope	Reported	USNTC
	Rock Island	Established	USNTC
	Schuyler	Reported	USNTC
	Union	Reported	USNTC
	Vermilion	Reported	USNTC
	Williamson	Established	(Nelson et al. 1984)
Indiana
	Bartholomew	Established	(Rynkiewicz and Clay 2014)
	Brown	Established	(Rynkiewicz and Clay 2014)
	Crawford	Reported	USNTC
	Lake	Established	USNTC
	Lawrence	Established	USNTC
	Monroe	Established	(Rynkiewicz and Clay 2014)
	Newton	Reported	USNTC
	Orange	Established	(Civitello et al. 2008)
	Porter	Reported	USNTC
	St. Joseph	Reported	USNTC
	Steuben	Reported	USNTC
Iowa
	Adair	Reporteda	(Lingren et al. 2005)
	Adams	Reporteda	(Lingren et al. 2005)
	Appanoose	Reporteda	(Lingren et al. 2005)
	Audubon	Reporteda	(Lingren et al. 2005)
	Benton	Reporteda	(Lingren et al. 2005)
	Black Hawk	Reporteda	(Lingren et al. 2005)
	Boone	Reporteda	(Lingren et al. 2005)
	Bremer	Reporteda	(Lingren et al. 2005)
	Buena Vista	Reporteda	(Lingren et al. 2005)
	Butler	Reporteda	(Lingren et al. 2005)
	Calhoun	Reporteda	(Lingren et al. 2005)
	Carroll	Reporteda	(Lingren et al. 2005)
	Cass	Reporteda	(Lingren et al. 2005)
	Cedar	Reporteda	(Lingren et al. 2005)
	Cerro Gordo	Reporteda	(Lingren et al. 2005)
	Cherokee	Reporteda	(Lingren et al. 2005)
	Chickasaw	Reporteda	(Lingren et al. 2005)
	Clarke	Reporteda	(Lingren et al. 2005)
	Clay	Reporteda	(Lingren et al. 2005)
	Clayton	Reporteda	(Lingren et al. 2005)
	Clinton	Reporteda	(Lingren et al. 2005)
	Crawford	Reporteda	(Lingren et al. 2005)
	Dallas	Reported	(Storm and Ritzi 2008)
	Decatur	Reporteda	(Lingren et al. 2005)
	Delaware	Reporteda	(Lingren et al. 2005)
	Des Moines	Reporteda	(Lingren et al. 2005)
	Dickinson	Reporteda	(Lingren et al. 2005)
	Dubuque	Reporteda	(Lingren et al. 2005)
	Fayette	Reporteda	(Lingren et al. 2005)
	Franklin	Reporteda	(Lingren et al. 2005)
	Fremont	Reporteda	(Lingren et al. 2005)
	Greene	Reporteda	(Lingren et al. 2005)
	Grundy	Reporteda	(Lingren et al. 2005)
	Guthrie	Reporteda	(Lingren et al. 2005)
	Hamilton	Reporteda	(Lingren et al. 2005)
	Hardin	Reporteda	(Lingren et al. 2005)
	Henry	Reporteda	(Lingren et al. 2005)
	Jackson	Reporteda	(Lingren et al. 2005)
	Jasper	Reporteda	(Lingren et al. 2005)
	Jefferson	Reporteda	(Lingren et al. 2005)
	Johnson	Reporteda	(Lingren et al. 2005)
	Jones	Reporteda	(Lingren et al. 2005)
	Kossuth	Reporteda	(Lingren et al. 2005)
	Lee	Reporteda	(Lingren et al. 2005)
	Linn	Reporteda	(Lingren et al. 2005)
	Louisa	Reporteda	(Lingren et al. 2005)
	Lucas	Reporteda	(Lingren et al. 2005)
	Lyon	Reporteda	(Lingren et al. 2005)
	Madison	Reporteda	(Lingren et al. 2005)
	Marion	Reporteda	(Lingren et al. 2005)
	Marshall	Reporteda	(Lingren et al. 2005)
	Mills	Reporteda	(Lingren et al. 2005)
	Mitchell	Reporteda	(Lingren et al. 2005)
	Monroe	Reporteda	(Lingren et al. 2005)
	Montgomery	Reporteda	(Lingren et al. 2005)
	Muscatine	Reporteda	(Lingren et al. 2005)
	O’Brien	Reporteda	(Lingren et al. 2005)
	Page	Reporteda	(Lingren et al. 2005)
	Polk	Establisheda	(Lingren et al. 2005)
	Pottawattamie	Reporteda	(Lingren et al. 2005)
	Ringgold	Reporteda	(Lingren et al. 2005)
	Sac	Reporteda	(Lingren et al. 2005)
	Scott	Establisheda	(Lingren et al. 2005)
	Shelby	Reporteda	(Lingren et al. 2005)
	Sioux	Reporteda	(Lingren et al. 2005)
	Story	Reporteda	(Lingren et al. 2005)
	Tama	Established	(Eddy and Joyce 1944)
	Taylor	Reporteda	(Lingren et al. 2005)
	Union	Reporteda	(Lingren et al. 2005)
	Wapello	Reporteda	(Lingren et al. 2005)
	Warren	Reporteda	(Lingren et al. 2005)
	Washington	Reporteda	(Lingren et al. 2005)
	Wayne	Reporteda	(Lingren et al. 2005)
	Webster	Reporteda	(Lingren et al. 2005)
	Winnebago	Reporteda	(Lingren et al. 2005)
	Winneshiek	Reporteda	(Lingren et al. 2005)
	Woodbury	Reporteda	(Lingren et al. 2005)
	Wright	Reporteda	(Lingren et al. 2005)
Kansas
	Barber	Reported	USNTC
	Bourbon	Established	(Savage et al. 2018a,b)
	Douglas	Reported	NEON; USNTC
	Jackson	Established	USNTC
	Jefferson	Reported	NEON
	Leavenworth	Established	USNTC
	Linn	Established	(Savage et al. 2018a,b)
	Lyon	Reported	USNTC
	Neosho	Established	USNTC
	Osage	Established	USNTC
	Riley	Reported	NEON
Kentucky
	Calloway	Established	USNTC
	Christian	Reported	USNTC
	Fayette	Reported	(Burg 2001)
	Hardin	Established	(Pagac et al. 2014)
	Hart	Reported	(Buchholz et al. 2018)
	Muhlenberg	Established	(Pagac et al. 2014)
Louisiana
	Acadia	Established	(Goddard and Bircham 2010)
Maine
	Androscoggin	Reporteda	(Smith et al. 1992, Rand et al. 2007)
	Aroostook	Reporteda	(Rand et al. 2007)
