100 Years of Drinking Water Outbreak Surveillance: Trends and Influence, United States, 1920–2020

Abstract

To examine trends in US drinking water outbreaks from 1920 to 2020, we evaluated US drinking water outbreaks documented in the National Outbreak Reporting System and from 4 historic publications dating back to 1920.

Before the drinking water regulations of the 1970s, enteric pathogens were the most reported (n = 393; 52%) etiologies. Most outbreaks occurred in individual or private water systems (n = 245; 32%). After major regulations, biofilm-associated outbreaks emerged (n = 343; 32%), and most outbreaks (n = 600; 55%) occurred in community water systems.

Biofilm-associated pathogens are an emerging concern in water systems. These findings highlight the importance of drinking water regulations, routine inspections of private wells, and building water management programs. Waterborne disease outbreak investigations and reporting to surveillance systems are essential to identifying factors contributing to outbreaks that can be addressed to improve drinking water policy, practices, and public understanding. As pathogens, drinking water distribution systems, and environmental factors change, identifying etiologic agents and contributing factors in waterborne outbreaks can guide future drinking water policies.

The United States at the turn of the 20th century experienced increasing urban population density and rapid industrial growth that demanded the expansion of reliable public water supplies and distribution systems.¹ Around 1900, waterborne diseases made up nearly 25% of all infectious disease deaths in urban areas.¹ Between 1900 and 1936, public health efforts such as hygiene education, sewage treatment, and drinking water filtration and disinfection programs in large cities dramatically decreased infectious waterborne diseases, including cholera, typhoid fever, and dysentery.^1–4

The US Public Health Service was the first (in 1914) to set drinking water quality standards regulating drinking water on interstate carriers like ships, trains, and buses (Appendix Table A, available as a supplement to the online version of this article at https://ajph.org).⁵ Early drinking water guidance was focused on bacterial pathogens associated with enteric or diarrheal illnesses; later it was revised and expanded as the growth of the US population and industries led to man-made chemicals finding their way into water supplies.⁵ These US Public Health Service standards ultimately served as a model for the Safe Drinking Water Act (SDWA) of 1974, which addressed both infectious disease pathogens and chemical contaminants in public water systems (Table A).^5,6

Despite the provision of safe drinking water and sanitation in the United States, waterborne disease outbreaks related to aging water infrastructure, chlorine-tolerant and biofilm-associated pathogens, and the use of private or unregulated water systems (e.g., private wells) persist.^7–10 Drinking water-related illnesses are associated with nearly $1.4 billion in direct health care costs annually.^8,11

Public health agencies and academic institutions have compiled data on waterborne outbreaks dating back to 1920.^3,12–14 In 1971, the Council of State and Territorial Epidemiologists, US Environmental Protection Agency (EPA), and Centers for Disease Control and Prevention (CDC) began collecting reports on drinking water outbreaks through the Waterborne Disease and Outbreak Surveillance System (WBDOSS; https://bit.ly/4tj8S4E; Table A). Since 2009, waterborne disease outbreaks are voluntarily reported electronically to CDC through the National Outbreak Reporting System (NORS).¹⁵ State and local health agencies are primarily responsible for investigating and reporting these outbreaks. Before the 1970s, trends for historical waterborne disease outbreaks in the United States were summarized in 10- to 15-year periods, but not regularly.^3,4,12,13 Since 1973, drinking water outbreaks have been summarized biennially or annually¹⁶ with the exception of 3 larger trend reports for the years 1920 to 2002,¹⁶ 1971 to 1998,¹⁰ and 1971 to 2006.¹⁵ The EPA published a 25-year (1974–1996) summary of drinking water outbreaks and violations since the SDWA⁵ and collaborated with CDC to report drinking water–associated hepatitis A outbreaks during 1971 to 2017.¹⁷

Provision of safe drinking water is one of most important public health interventions of the last century, responsible for half the decrease in overall mortality and three quarters of the decline in infant mortality.¹ Over the years, the complexity of drinking water distribution systems and the variety of ways in which we use water have posed new challenges in maintaining safe water while public health and utilities respond to the threats of emerging waterborne pathogens. Surveillance data from waterborne disease outbreaks have been critical to understanding the risks from unsafe water, evaluating the impact of public health interventions, and guiding public health officials in responding to new threats. We sought to identify all reported drinking water–associated outbreaks in the United States that occurred between 1920 and 2020, perform descriptive analytics, identify trends before and after the 1974 SDWA, and highlight emerging issues.

