Surveillance of Waterborne Disease Outbreaks Associated with Drinking Water — United States, 2015–2020

Results

All Outbreaks

Public health officials from 28 states reported 214 outbreaks associated with drinking water during the surveillance period (Tables 1, 2, 3, 4, 5, and 6) (Figure 1). Reported outbreaks included 187 biofilm-associated, 24 enteric illness-associated, and three other (two unknown and one chemical or toxin) etiologies (Table 7) (Figure 2). Outbreaks resulted in at least 2,140 cases of illness, 563 hospitalizations (26% of cases), and 88 deaths (4% of cases). At least one etiologic agent was identified in 212 (99%) outbreaks (Tables 1, 2, 3, 4, 5, and 6).

Water Systems, Sources, and Contributing Factors

Community or noncommunity water systems (i.e., public) were linked with 172 (80%) outbreaks, 22 (10%) outbreaks with unknown water systems, 17 (8%) with individual or private systems (i.e., unregulated), and two (0.9%) with other systems; one system type (0.5%) was not reported. Water from individual or private water systems was implicated in 944 (44%) cases, 52 (9%) hospitalizations, and 14 (16%) deaths (Tables 1–6). Drinking water systems with groundwater sources accounted for 82 (38%) outbreaks, surface water sources accounted for 57 (27%) outbreaks, and unknown water sources accounted for 61 (29%) outbreaks (Table 7). A total of 454 contributing factors (practices and factors that led to the outbreak) were reported for 144 (67%) outbreaks (Tables 8 and 9). A total of 393 contributing factors were reported for biofilm-associated outbreaks and 61 for enteric illness-associated outbreaks.

Enteric Illness-Associated Etiologies

Outbreaks of enteric illness included 24 (11%) reports implicating Campylobacter (n = 2; 1%), Cryptosporidium (n = 2; 1%), Escherichia coli (n = 1; 0.5%), Giardia (n = 3; 1%), norovirus (n = 7; 3%), Shigella (n = 4; 2%), and multiple etiologies (n = 5; 2%) (Tables 1, 2, 3, 4, 5, and 6). The enteric illness outbreaks resulted in 1,299 (61%) cases, 10 (2%) hospitalizations, and no deaths. Seventeen outbreaks were linked to norovirus, Shigella, Campylobacter, or multiple etiology outbreaks including these three pathogens and were implicated in 1,225 (57%) cases.

Water System and Water Source

The largest number of cases reported for a single outbreak was 693 (32%). This outbreak was linked to water from an individual or private water system that was contaminated with norovirus and enteropathogenic E. coli (Table 4). When water source (e.g., groundwater) was known (n = 14), wells were identified in 13 (93%) of enteric illness outbreaks, regardless of water system (Tables 1, 2, 3, 4, 5, and 6).

Contributing Factors

A total of 61 (13%) contributing factors were reported for enteric illness outbreaks (Table 9). Water source was the most cited contributing factor type for enteric illness outbreaks, described by 34 (56%) individual contributing factors. Contamination through limestone or fissured rock (e.g., karst) (n = 7; 11%), improper construction or location of a well or spring (n = 7; 11%), and flooding or heavy rains (n = 5; 8%) were the most reported source water contributing factors for enteric illness outbreaks. No disinfection (n = 9, 15%), no filtration (n = 5, 8%), and chronically inadequate disinfection (n = 2; 3%) were the most frequently reported treatment contributing factors for enteric disease outbreaks (Table 9).

Biofilm-Associated Etiologies

Biofilm-associated outbreaks comprised 184 Legionella (86%), two NTM (1%), and one Pseudomonas (0.5%) outbreaks (Table 7). Legionella-associated outbreaks generally increased in number over the study period (14 in 2015, 31 in 2016, 30 in 2017, 34 in 2018, 33 in 2019, and 18 in 2020). The Legionella outbreaks resulted in 786 (37%) cases (Table 7), 544 (97%) hospitalizations, and 86 (98%) deaths (Tables 1, 2, 3, 4, 5, and 6).

Water System

Legionella was the most implicated etiology in public water system outbreaks, associated with 160 (92%) outbreaks, 666 (60%) cases, 462 (97%) hospitalizations, and 68 (97%) deaths related to community and noncommunity water systems (Tables 1, 2, 3, 4, 5, and 6). Individual or private water system outbreaks associated with Legionella resulted in 71 (8%) cases, 48 (92%) hospitalizations, and 14 (100%) deaths.

Contributing Factors

A total of 393 (87%) contributing factors were reported for Legionella and other biofilm pathogen-associated outbreaks (Table 8). Premise or point of use was the most cited contributing factor type for all biofilm-associated pathogen outbreaks and was linked with 287 (73%) individual contributing factors. The most reported premise or point of use contributing factors were Legionella species in water system (n = 67; 17%), Legionella growth-promoting water temperatures and permissive chlorine levels within the building potable water system (n = 35; 9%), water temperature ≥86°F (≥30°C) (n = 28; 7%), and aging plumbing components (e.g., pipes, tanks, and valves) (n = 27; 7%) (Table 8).

Water Treatment and Water Treatment Methods

A total of 183 (86%) outbreak reports contained information about water treatment. Among all outbreaks, disinfection was the reported water treatment for 116 (54%) drinking water systems, unknown water treatment for 49 (23%) drinking water systems, and no water treatment for 17 (8%) drinking water systems (Tables 1, 2, 3, 4, 5, and 6). Seventy-nine outbreak reports (37%) indicated that chlorine was the water treatment method (e.g., description), 99 (46%) reported unknown or no treatment description, and 12 (6%) reported chloramine as the treatment description.

Settings

Hospital or health care facility, long-term care facility, and assisted living or rehabilitation facility (i.e., health care) were identified as the exposure settings in 113 (53%) outbreaks, 456 (21%) cases, 372 (66%) hospitalizations, and 75 (87%) deaths (Figure 3). Furthermore, in the health care facility setting, Legionella was implicated in 111 (52%) outbreaks, 444 (21%) cases, 364 (65%) hospitalizations, and 73 (85%) deaths. Hotels, motels, lodges, or inns were implicated in 35 (16%) outbreaks, 225 (11%) cases, 85 (15%) hospitalizations, and three (3%) deaths, all of which were caused by Legionella (Tables 1, 2, 3, 4, 5, and 6). Finally, Legionella was reported to NORS in 2015 and 2019 as the cause of three outbreaks in private residences resulting in seven (0.3%) cases, four (0.7%) hospitalizations, and no deaths (Tables 1 and 4) (Figure 3) (2).