	Cumberland	Establisheda	(Rand et al. 2007)
	Franklin	Reporteda	(Smith et al. 1992, Rand et al. 2007)
	Hancock	Reporteda	(Rand et al. 2007)
	Kennebec	Reporteda	(Smith et al. 1992, Rand et al. 2007)
	Knox	Reporteda	(Smith et al. 1992, Rand et al. 2007)
	Lincoln	Establisheda	(Rand et al. 2007)
	Oxford	Reporteda	(Smith et al. 1992, Rand et al. 2007)
	Penobscot	Establisheda	(Rand et al. 2007)
	Sagadahoc	Reporteda	(Smith et al. 1992, Rand et al. 2007)
	Somerset	Reporteda	(Smith et al. 1992)
	Waldo	Reporteda	(Rand et al. 2007)
	Washington	Reporteda	(Smith et al. 1992, Rand et al. 2007)
	York	Established	(Rand et al. 2007, Kaufman et al. 2018)
Maryland
	Anne Arundel	Reported	NEON
	Baltimore	Established	(Ammerman et al. 2004)
	Calvert	Established	(Ammerman et al. 2004)
	Charles	Established	(Ammerman et al. 2004)
	Dorchester	Established	(Harman et al. 1984)
	Frederick	Established	(Durden 1992)
	Kent	Reported	USNTC
	Montgomery	Established	(Carroll et al. 1989)
	Prince George’s	Reported	(Ammerman et al. 2004)
	St. Mary’s	Established	(Ammerman et al. 2004)
	Talbot	Established	USNTC
	Worcester	Established	(Oliver et al. 1999)
Massachusetts
	Barnstable	Established	(Feng et al. 1980)
	Dukes	Established	(Lyon et al. 1998, Goethert et al. 2004)
	Essex	Established	(Goethert and Telford 2005)
	Hampshire	Reported	(Coher and Shaw 1951)
	Middlesex	Established	USNTC
	Nantucket	Established	(Spielman 1976)
	Worcester	Established	(Perla and Thomas 2001)
Michigan
	Alger	Reporteda	(Walker et al. 1998)
	Allegan	Reporteda	(Walker et al. 1998)
	Alpena	Reporteda	(Walker et al. 1998)
	Baraga	Establisheda	(Walker et al. 1998)
	Barry	Reporteda	(Walker et al. 1998)
	Bay	Reporteda	(Walker et al. 1998)
	Berrien	Reporteda	(Walker et al. 1998)
	Branch	Reporteda	(Walker et al. 1998)
	Calhoun	Reporteda	(Walker et al. 1998)
	Cass	Reporteda	(Walker et al. 1998)
	Charlevoix	Reporteda	(Walker et al. 1998)
	Cheboygan	Reporteda	(Walker et al. 1998)
	Chippewa	Reporteda	(Walker et al. 1998)
	Clare	Reporteda	(Walker et al. 1998)
	Clinton	Reporteda	(Walker et al. 1998)
	Crawford	Reporteda	(Walker et al. 1998)
	Delta	Establisheda	(Walker et al. 1998)
	Dickinson	Establisheda	(Walker et al. 1998)
	Eaton	Reporteda	(Walker et al. 1998)
	Emmet	Reporteda	(Walker et al. 1998)
	Grand Traverse	Reporteda	(Walker et al. 1998)
	Gladwin	Reporteda	(Walker et al. 1998)
	Gogebic	Established	NEON
	Grand Traverse	Reporteda	(Walker et al. 1998)
	Hillsdale	Reporteda	(Walker et al. 1998)
	Houghton	Reporteda	(Walker et al. 1998)
	Huron	Reporteda	(Walker et al. 1998)
	Ingham	Reporteda	(Walker et al. 1998)
	Ionia	Reporteda	(Walker et al. 1998)
	Iosco	Reporteda	(Walker et al. 1998)
	Iron	Establisheda	(Walker et al. 1998)
	Isabella	Reporteda	(Walker et al. 1998)
	Jackson	Establisheda	(Walker et al. 1998)
	Kalamazoo	Reporteda	(Walker et al. 1998)
	Kent	Reporteda	(Walker et al. 1998)
	Leelanau	Reporteda	(Walker et al. 1998)
	Lake	Reporteda	(Walker et al. 1998)
	Lapeer	Reporteda	(Walker et al. 1998)
	Leelanau	Reporteda	(Walker et al. 1998)
	Lenawee	Establisheda	(Walker et al. 1998)
	Livingston	Reporteda	(Walker et al. 1998)
	Luce	Reporteda	(Walker et al. 1998)
	Mackinac	Reporteda	(Walker et al. 1998)
	Macomb	Reporteda	(Walker et al. 1998)
	Manistee	Reporteda	(Walker et al. 1998)
	Marquette	Establisheda	(Walker et al. 1998)
	Mason	Reporteda	(Walker et al. 1998)
	Mecosta	Reporteda	(Walker et al. 1998)
	Menominee	Establisheda	(Walker et al. 1998)
	Midland	Reporteda	(Walker et al. 1998)
	Monroe	Establisheda	(Walker et al. 1998)
	Montcalm	Reporteda	(Walker et al. 1998)
	Muskegon	Reporteda	(Walker et al. 1998)
	Newaygo	Reporteda	(Walker et al. 1998)
	Oakland	Establisheda	(Walker et al. 1998)
	Oceana	Reporteda	(Walker et al. 1998)
	Ogemaw	Reporteda	(Walker et al. 1998)
	Ontonagon	Reporteda	(Walker et al. 1998)
	Osceola	Reporteda	(Walker et al. 1998)
	Otsego	Reporteda	(Walker et al. 1998)
	Ottawa	Reporteda	(Walker et al. 1998)
	Presque Isle	Reporteda	(Walker et al. 1998)
	Roscommon	Reporteda	(Walker et al. 1998)
	Saginaw	Reporteda	(Walker et al. 1998)
	St. Clair	Reporteda	(Walker et al. 1998)
	St. Joseph	Establisheda	(Walker et al. 1998)
	Sanilac	Reporteda	(Walker et al. 1998)
	Schoolcraft	Establisheda	(Walker et al. 1998)
	Shiawassee	Reporteda	(Walker et al. 1998)
	Tuscola	Reporteda	(Walker et al. 1998)
	Van Buren	Reporteda	(Walker et al. 1998)
	Washtenaw	Reporteda	(Walker et al. 1998)
	Wayne	Reporteda	(Walker et al. 1998)