METHODS

This summary characterizes reported waterborne outbreaks associated with drinking water for the 100-year study period from 1920 to 2020 finalized in NORS as of June 5, 2024 (further described in the Supplemental Methods section of the Appendix) and line lists from 4 historic publications.^3,12–14 We cleaned and analyzed data and calculated descriptive statistics using SAS (version 9.4; SAS Institute, Cary, NC) and Microsoft Excel for Microsoft 365 Microsoft Office (version 2408; Microsoft Corporation, Redmond, WA). For this analysis, Legionella pneumophila, nontuberculous mycobacteria, and Pseudomonas aeruginosa were analyzed separately from other bacterial outbreaks and categorized as biofilm-associated pathogens.^11,18 Campylobacter, Cryptosporidium, Giardia, Escherichia coli, norovirus, rotavirus, Salmonella, and Shigella were defined as infectious enteric pathogens.¹¹ The remaining outbreak categories comprised undetermined, parasites, other virus or bacteria, and chemical or toxin (further described in the Supplemental Methods).

RESULTS

During 1920 to 2020, there were 1840 outbreaks (Table 1), 866 506 cases of illness (Table 2), and 1207 deaths (Figure 1) reported as associated with drinking water. All 50 states, the Northern Mariana Islands, Puerto Rico, the Republic of Palau, the Republic of the Marshall Islands, the US Virgin Islands, and Washington, DC, reported at least 1 outbreak (Appendix Figure A). Outbreaks most frequently occurred during the months of June through September (n = 890; 48%) but were reported across all months (Appendix Figure B).

TABLE 1—

Waterborne Disease Outbreaks Associated With Drinking Water, by Water Source and Description: United States, 1920–2020

Water Source and Description	Outbreaks, No. (%)
	1920–1974	1975–2020	All Years
Ground water	448 (59)	597 (55)	1045 (57)
Well	361 (81)	477 (80)	838 (80)
Spring	68 (15)	64 (11)	132 (13)
Unknown	4 (< 1)	35 (6)	39 (4)
Othera	15 (3)	21 (4)	36 (3)
Surface water	182 (24)	319 (29)	501 (27)
River or stream	114 (63)	130 (41)	244 (49)
Lake, reservoir, or impoundment	46 (25)	158 (50)	204 (41)
Unknown	19 (10)	24 (8)	43 (9)
Pond	…	5 (2)	5 (1)
Otherb	1 (< 1)	2 (< 1)	3 (< 1)
Puddle, canal, or swamp	2 (1)	…	2 (< 1)
Unknown	41 (5)	151 (14)	192 (10)
Ground water under the influence of surface water	45 (6)	8 (< 1)	53 (3)
Well	30 (67)	4 (50)	34 (64)
Spring	11 (24)	1 (13)	12 (23)
Unknown	2 (4)	2 (25)	4 (8)
Ground water and surface water	39 (5)	10 (1)	49 (3)
River or stream and well	21 (54)	2 (20)	23 (47)
Lake, reservoir, or impoundment and well	8 (21)	3 (30)	11 (22)
Otherc	10 (26)	2 (20)	12 (25)
Unknown	…	3 (30)	3 (6)
Otherd	2 (4)	1 (13)	3 (6)
Total	755	1085	1840

^aAquifer (n = 14), well and spring (n = 11), cistern (n = 5), not specified (n = 4), drift gallery (n = 1), and well and cistern (n = 1).

^bDrift gallery (n = 1), rainwater (n = 1), and not specified (n = 1).

^cRiver or stream and spring (n = 3); other surface water and well (n = 4); lake, reservoir, or impoundment and spring (n = 4); and puddle, canal, or swamp and well (n = 1).

^dCistern (n = 1), well and drift gallery (n = 1), and not specified (n = 1).