	Wexford	Reporteda	(Walker et al. 1998)
Minnesota
	Aitkin	Reported	USNTC
	Anoka	Reported	USNTC
	Blue Earth	Reported	USNTC
	Clearwater	Reported	USNTC
	Itasca	Reported	USNTC
	Kanabec	Reported	USNTC
	Koochiching	Reported	USNTC
	Lyon	Reported	USNTC
	Morrison	Established	(Mcnemee et al. 2003)
	Pine	Reported	USNTC
Mississippi
	Copiah	Reported	(Goddard et al. 2003)
	Greene	Reported	USNTC
	Hinds	Reported	(Goddard et al. 2003)
	Marshall	Established	(Goddard et al. 2003)
	Rankin	Established	(Goddard et al. 2011)
	Wayne	Reported	(Goddard et al. 2003)
Missouri
	Adair	Established	(Dallas et al. 2012, Hudman and Sargentini 2016)
	Andrew	Established	(Savage et al. 2013, 2016)
	Barry	Reported	(Steiert and Gilfoy 2002)
	Carter	Reported	(Brown et al. 2011)
	Christian	Established	(Steiert and Gilfoy 2002)
	Dade	Established	(Steiert and Gilfoy 2002)
	Dallas	Established	(Steiert and Gilfoy 2002)
	Dent	Reported	(Brown et al. 2011)
	Gentry	Established	(Brown et al. 2011)
	Greene	Reported	(Steiert and Gilfoy 2002)
	Hickory	Reported	(Brown et al. 2011)
	Johnson	Reported	(Brown et al. 2011)
	Laclede	Reported	(Brown et al. 2011)
	Lawrence	Established	(Steiert and Gilfoy 2002)
	Nodaway	Established	(Savage et al. 2013, 2016)
	Polk	Established	(Steiert and Gilfoy 2002)
	St. Louis	Established	(Santanello et al. 2018)
	Stoddard	Established	USNTC
	Stone	Reported	(Steiert and Gilfoy 2002)
	Taney	Established	USNTC
	Washington	Reported	USNTC
	Webster	Established	(Steiert and Gilfoy 2002)
	Worth	Established	(Savage et al. 2013)
Montana
	Big Horn	Established	USNTC
	Musselshell	Reported	USNTC
	Rosebud	Reported	USNTC
	Treasure	Reported	USNTC
	Yellowstone	Established	(Araya-Anchetta et al. 2013)
Nebraska
	Adams	Reported	(Cortinas and Spomer 2014, Kaufman et al. 2018)
	Antelope	Reporteda	(Cortinas and Spomer 2014)
	Blaine	Reporteda	(Cortinas and Spomer 2014)
	Boyd	Reporteda	(Cortinas and Spomer 2014)
	Brown	Reporteda	(Cortinas and Spomer 2014)
	Buffalo	Reported	(Cortinas and Spomer 2014, Kaufman et al. 2018)
	Burt	Reporteda	(Cortinas and Spomer 2014)
	Butler	Reporteda	(Cortinas and Spomer 2014)
	Cass	Reporteda	(Cortinas and Spomer 2014)
	Cedar	Reporteda	(Cortinas and Spomer 2014)
	Chase	Reporteda	(Cortinas and Spomer 2014)
	Cherry	Reporteda	(Cortinas and Spomer 2014)
	Cheyenne	Reporteda	(Cortinas and Spomer 2014)
	Clay	Reporteda	(Cortinas and Spomer 2014)
	Colfax	Reporteda	(Cortinas and Spomer 2014)
	Cuming	Reporteda	(Cortinas and Spomer 2014)
	Custer	Reporteda	(Cortinas and Spomer 2014)
	Dakota	Reporteda	(Cortinas and Spomer 2014)
	Dawes	Reported	(Cortinas and Spomer 2014, Kaufman et al. 2018)
	Dawson	Reported	(Cortinas and Spomer 2014, Kaufman et al. 2018)
	Dixon	Reporteda	(Cortinas and Spomer 2014)
	Dodge	Reporteda	(Cortinas and Spomer 2014)
	Douglas	Reporteda	(Cortinas and Spomer 2014)
	Dundy	Reporteda	(Cortinas and Spomer 2014)
	Franklin	Reporteda	(Cortinas and Spomer 2014)
	Frontier	Reporteda	(Cortinas and Spomer 2014)
	Furnas	Reported	(Cortinas and Spomer 2014, Kaufman et al. 2018)
	Gage	Reporteda	(Cortinas and Spomer 2014)
	Garfield	Reporteda	(Cortinas and Spomer 2014)
	Greeley	Reporteda	(Cortinas and Spomer 2014)
	Hall	Reporteda	(Cortinas and Spomer 2014)
	Hamilton	Reporteda	(Cortinas and Spomer 2014)
	Harlan	Reporteda	(Cortinas and Spomer 2014)
	Hitchcock	Reporteda	(Cortinas and Spomer 2014)
	Hooker	Reporteda	(Cortinas and Spomer 2014)
	Howard	Reporteda	(Cortinas and Spomer 2014)
	Jefferson	Reporteda	(Durden and Richardson 2013, Cortinas and Spomer 2014)
	Johnson	Reporteda	(Cortinas and Spomer 2014)
	Kearney	Reporteda	(Cortinas and Spomer 2014)
	Keith	Reported	(Durden and Richardson 2013, Cortinas and Spomer 2014)
	Keya Paha	Reporteda	(Cortinas and Spomer 2014)
	Knox	Reporteda	(Cortinas and Spomer 2014)
	Lancaster	Reporteda	(Cortinas and Spomer 2014)
	Lincoln	Reporteda	(Cortinas and Spomer 2014)
	Loup	Reporteda	(Cortinas and Spomer 2014)
	Madison	Reporteda	(Cortinas and Spomer 2014)
	Merrick	Reporteda	(Cortinas and Spomer 2014)
	Morrill	Reporteda	(Cortinas and Spomer 2014)
	Nance	Reporteda	(Cortinas and Spomer 2014)
	Nemaha	Reporteda	(Cortinas and Spomer 2014)
	Nuckolls	Reporteda	(Cortinas and Spomer 2014)
	Otoe	Reporteda	(Cortinas and Spomer 2014)
	Pawnee	Reporteda	(Cortinas and Spomer 2014)
	Perkins	Reporteda	(Cortinas and Spomer 2014
	Phelps	Reported	(Cortinas and Spomer 2014, Kaufman et al. 2018)
	Platte	Reporteda	(Cortinas and Spomer 2014)
	Polk	Reporteda	(Cortinas and Spomer 2014)
	Red Willow	Reporteda	(Cortinas and Spomer 2014)
	Richardson	Reporteda	(Cortinas and Spomer 2014)
	Rock	Reporteda	(Cortinas and Spomer 2014)
	Saline	Reporteda	(Cortinas and Spomer 2014)
	Sarpy	Reporteda	(Cortinas and Spomer 2014)
	Saunders	Reporteda	(Cortinas and Spomer 2014)
	Scotts Bluff	Reporteda	(Cortinas and Spomer 2014)
	Seward	Reporteda	(Cortinas and Spomer 2014)
	Sheridan	Reporteda	(Cortinas and Spomer 2014)
	Sherman	Reporteda	(Cortinas and Spomer 2014)
	Sioux	Reported	(Cortinas and Spomer 2014, Kaufman et al. 2018)
	Thomas	Reporteda	(Cortinas and Spomer 2014)
	Thurston	Reporteda	(Cortinas and Spomer 2014)
	Washington	Reporteda	(Cortinas and Spomer 2014)
	Wayne	Reporteda	(Cortinas and Spomer 2014)
	Webster	Reported	(Cortinas and Spomer 2014, Kaufman et al. 2018)
	York	Reporteda	(Cortinas and Spomer 2014)
New Hampshire
	Carroll	Established	NEON
	Grafton	Reported	USNTC
New Jersey
	Burlington	Reported	USNTC
	Middlesex	Reported	USNTC
	Monmouth	Established	(Smart and Caccamise 1988)
New Mexico
	Dona Ana	Reported	USNTC
New York
	Bronx	Established	(Salgo et al. 1988)
	Nassau	Established	(Benach et al. 1977)
	Queens	Reported	USNTC
	Suffolk	Established	(Benach et al. 1977, Tokarz et al. 2018)
	Westchester	Established	(Fish and Dowler 1989)
North Carolina
	Alamance	Reported	(Kakumanu et al. 2018)
	Anson	Reported	(Kakumanu et al. 2018)
	Bladen	Reported	(Kakumanu et al. 2018)
	Brunswick	Reported	(Kakumanu et al. 2018)
	Camden	Reported	(Kakumanu et al. 2018)
	Carteret	Reported	(Kakumanu et al. 2018)
	Chatham	Established	(Apperson et al. 2008, Smith et al. 2010)
	Columbus	Reported	(Kakumanu et al. 2018)
	Craven	Reported	(Kakumanu et al. 2018)
	Cumberland	Established	(Solberg et al. 1996)
	Dare	Reported	USNTC
	Durham	Reported	USNTC
	Franklin	Reported	(Kakumanu et al. 2018)
	Gates	Reported	(Kakumanu et al. 2018)
	Greene	Reported	(Kakumanu et al. 2018)
	Guilford	Reported	(Kakumanu et al. 2018)
	Halifax	Reported	(Kakumanu et al. 2018)
	Harnett	Reported	(Kakumanu et al. 2018)
	Haywood	Reported	USNTC
	Hoke	Reported	(Kakumanu et al. 2018)
	Johnston	Reported	(Kakumanu et al. 2018)
	Jones	Reported	(Kakumanu et al. 2018)
	Martin	Reported	(Kakumanu et al. 2018)
	Northampton	Reported	(Kakumanu et al. 2018)
	Onslow	Established	(Apperson et al. 1993, Ouellette et al. 1997)
	Perquimans	Reported	(Kakumanu et al. 2018)
	Sampson	Reported	(Kakumanu et al. 2018)
	Scotland	Reported	(Kakumanu et al. 2018)
	Surry	Reported	(Kakumanu et al. 2018)
	Tyrrell	Reported	(Kakumanu et al. 2018)
	Vance	Reported	(Kakumanu et al. 2018)
	Wake	Reported	(Kakumanu et al. 2018)
	Warren	Reported	(Kakumanu et al. 2018)
	Wayne	Reported	(Kakumanu et al. 2018)
North Dakota
	Billings	Established	(Russart et al. 2014)
	Eddy	Established	(Russart et al. 2014)
	Grand Forks	Established	(Russart et al. 2014)
	McKenzie	Reported	(Russart et al. 2014)
	Morton	Established	NEON
	Pembina	Established	(Russart et al. 2014)
	Ramsey	Established	(Russart et al. 2014
	Rolette	Established	(Russart et al. 2014)
	Slope	Reported	USNTC
	Steele	Established	(Russart et al. 2014)
	Stutsman	Established	NEON
	Ward	Established	USNTC
Ohio
	Adams	Establisheda	(Pretzman et al. 1990)
	Allen	Reporteda	(Pretzman et al. 1990)
	Ashland	Reporteda	(Pretzman et al. 1990)
	Ashtabula	Reporteda	(Pretzman et al. 1990)
	Athens	Establisheda	(Pretzman et al. 1990)
	Auglaize	Reporteda	(Pretzman et al. 1990)
	Belmont	Establisheda	(Pretzman et al. 1990)
	Brown	Reporteda	(Pretzman et al. 1990)
	Butler	Establisheda	(Pretzman et al. 1990)
	Carroll	Reporteda	(Pretzman et al. 1990)
	Champaign	Establisheda	(Pretzman et al. 1990)
	Clark	Establisheda	(Pretzman et al. 1990)
	Clermont	Establisheda	(Pretzman et al. 1990)
	Clinton	Establisheda	(Pretzman et al. 1990)
	Columbiana	Reporteda	(Pretzman et al. 1990)
	Coshocton	Reporteda	(Pretzman et al. 1990)
	Crawford	Reporteda	(Pretzman et al. 1990)
	Cuyahoga	Reporteda	(Pretzman et al. 1990)
	Darke	Reporteda	(Pretzman et al. 1990)
	Defiance	Establisheda	(Pretzman et al. 1990)
	Delaware	Established	(Harlan and Foster 1990, Pretzman et al. 1990)
	Erie	Reporteda	(Pretzman et al. 1990)
	Fairfield	Establisheda	(Pretzman et al. 1990)
	Fayette	Establisheda	(Pretzman et al. 1990)
	Franklin	Establisheda	(Pretzman et al. 1990)
	Fulton	Reporteda	(Pretzman et al. 1990)
	Gallia	Reporteda	(Pretzman et al. 1990)
	Geauga	Reporteda	(Pretzman et al. 1990)
	Greene	Establisheda	(Pretzman et al. 1990)