TABLE 2—

Waterborne Disease Outbreaks (OBs) and Cases Associated With Drinking Water, by Etiologic Agent and Water System: United States, 1920–2020

Water Systema	Enteric Pathogens,b No. (%)		Biofilm-Associated Pathogens,c No. (%)		Chemicals or Toxins, No. (%)		Other Viruses and Bacteria, No. (%)		Parasites, No. (%)		Undetermined, No. (%)		All Etiologies No. (%)
	OBs	Cases	OBs	Cases	OBs	Cases	OBs	Cases	OBs	Cases	OBs	Cases	OBs	Cases
Community	236 (33)	581 239 (91)	280 (81)	1 355 (74)	61 (67)	3 666 (82)	98 (32)	104 712 (82)	8 (53)	548 (26)	110 (29)	54 368 (59)	793 (43)	745 888 (86)
Individual or private	142 (20)	5843 (< 1)	9 (3)	78 (4)	15 (16)	113 (3)	134 (44)	7 415 (6)	4 (27)	50 (2)	36 (10)	1 206 (1)	340 (19)	14 705 (2)
Noncommunity	113 (16)	16 313 (3)	7 (2)	21 (1)	10 (11)	710 (16)	11 (4)	431 (< 1)	2 (13)	80 (4)	220 (58)	33 527 (36)	363 (20)	51 082 (6)
Unknown	216 (30)	33 459 (5)	42 (12)	331 (18)	1 (1)	2 (< 1)	56 (19)	15 800 (12)	1 (7)	1 409 (68)	5 (3)	3 056 (3)	321 (17)	54 057 (6)
Otherd	3 (<1)	175 (<1)	6 (2)	39 (2)	4 (4)	9 (< 1)	3 (1)	116 (< 1)	…	…	7 (2)	435 (< 1)	23 (1)	774 (< 1)
Total	710 (39)	637 029 (74)	344 (19)	1 824 (< 1)	91 (5)	4 500 (< 1)	302 (16)	128 474 (15)	15 (< 1)	2 087 (< 1)	378 (21)	92 592 (11)	1840	866 506

^aCommunity and noncommunity water systems are defined by the Environmental Protection Agency as public water systems that have ≥ 15 service connections or serve an average of ≥ 25 residents for ≥ 60 days per year. A community water system serves year-round residents of a community, subdivision, or mobile home park. A noncommunity water system serves an institution, industry, camp, park, hotel, or business and can be nontransient or transient. Nontransient systems serve ≥ 25 of the same persons for ≥ 6 months of the year but not year-round (e.g., factories and schools), whereas transient systems provide water to places in which persons do not remain for long periods (e.g., restaurants, highway rest stations, and parks). Individual water systems are small systems not owned or operated by a water utility that have < 15 connections or serve < 25 persons.

^bCampylobacter, Cryptosporidium, Giardia, Escherichia coli, norovirus, rotavirus, Salmonella, and Shigella.

^cLegionella pneumophila, nontuberculous mycobacteria, and Pseudomonas aeruginosa.

^dIncludes commercially bottled (n = 16), other–not specified (n = 6), and bulk water purchase (n = 1).

FIGURE 1— Waterborne Disease Outbreaks, Cases, and Deaths AssociatedWith Drinking Water, by Year: United States, 1920–2020.

Note. Increase in cases and deaths associated with a 1993 Cryptosporidium outbreak (> 400 000 cases) in Milwaukee, WI.¹⁹

Etiologies

Among known etiologic agents, enteric pathogens were linked to most outbreaks (n = 710; 39%), illnesses (n = 637 029; 74%; Table 2), and deaths (n = 864; 72%; not shown). The second-most reported etiologic agents in outbreaks were biofilm-associated pathogens (n = 344; 19%). Biofilm pathogen–associated outbreaks were composed mostly of Legionella outbreaks (n = 338; 98%) and resulted in 1824 illnesses (< 1%; Table 2) and 217 deaths (18%; not shown); however, Legionella was not identified until 1977 (Table A). Enteric pathogens (n = 393; 52%) were the most reported etiology between 1920 and 1974, but during 1975 to 2020, biofilm pathogens (n = 343; 32%) were the most reported etiology while enteric pathogens (n = 317; 29%) were associated with a similar number of outbreaks (Figure 2). Enteric pathogens were linked to the most illnesses (n = 486 084; 85%) associated with drinking water outbreaks during 1975 to 2020, and biofilm pathogens were associated with the most deaths (n = 66; 22%).