	Guernsey	Establisheda	(Pretzman et al. 1990)
	Hamilton	Establisheda	(Pretzman et al. 1990)
	Hancock	Reporteda	(Pretzman et al. 1990)
	Hardin	Reporteda	(Pretzman et al. 1990)
	Harrison	Reporteda	(Pretzman et al. 1990)
	Henry	Reporteda	(Pretzman et al. 1990)
	Highland	Establisheda	(Pretzman et al. 1990)
	Hocking	Establisheda	(Pretzman et al. 1990)
	Holmes	Reporteda	(Pretzman et al. 1990)
	Huron	Reporteda	(Pretzman et al. 1990)
	Jackson	Establisheda	(Pretzman et al. 1990)
	Jefferson	Reporteda	(Pretzman et al. 1990)
	Knox	Establisheda	(Pretzman et al. 1990)
	Lake	Reporteda	(Pretzman et al. 1990)
	Lawrence	Reporteda	(Pretzman et al. 1990)
	Licking	Establisheda	(Pretzman et al. 1990)
	Logan	Establisheda	(Pretzman et al. 1990)
	Lorain	Reporteda	(Pretzman et al. 1990)
	Lucas	Established	(Conlon and Rockett 1982, Pretzman et al. 1990)
	Madison	Establisheda	(Pretzman et al. 1990)
	Mahoning	Reporteda	(Pretzman et al. 1990)
	Marion	Establisheda	(Pretzman et al. 1990)
	Medina	Reporteda	(Pretzman et al. 1990)
	Meigs	Reporteda	(Pretzman et al. 1990)
	Mercer	Reporteda	(Pretzman et al. 1990)
	Miami	Reporteda	(Pretzman et al. 1990)
	Monroe	Reporteda	(Pretzman et al. 1990)
	Montgomery	Establisheda	(Pretzman et al. 1990)
	Morgan	Reporteda	(Pretzman et al. 1990)
	Morrow	Establisheda	(Pretzman et al. 1990)
	Muskingum	Establisheda	(Pretzman et al. 1990)
	Noble	Reporteda	(Pretzman et al. 1990)
	Ottawa	Reporteda	(Pretzman et al. 1990)
	Paulding	Reporteda	(Pretzman et al. 1990)
	Perry	Reporteda	(Pretzman et al. 1990)
	Pickaway	Establisheda	(Pretzman et al. 1990)
	Pike	Reporteda	(Pretzman et al. 1990)
	Portage	Establisheda	(Pretzman et al. 1990)
	Preble	Reporteda	(Pretzman et al. 1990)
	Putnam	Reporteda	(Pretzman et al. 1990)
	Richland	Reporteda	(Pretzman et al. 1990)
	Ross	Establisheda	(Pretzman et al. 1990)
	Sandusky	Reporteda	(Pretzman et al. 1990)
	Scioto	Reporteda	(Pretzman et al. 1990)
	Seneca	Reporteda	(Pretzman et al. 1990)
	Shelby	Reporteda	(Pretzman et al. 1990)
	Stark	Reporteda	(Pretzman et al. 1990)
	Summit	Reporteda	(Pretzman et al. 1990)
	Trumbull	Reporteda	(Pretzman et al. 1990)
	Tuscarawas	Establisheda	(Pretzman et al. 1990)
	Union	Establisheda	(Pretzman et al. 1990)
	Van Wert	Reporteda	(Pretzman et al. 1990)
	Vinton	Establisheda	(Pretzman et al. 1990)
	Warren	Establisheda	(Pretzman et al. 1990)
	Washington	Establisheda	(Pretzman et al. 1990)
	Wayne	Reporteda	(Pretzman et al. 1990)
	Williams	Establisheda	(Pretzman et al. 1990)
	Wood	Reporteda	(Mitcham et al. 2017)
	Wyandot	Establisheda	(Mitcham et al. 2017)
Oklahoma
	Beckham	Reported	(Mitcham et al. 2017)
	Blaine	Reported	(Mitcham et al. 2017)
	Caddo	Established	(Mitcham et al. 2017)
	Cherokee	Established	USNTC
	Cleveland	Established	(Mitcham et al. 2017)
	Coal	Reported	(Mitcham et al. 2017)
	Comanche	Established	USNTC
	Cotton	Reported	(Mitcham et al. 2017)
	Custer	Reported	(Mitcham et al. 2017)
	Delaware	Reported	(Mitcham et al. 2017)
	Dewey	Reported	(Mitcham et al. 2017)
	Ellis	Reported	(Mitcham et al. 2017)
	Garfield	Established	(Mitcham et al. 2017)
	Garvin	Reported	(Mitcham et al. 2017)
	Grady	Established	(Mitcham et al. 2017)
	Grant	Reported	(Mitcham et al. 2017)
	Harmon	Reported	(Mitcham et al. 2017)
	Harper	Established	USNTC
	Hughes	Reported	(Mitcham et al. 2017)
	Jefferson	Reported	(Mitcham et al. 2017)
	Kingfisher	Reported	(Mitcham et al. 2017)
	Kiowa	Reported	(Mitcham et al. 2017)
	Le Flore	Established	(Koch and Dunn 1980)
	Love	Reported	(Mitcham et al. 2017)
	Major	Reported	(Mitcham et al. 2017)
	McClain	Established	(Gage et al. 1992)
	Murray	Reported	(Noden et al. 2017)
	Muskogee	Reported	USNTC
	Nowata	Established	(Semtner and Hair 1975)
	Oklahoma	Established	(Barrett et al. 2014)
	Osage	Established	(Mitcham et al. 2017)
	Pawnee	Established	(Mitcham et al. 2017)
	Payne	Established	(Barrett et al. 2014)
	Pottawatomie	Reported	(Mitcham et al. 2017)
	Roger Mills	Reported	(Mitcham et al. 2017)
	Stephens	Reported	(Mitcham et al. 2017)
	Tulsa	Reported	USNTC
	Washington	Reported	(Mitcham et al. 2017)
	Washita	Established	NEON
	Woodward	Reported	(Mitcham et al. 2017)
Oregon
	Baker	Established	USNTC
	Hood River	Reported	USNTC
	Jackson	Established	(Easton et al. 1977)
	Umatilla	Reported	USNTC
	Washington	Reported	USNTC
Pennsylvania
	Adams	Establisheda	(Snetsinger et al. 1993)
	Allegheny	Reporteda	(Snetsinger et al. 1993)
	Armstrong	Reporteda	(Snetsinger et al. 1993)
	Beaver	Reporteda	(Snetsinger et al. 1993)
	Bedford	Reporteda	(Snetsinger et al. 1993)
	Berks	Establisheda	(Snetsinger et al. 1993)
	Blair	Reporteda	(Snetsinger et al. 1993)
	Bradford	Reporteda	(Snetsinger et al. 1993)
	Bucks	Establisheda	(Snetsinger et al. 1993)
	Butler	Reporteda	(Snetsinger et al. 1993)
	Cambria	Reporteda	(Snetsinger et al. 1993)
	Carbon	Reporteda	(Snetsinger et al. 1993)
	Centre	Reporteda	(Snetsinger et al. 1993)
	Chester	Establisheda	(Snetsinger et al. 1993)
	Clarion	Reporteda	(Snetsinger et al. 1993)
	Clearfield	Reporteda	(Snetsinger et al. 1993)
	Clinton	Reporteda	(Snetsinger et al. 1993)
	Columbia	Reporteda	(Snetsinger et al. 1993)
	Crawford	Reporteda	(Snetsinger et al. 1993)
	Cumberland	Reporteda	(Snetsinger et al. 1993)
	Dauphin	Establisheda	(Snetsinger et al. 1993)
	Delaware	Establisheda	(Snetsinger et al. 1993)
	Erie	Reporteda	(Snetsinger et al. 1993)
	Fayette	Reporteda	(Snetsinger et al. 1993)
	Forest	Reporteda	(Snetsinger et al. 1993)
	Franklin	Establisheda	(Snetsinger et al. 1993)
	Fulton	Reporteda	(Snetsinger et al. 1993)
	Greene	Establisheda	(Snetsinger et al. 1993)
	Huntingdon	Reporteda	(Snetsinger et al. 1993)
	Indiana	Reporteda	(Snetsinger et al. 1993)
	Jefferson	Reporteda	(Snetsinger et al. 1993)
	Juniata	Reporteda	(Snetsinger et al. 1993)
	Lackawanna	Reporteda	(Snetsinger et al. 1993)
	Lancaster	Establisheda	(Snetsinger et al. 1993)
	Luzerne	Reporteda	(Snetsinger et al. 1993)
	Lebanon	Reporteda	(Snetsinger et al. 1993)
	Lehigh	Reporteda	(Snetsinger et al. 1993)
	Luzerne	Reporteda	(Snetsinger et al. 1993)
	Lycoming	Reporteda	(Snetsinger et al. 1993)
	McKean	Reporteda	(Snetsinger et al. 1993)
	Mercer	Reporteda	(Snetsinger et al. 1993)
	Mifflin	Reporteda	(Snetsinger et al. 1993)
	Monroe	Reporteda	(Snetsinger et al. 1993)
	Montgomery	Establisheda	(Snetsinger et al. 1993)
	Montour	Reporteda	(Snetsinger et al. 1993)
	Northampton	Reporteda	(Snetsinger et al. 1993)
	Northumberland	Reporteda	(Snetsinger et al. 1993)
	Perry	Reporteda	(Snetsinger et al. 1993)
	Philadelphia	Establisheda	(Snetsinger et al. 1993)
	Pike	Establisheda	(Snetsinger et al. 1993)
	Potter	Reporteda	(Snetsinger et al. 1993)
	Schuylkill	Reporteda	(Snetsinger et al. 1993)
	Snyder	Reporteda	(Snetsinger et al. 1993)
	Somerset	Reporteda	(Snetsinger et al. 1993)
	Sullivan	Reporteda	(Snetsinger et al. 1993)
	Union	Reporteda	(Snetsinger et al. 1993)
	Venango	Reporteda	(Snetsinger et al. 1993)
	Warren	Reporteda	(Snetsinger et al. 1993)
	Washington	Establisheda	(Snetsinger et al. 1993)
	Wayne	Reporteda	(Snetsinger et al. 1993)
	Westmoreland	Reporteda	(Snetsinger et al. 1993)
	Wyoming	Reporteda	(Snetsinger et al. 1993)
	York	Establisheda	(Snetsinger et al. 1993)
Rhode Island
	Newport	Established	(Mather and Mather 1990)
South Carolina
	Aiken	Reported	USNTC
	Charleston	Established	(Burgdorfer et al. 1975)
	Chester	Established	(Clark et al. 1998)
	Georgetown	Established	(Clark et al. 1998)
	Sumter	Established	(Clark et al. 1998)
South Dakota
	Brookings	Established	USNTC
	Butte	Reported	(Easton 1983)
	Clark	Reported	(Easton 1983)
	Clay	Reported	(Easton 1983)
	Codington	Reported	(Easton 1983)
	Davison	Reported	(Easton 1983)
	Day	Established	USNTC
	Deuel	Established	USNTC
	Fall River	Reported	(Easton 1983)
	Grant	Reported	(Easton 1983)
	Gregory	Reported	(Easton 1983)
	Haakon	Reported	USNTC
	Hamlin	Reported	(Easton 1983)
	Hand	Reported	(Easton 1983)
	Hughes	Reported	(Easton 1983)
	Jackson	Established	USNTC
	Kingsbury	Reported	(Easton 1983)
	Lawrence	Established	USNTC
	Lincoln	Reported	(Easton 1983)
	Marshall	Established	USNTC
	Meade	Established	USNTC
	Minnehaha	Reported	USNTC
	Pennington	Reported	USNTC
	Perkins	Reported	(Easton 1983)
	Roberts	Established	USNTC
	Shannon	Reported	(Easton 1983)
	Spink	Reported	USNTC
	Stanley	Reported	USNTC; (Easton 1983)
	Union	Established	USNTC
	Ziebach	Reported	USNTC; (Easton 1983)
Tennessee
	Anderson	Established	NEON
	Blount	Established	(Cohen et al. 2010)
	Bradley	Established	(Cohen et al. 2010)
	Carroll	Reported	(Cohen et al. 2010)
	Carter	Established	(Cohen et al. 2010)
	Cheatham	Established	(Cohen et al. 2010)
	Chester	Reported	(Cohen et al. 2010)
	Cumberland	Established	(Cohen et al. 2010)
	Davidson	Established	(Durden and Wilson 1990, Cohen et al. 2010)
	Decatur	Reported	(Cohen et al. 2010)
	DeKalb	Established	(Cohen et al. 2010)
	Dickson	Established	(Cohen et al. 2010)
	Fayette	Established	(Mays et al. 2016, Pompo et al. 2016, Trout Fryxell et al. 2017)
	Giles	Established	(Mays et al. 2016)