FIGURE 2— Waterborne Disease Outbreaks Associated With Drinking Water, by Etiologic Agents: United States, 1920–2020.

Note. Enteric pathogens include Campylobacter, Cryptosporidium, Giardia, Escherichia coli, norovirus, rotavirus, Salmonella, and Shigella. Legionella, Pseudomonas, and nontuberculous mycobacteria were categorized as biofilm-associated pathogens.

Water Systems

For all years, most outbreaks (n = 793; 43%) and cases (n = 745 888; 86%) occurred in community water systems (Table 2). Noncommunity (n = 363; 20%) and individual or private (n = 340; 19%) were the second most reported systems in outbreaks (Table 2). Most biofilm-associated pathogen (n = 280; 81%), enteric pathogen (n = 236; 33%), and chemical or toxins (n = 61; 67%) outbreaks during 1920 to 2020 were associated with community water systems (Table 2). Individual or private water systems were identified in 142 (20%) enteric pathogen outbreaks. Noncommunity systems were associated with 113 (16%) enteric pathogen outbreaks (Table 2).

During 1920 to 1974, when the water system was known, most outbreaks occurred in individual or private water systems (n = 245; 33%; Appendix Figure C). Community water systems (n = 193; 26%) were the second-most reported type (Figure C). During 1975 to 2020, where the water system was known, most outbreaks occurred in community water systems (n = 600; 55%; Figure C). Noncommunity systems (n = 313; 29%) were the second-most reported type (Figure C).

Water Sources and Types

For all years, ground water (n = 1045; 57%) was the most frequently reported water source in outbreaks followed by surface water (n = 501; 27%; Table 1). Well water (n = 838; 80%) was the most frequently reported ground water type (Table 1). River or stream (n = 244; 49%) and lake, reservoir, or impoundment (n = 204; 41%) were the most frequently reported surface water types (Table 1). Additional results are described in the Supplemental Results section of the Appendix.

Water Treatments

Water treatment type documented “no treatment” (n = 437; 24%) was most reported in outbreaks for all years (Appendix Table B). Among known water systems, individual or private systems outbreaks had “no treatment” documented more often (n = 191) compared with other systems (Appendix Figure D). Treatment by disinfection was associated with 292 outbreaks (16%), mostly reporting chlorine disinfection (n = 270; 92%) (Table B). “No treatment” (n = 405; 54%) was most reported in outbreaks during 1920 to 1974 (Table B). Treatment by disinfection was documented in 151 outbreaks (20%) during 1920 to 1974, all of which used chlorine disinfection (Table B). During 1975 to 2020, treatment by disinfection was most reported (n = 160; 15%) in outbreaks (Table B). Chlorine disinfection was most identified as the treatment type (n = 119; 84%). “No treatment” was reported in 32 outbreaks (3%; Table B).

Settings

For all years, among known settings, health care settings (e.g., hospitals, nursing homes, assisted living) were the most reported (n = 223; 12%), followed by community (n = 192; 10%); hotel, motel, lodge, or inn (n = 189; 10%); and camp or cabin (n = 178; 10%; Appendix Figure E). Within health care waterborne associated outbreaks, biofilm pathogens were the primary etiology (n = 213; 96%; Appendix Figure F). From 1920 to 1974, hotel, motel, lodge, or inn (n = 59; 8%) was the most reported setting; followed by camp or cabin setting (n = 46; 6%); then school, college, or university (n = 45; 6%) for outbreaks (Figure E). Most factory or industrial facility settings (n = 41/46; 89%) were also reported during this period. From 1975 to 2020, health care facilities (n = 218; 20%) were the most reported, followed by community (n = 166; 15%), then camp or cabin (n = 132; 12%), and hotel, motel, lodge, or inn (n = 130; 12%; Figure E).