	Greene	Established	(Cohen et al. 2010)
	Hamilton	Established	(Cohen et al. 2010)
	Hardeman	Established	(Mays et al. 2016)
	Hardin	Reported	(Cohen et al. 2010)
	Henry	Reported	(Cohen et al. 2010)
	Humphreys	Reported	(Cohen et al. 2010)
	Jefferson	Established	(Cohen et al. 2010)
	Johnson	Reported	(Cohen et al. 2010)
	Knox	Established	(Cohen et al. 2010)
	Loudon	Established	(Cohen et al. 2010)
	McMinn	Reported	(Cohen et al. 2010)
	McNairy	Reported	(Cohen et al. 2010)
	Madison	Established	(Cohen et al. 2010)
	Monroe	Established	(Cohen et al. 2010)
	Montgomery	Reported	(Cohen et al. 2010)
	Morgan	Reported	(Cohen et al. 2010)
	Perry	Reported	(Cohen et al. 2010)
	Roane	Established	(Cohen et al. 2010)
	Scott	Established	(Cohen et al. 2010)
	Sevier	Established	(Cohen et al. 2010)
	Shelby	Established	(McLean et al. 1985, Kollars 1993, 1996, Kollars and Ladine 1999, Kollars and Kengluecha 2001)
	Smith	Reported	(Cohen et al. 2010)
	Stewart	Established	(Cooney and Burgdorfer 1974, Zimmerman et al. 1987)
	Sullivan	Established	(Cohen et al. 2010)
	Sumner	Established	(Cohen et al. 2010)
	Unicoi	Reported	(Cohen et al. 2010)
	Washington	Established	(Cohen et al. 2010)
	Weakley	Established	(Cohen et al. 2010)
	Williamson	Reported	(Cohen et al. 2010)
Texas
	Angelina	Reported	USNTC
	Aransas	Reported	(Eads 1956)
	Bee	Established	USNTC; (Eads 1956)
	Bexar	Reported	USNTC; (Eads 1948)
	Brazos	Established	(Sanders et al. 2013)
	Cameron	Established	(Corn et al. 2016)
	Floyd	Reported	(Eads 1948)
	Frio	Reported	USNTC
	Hidalgo	Reported	USNTC
	Jackson	Reported	USNTC
	Jefferson	Reported	USNTC
	Jim Wells	Established	USNTC
	Johnson	Reported	USNTC
	Kenedy	Established	USNTC
	Kerr	Reported	USNTC
	Kleberg	Reported	USNTC
	Liberty	Reported	USNTC
	Live Oak	Reported	USNTC
	Mason	Reported	USNTC
	Milam	Reported	(Eads et al. 1956)
	Newton	Reported	(Sanders et al. 2013)
	Nueces	Reported	USNTC
	Refugio	Established	(Eads et al. 1956)
	San Patricio	Established	(Sanders et al. 2013)
	Shelby	Established	(Eads et al. 1956)
	Travis	Established	(Eads et al. 1956)
	Uvalde	Established	USNTC
	Victoria	Established	USNTC
	Webb	Reported	(Lin et al. 2005)
	Wichita	Established	(Eads et al. 1956)
	Wilbarger	Reported	(Eads 1948)
	Willacy	Established	USNTC
	Zavala	Reported	USNTC; (Eads et al. 1956)
Vermont
	Bennington	Reported	USNTC
	Caledonia	Reported	Serra et al. 2013)
	Orange	Reported	(Serra et al. 2013)
Virginia
	Accomack	Established	(Orr et al. 2013)
	Clarke	Reported	NEON
	Dinwiddie	Reported	USNTC
	Fairfax	Established	(Orr et al. 2013, Henning et al. 2014)
	Hanover	Established	(Sonenshine and Atwood 1967, Sonenshine 1979a)
	Newport News	Established	(Miller et al. 2016)
	Nottoway	Established	(Stein et al. 2008)
	Prince George	Reported	USNTC
	Prince William	Established	(Johnson et al. 2017)
	Shenandoah	Reported	USNTC
	Sussex	Established	USNTC
	Warren	Established	NEON
	Westmoreland	Established	USNTC
Washington
	Lincoln	Established	(Araya-Anchetta et al. 2013)
	Whitman	Established	(Stout et al. 1971)
	Yakima	Reported	(Stout et al. 1971)
West Virginia
	Mason	Established	(Joy and Briscoe 1994)
Wisconsin
	Ashland	Reported	(Manville 1978)
	Bayfield	Reported	(Manville 1978)
	Douglas	Reported	USNTC
	Iowa	Established	(Jackson and DeFoliart 1975)
	Iron	Established	(Manville 1978)
	Langlade	Reported	USNTC
	Lincoln	Established	NEON
	Oneida	Established	USNTC
	Outagamie	Reported	USNTC
	Price	Reported	NEON
	Sawyer	Established	(Brackney et al. 2008)
	Trempealeau	Reported	USNTC
	Vernon	Reported	USNTC
	Vilas	Reported	USNTC
	Washburn	Established	(Brackney et al. 2008)
Wyoming
	Campbell	Reported	USNTC
	Sheridan	Reported	USNTC

^aCounties included using expanded inclusion criteria.

Fig. 1.

Distribution by county of recorded presence of D. variabilis in the contiguous United States based on strict (A) or expanded (B) inclusion criteria. Established counties (in red) have records of six or more ticks or two or more life stages recorded in 1 yr, or using the expanded criteria ≥50 ticks recorded over ≥5 yr if from a state-wide surveillance program. Reported counties (in blue) have records of fewer than six ticks in 1 yr, no documentation of number of ticks, or using the expanded criteria <50 ticks over ≥5 yr if a state-wide surveillance program. Counties shown in white indicate ‘no records’. Notably, an absence of records derived through our review is not indicative of the tick’s absence in a particular county.

When using our expanded inclusion criteria, D. variabilis has been recorded in a total of 928 counties (29.8%) (Tables 1 and 2; Fig. 1B); three states in the contiguous 48 states do not have records of the tick, these include: Arizona, Nevada, and Utah. In total, 316 counties (10.2%) distributed across 42 states (Tables 1 and 2; Fig. 1B) met the criteria to be classified as established. The majority of these counties are east of the Rocky Mountains although states west of the Rockies including California, Oregon, Washington, and Idaho have documented established populations. Dermacentor variabilis is now classified as reported in 612 (19.7%) counties, distributed across 41 states (Tables 1 and 2; Fig. 1B).

Discussion

Among medically important ticks in the contiguous United States, D. variabilis has one of the broadest geographic ranges (Hooker 1909, Bishopp and Trembley 1945, James et al. 2015). Our data show widespread, yet sporadic county records for this tick and suggest it is currently underreported, perhaps in part because of a lack of systematic active surveillance efforts focused on this tick species. Whether based on statistical modeling or expert opinion guided by collection records, what we refer to herein as ‘species range maps’ represent areas where, hypothetically, the tick could survive and reproduce if introduced; such maps can be useful for targeting surveillance efforts that reveal whether or not the tick is present within specific parts of the tick’s range. In contrast to range maps, maps showing the reported distribution of a species depict areas where the tick has been collected. In general, information on tick abundance, rather than simple measures of tick presence or estimated ranges provides better information on the likelihood of human encounters with ticks. However, accurately estimating tick abundance over large geographic areas is time-consuming, costly, and rarely done. In this study, we used the Dennis et al. (1998) classification criteria to categorize counties based on where 1) tick records are lacking, 2) the tick has been found (reported), but not necessarily established, and 3) the tick is established, and likely able to survive and reproduce.

In addition to species range maps, accurate distribution maps of medically important ticks are increasingly important as tick-borne disease incidence increases and cases are reported over expanded geographic areas (Dantas-Torres 2007, Piesman and Eisen 2008, Nicholson et al. 2010, Dantas-Torres et al. 2012, Eisen et al. 2017, Rosenberg et al. 2018). Such information can be useful in implicating ticks as potential vectors of newly discovered human pathogens (e.g., Heartland virus, Bourbon virus, Borrelia miyamotoi, and Borrelia mayonii), for identifying risk for potential exposure to tick-borne pathogens across regions, and for raising public awareness of which ticks and potentially which tick-borne pathogens are in the area and the importance of practicing tick bite prevention strategies. Long-term systematic active surveillance of ticks is important as species distributions shift due to a variety of factors including changes in land use, habitat and host availability, and climate (Childs and Paddock 2003, Estrada-Peña et al. 2006, Gray et al. 2009, James et al. 2015, Eisen et al. 2016).

James et al. (2015) used statistical modeling to predict the range in suitable habitat for D. variabilis in the United States, based largely on elevation and temperature. Highly suitable habitat included areas in: coastal California, the Pacific Northwest, the Midwest, the Gulf Coast, Florida, and the Atlantic seaboard (James et al. 2015). Regions of low suitability included much of the Western United States, areas in the South, and northwestern inland regions of New England. The distribution map we present here is comparable to the James et al. (2015) species range map.

Our distribution map using the strict criteria (Fig. 1A) shows records of the American dog tick throughout the United States particularly along the West, Gulf, and East coasts and in areas of low elevation in the central regions of the country; these records generally align with the suitable habitats delineated in the James et al. (2015) species range map. Additionally, the range map indicates low suitability in the Rocky Mountain Region (James et al. 2015), where our distribution map shows a noticeable lack of records. There is discordance between the maps where we report records in medium–low suitable habitats (James et al. 2015), such as in parts of Oklahoma and Wisconsin. However, what appears most telling is the lack of records in our distribution map in areas that James et al. (2015) note as highly suitable habitat. For example, we report only two counties in Iowa with records of D. variabilis even though most of Iowa is classified by the model as moderately or highly suitable habitat (James et al. 2015) for the tick. Additionally, we report no records in eastern Michigan which is classified as highly suitable (James et al. 2015), the same is true for portions of the Atlantic seaboard and Gulf Coast.

Using our expanded criteria to include counties for which there was compelling evidence that D. variabilis is present we created our expanded distribution map (Fig. 1B). This map includes the addition of county records from the following states: Georgia, Iowa, Maine, Michigan, Nebraska, Ohio, and Pennsylvania (Tables 1 and 2; Fig. 1B). Compared to the strict distribution map, the expanded map displays greater concordance with the species range map (James et al. 2015) as we add records in highly suitable habitats including: Iowa, eastern Michigan, and northern Ohio. However, we still observe areas of high to medium habitat suitability (James et al. 2015) with few collection records such as along the Atlantic seaboard from New Jersey down through the Carolinas.

The variation between our distribution maps and the species range map (James et al. 2015) may arise from a lack of systematic active sampling efforts specifically targeting collection of D. variabilis in vegetation where it is most commonly found, such as low-elevation grasslands and boundaries of forests and trails (Wilkinson 1967, Sonenshine 1979b, McDade and Newhouse 1986, Dergousoff et al. 2013). Approximately one third of D. variabilis records used here originated from sources in which D. variabilis collections were incidental during collections focused largely on woodland-associated ticks, namely I. scapularis and A. americanum. Consequently, although tick sampling was conducted in numerous counties throughout the United States, efforts targeting I. scapularis and A. americanum may underestimate the number of established D. variabilis populations either because incidental species are not reported in publications, or because tick abundance is generally lower in woodlands where I. scapularis and A. americanum are common compared with grasslands where D. variabilis is more abundant. Additionally, by limiting our records primarily to those in the peer-reviewed literature it is quite likely that records of this tick being established in more counties is likely. In an effort to disseminate current and accurate tick distribution maps, the U.S. Centers for Disease Control and Prevention recently issued guidance on tick surveillance methods and have started to collect tick distribution and abundance records from state health departments through the ArboNET database (CDC 2018a; https://wwwn.cdc.gov/arbonet/).

Species distribution maps, developed based on spatial modeling, collection records and expert knowledge are powerful tools for assessing risk for human exposure to medically important ticks. We recognize the increasing need for accurate distribution maps of medically important ticks as tick-borne disease incidence increases, cases are reported over expanded geographic areas, and increasing numbers of people are exposed to potentially infectious tick bites. Importantly, there is a need to enhance 1) active and passive tick surveillance, particularly at the county level, 2) reporting of medically important ticks in peer-reviewed literature and through public health databases such as ArboNET, and 3) use of common classification criteria such as the Dennis et al. (1998) criteria.