DISCUSSION

During 100 years of drinking water outbreak reporting, we note fluctuations in the number of reported outbreaks yet a decrease in deaths (Figure 1). The rise and fall in outbreak numbers may have been related to several factors. Public health efforts regarding personal hygiene and water quality standards introduced in the early 1900s likely contributed to reduced drinking water outbreaks,⁵ but there was a steady number of outbreaks from 1920 to the late 1930s.³ Some outbreaks may have persisted as community water systems were often affected by the discharge of wastewater into source waters used for drinking.¹ Fecal contamination of drinking water was associated with 3 outbreaks in Chicago from 1933 to 1934. In 1 year, cross-connections and sewage contamination resulted in 1700 cases of amebic dysentery, 100 cases of typhoid fever, and an estimated 1000 diarrheal infections from secondary transmission.³

There was a marked decrease in outbreak reports in the 1940s, during World War II (Figure 1). A 1948 review documented 327 drinking water outbreaks during 1938 to 1945.¹² More than 40% of those outbreaks were associated with cross connections or backflow that was compounded by failures in filtration or disinfection.¹² Reporting increased after World War II through the 1960s, during which time, source water pollution, contamination of the distribution system, and inadequate or no water treatment was associated with most of the outbreaks.^13,14 One such event took place in Riverside, California, during 1965, in which more than 16 000 people were sickened with salmonellosis after drinking untreated municipal water.²⁰

A steady annual increase in the number of reported outbreaks occurred during the 1970s to 1980s. This period coincided with the formalization of WBDOSS in 1971, a subsequent waterborne disease outbreak–specific reporting form in 1975, and the enactment of SDWA.¹⁵ We hypothesize that the availability of a dedicated national waterborne outbreak surveillance system and waterborne-specific reporting form facilitated public health reporting. Whatever the reason for the increase, inadequate water treatment and distribution system deficiencies were primarily associated with outbreaks during this time.¹⁵ Illness cases nearly doubled after SDWA, but there were two thirds fewer drinking water–associated deaths after SDWA compared with the period before (Figure 1). The larger case count after SDWA may be attributed to the limited ability to track cases before WBDOSS. Another factor was the 1993 cryptosporidiosis outbreak associated with a filtration failure at a water treatment plant that resulted in more than 400 000 illnesses.¹⁹ That outbreak alone accounted for roughly 70% of the 569 356 illness cases reported during 1975 to 2020.

Etiologies

Enteric pathogens were associated with the most drinking water outbreaks for all years when etiology was known, yet there was a large reduction in the number of enteric pathogen outbreaks in community water systems after SDWA (Table 2 and Figure 2). Since SDWA and subsequent drinking water regulations were implemented (Table A), Legionella and other biofilm-associated pathogens, emerging concerns in water systems, were implicated in more outbreaks (n = 343) compared with enteric pathogens (n = 317; Figure 2). While enteric pathogens were linked to the most illnesses, largely driven by the 1993 cryptosporidiosis outbreak, biofilm-associated pathogens were associated with most deaths since SDWA. This number is even more surprising when taking into account that waterborne associated Legionella outbreaks have been reportable only since 2001.¹⁵ Notably, more than 80% of Legionella-associated outbreaks occurred in community water systems, while most enteric pathogen–associated outbreaks occurred in individual or private water systems for all years (Table 2). We also note that a large portion of biofilm pathogen–associated drinking water outbreaks occurred in health care facilities and hotels, motels, lodges, or inns (Figure F).

Legionella, along with nontuberculous mycobacteria and Pseudomonas, can survive in the biofilm of distribution systems and premise plumbing.^18,21 Hospitalizations and deaths from these pathogens cost residents of the United States almost $2.4 billion annually.^8,11 Biofilm pathogens differ from many enteric pathogens as they are naturally occurring in soil and water and do not require distribution intrusion or contamination events to be present in water systems.²¹ Water management best practices, including distribution system maintenance and disinfectant residuals, have been shown to be effective for control of Legionella in plumbing.^21,22 There has been an increase in the proportion of outbreaks associated with premise plumbing deficiencies,¹⁵ and a summary of Legionella outbreaks since 2000 identified at least 1 maintenance deficiency or process failure that led to the event.²³

Water Systems, Sources, and Treatments

Among known water systems, individual or private systems were associated with the most reported outbreaks before SDWA; thereafter, the trend shifted to community water systems (Figure E). While some may interpret this shift as a failure of SDWA, the trend must be interpreted considering the greaterthan-50-fold increase in community water systems during the 20th century. In 1900, there were an estimated 3000 public water systems, and that number grew to 19 000 in the 1960s and to 62 000 by 1980.⁵ By 2004, there were more than 170 000 public water systems in the United States.⁶ Despite this expansion of public drinking water distribution systems, there were fewer than 600 community water system outbreaks (< 4 for every 1000 water systems) during 1974 to 2024 (Figure E). Moreover, these data note “no treatment” as a frequent contributor to outbreaks in individual or private and unknown systems (Figure D).

The analysis does not show a large difference in water sources implicated in outbreaks before and after SDWA (Table 1). Ground water remains the most frequent source associated with outbreaks and well water as the most frequent type for all years.^3,4,13,14 However, the proportion of outbreaks associated with untreated ground water has not increased.¹⁶ Moreover, despite the increase in the use of surface water sources to meet demands, overall outbreaks associated with these sources increased from 24% to just 29% after SDWA (Table 1), but the proportion associated with contaminated untreated surface water has decreased.¹⁶ By 2015, nearly 75% of US drinking water was sourced from surface water.²⁴ Some credit may be attributable to the protections of the Clean Water Act, Surface Water Treatment Rule, and other SDWA amendments (Table A).¹⁶

Most water systems serving fewer than 10 000 people use ground water as their source,²⁵ and approximately 43 million people are served by private wells that do not fall under the SDWA regulations.^26,27 Furthermore, there has been an increase in the annual proportion of outbreaks reported in individual water systems since 1971.¹⁵ State and local regulations for private wells vary and often only address the drilling and testing of new wells.^26,28 This lack of guidance leaves millions of people vulnerable to exposure to pathogens and chemicals in their drinking water. From 1971 to 2008, nearly one third (30%) of waterborne disease outbreaks in the United States were attributed to drinking untreated groundwater, including from private wells.⁷ In a systematic review of waterborne disease outbreaks related to small, noncommunity drinking water systems, most outbreaks that reported a water source were attributed to ground water.²⁹ One study estimated 1.29 million annual cases of acute gastrointestinal illness in the United States may be attributed to untreated private well water consumption.²⁶

Among outbreaks for all years in which treatment type was reported, “no treatment” was the most reported (Table B). The use of filtration and chlorination in early 20th century water distribution systems has been credited with reducing total US mortality by almost half.¹ The 1974 SDWA regulations may have also contributed to the decline as this analysis notes “no treatment” outbreaks decreased after SDWA (Table B). When treatment was reported, chlorine disinfection was most identified in post-SDWA outbreaks.

This does not suggest that chlorination is ineffective but, rather, that some outbreaks may be associated with process failures and drops in disinfectant residuals. Instances like these were noted in a 1971–1998 summary that reported that inadequate treatment or disinfection and distribution system deficiencies were the primary contributors of US drinking water outbreaks.¹⁰ Inadequate disinfection was also highlighted as the primary cause of US drinking water outbreaks during 1971 to 2006.¹⁵ These outbreaks are not surprising because it has been shown that viral intrusions into nondisinfected drinking water distribution systems can cause gastroenteritis.³⁰ Furthermore, biofilm-associated pathogens have exhibited some disinfectant resistance because of the protective nature of the biofilm.^18,21

Water Settings

Among known settings, community; hotel, motel, lodge, or inn; and camp or cabin were the most common settings reported for all years, but we note an alarming trend with the emergence of outbreaks in health care facilities after SDWA (Figure C). The aging complex water systems in health care facilities can act as reservoirs and vectors for pathogens that may infect patients with compromised immune systems.³¹ In a recent summary, health care facilities were identified as the exposure settings for more than half of drinking water outbreaks during 2015 to 2020.⁹ Furthermore, those hospital outbreaks resulted in 66% of hospitalizations and 87% of deaths associated with drinking water outbreaks during that period.⁹ During 2001 to 2006, nearly 70% of Legionella outbreaks associated with drinking water occurred in health care facilities.¹⁵ Moreover, Legionella outbreaks in health care facilities result in more deaths than other outbreaks.^9,23 Health care settings are unique in the ways water is used, and the population is more vulnerable to infection. For these reasons, health care facilities could consider reducing water-associated risks through infection prevention practices and water management programs that address premise plumbing systems in hospital buildings (https://bit.ly/3QZhdg9 and https://bit.ly/4umMurY).

Emerging Issues

Over the last 100 years, drinking water treatment has evolved, as have the challenges. In the early 1900s, treatment was focused on bacterial pathogens. Then, around midcentury, the control of viral pathogens was also addressed, and in the 1980s to 1990s, protozoan pathogens were also targeted.²¹ This summary highlights the emergence of opportunistic biofilm pathogens like Legionella, yet other challenges have appeared as well. Here we briefly highlight challenges that are likely to continue to influence future drinking water outbreaks.

Extreme weather shifts and events.

Extreme weather events and changes in temperature in temperate climates, like in the United States, are likely to change the distribution of pathogen habitats and introduce diseases into new locations.³² The increase in US freshwater temperatures is positively correlated with diarrhea from bacterial pathogens like Campylobacter.³² Warmer water temperatures are also predictors of E coli in surface waters used for drinking water supplies.³³ Besides increasing water temperatures, heavy rainfall is also a driver of waterborne pathogens and diseases via agricultural runoff and system overflow.^32,33 Moreover, studies have shown that distribution system water can reach higher temperatures than previously expected, altering disinfection decay rates.³⁴

Built environment.

Regulation of a water distribution system generally ends at the point of a home’s water meter. This leaves household premise plumbing vulnerable to such threats as growth and protection of opportunistic pathogens in biofilms.¹⁸ Those at higher risk for severe illness include persons aged 65 years or older, young children, pregnant women, and people with weakened immune systems. A 2015–2020 outbreak summary cited the premise or point-of-use site most often as the contributing factor for Legionella and other biofilm-associated pathogen outbreaks in the United States.⁹ Growth of Legionella in premise plumbing contributed to one third of deficiencies reported in other drinking water outbreaks during 1971 to 2006.¹⁵ Legionella was also first reported as the cause of 3 outbreaks in private residences during this time.⁹ Homeowners can take steps to reduce their risk of exposure to these pathogens including flushing faucets and showerheads, properly maintaining water heaters, and not using tap water in aerosolizing devices (https://bit.ly/4n3ddHi).

Aging infrastructure.

Water distribution systems around the country are aging, and some of the drinking water infrastructure has reached the end of its useful life and would cost approximately $625 billion to renovate.²⁵ It is estimated that there are approximately 240 000 water main breaks annually, not including planned repairs (https://bit.ly/4cZr5hg).²⁵ These disruptions leave the distribution systems vulnerable to contamination and result in increased risks of gastrointestinal illness among users.³⁵

Limitations

The findings in this report are subject to several limitations (further described in the Supplemental Limitations section of the Appendix). First, the absence of a formal national waterborne disease surveillance system before 1971 limited documentation of outbreaks. Second, the combining of previous outbreak documentation from multiple sources may have resulted in missing data. Third, the historical line lists and NORS do not include reports of endemic or sporadic waterborne disease, which therefore were not included in our analysis. Fourth, classifications of outbreaks have evolved. Fifth, NORS reporting is voluntary, and reporting capabilities vary by jurisdiction. Despite these limitations, this is the best waterborne outbreak data available, and one of the strengths is its longevity; therefore, we feel the trends can be described with reasonable confidence.

Conclusions

This report summarizes trends in the burden and etiology of reported waterborne disease outbreaks in the United States. Continued surveillance for waterborne disease outbreaks can help identify emerging waterborne disease threats and be used to inform and assess outbreak prevention measures. Public health interventions like surveillance; system-level approaches such as source water protection, filtration, or disinfection; interventions at buildings or homes such as water management programs and routine inspection and maintenance of well water systems; and public health communications to reduce risk for persons more susceptible to disease must remain a priority for the next 100 years.