2021 Waterborne Disease Outbreaks Annual Surveillance Report

Drinking Water Exposures

Drinking water exposure was associated with 29% (15/52) of reported waterborne disease outbreaks, which were associated with 42% (214/511) of cases, 54% (56/104) of hospitalizations, and seven deaths (Table 1). Drinking water–associated outbreaks most frequently started in August, followed by July and April (Figure 5). None of the reported outbreaks started in January or September–December.

Among the drinking water–associated outbreaks, 67% (10/15) of outbreaks were confirmed or suspected to be caused by Legionella (Table 10). Another 20% (3/15) of outbreaks were confirmed or suspected to be caused by Campylobacter. Of the remaining two outbreaks, one was caused by Pseudomonas and the other was suspected to be caused by multiple etiologies, including Campylobacter and norovirus. The latter resulted in 43% (91/214) of cases. Outbreaks confirmed or suspected to be caused by Legionella were associated with 18% (39/214) of cases but 63% (35/56) of hospitalizations and 43% (3/7) of deaths. The outbreak caused by Pseudomonas was associated with the other 57% (4) of deaths.

By definition, a public water system is a system that supplies water to the same population year-round; the categories of public water systems, community and non-community (which includes transient and non-transient), are mutually exclusive (2). Among drinking water–associated outbreaks, about 60% (9/15) were associated with community water systems (Table 11). Individual/private water systems (e.g., private wells) were associated with two outbreaks (13%, 2/15).

An unknown water source was associated with 40% (6/15) of drinking water–associated outbreaks, resulting in 50% (106/214) of cases, while surface water was reported for 27% (4/15) of outbreaks, resulting in 15% (33/214) of cases. Ground water was reported for 20% (3/15) of outbreaks, resulting in 32% (68/214) of cases (Table 12).

The most frequently reported setting for drinking water–associated outbreaks was hospital/healthcare facility (47%, 7/15). One outbreak associated with a camp/cabin setting resulted in 43% (91/214) of cases and another outbreak associated with a community/municipality resulted in 30% (64/214) of cases (Table 13).

Most outbreaks were caused by Legionella (10/15) and occurred in a hospital/healthcare facility (Table 14). One Campylobacter outbreak was responsible for 30% (64/214) of cases and occurred in a community/municipality setting using ground water supplied by a well that was reported to be chlorinated for about two weeks twice per year as part of routine maintenance. Another outbreak suspected to be caused by Campylobacter and norovirus resulted in 43% (91/214) of cases and occurred in a camp/cabin setting with an unknown water system.

Other Exposures to Water

Non-drinking, non-recreational exposure to water, referred to as “other exposure to water” (e.g., industrial water exposure, flood water), was associated with one (2%) of the reported waterborne disease outbreaks (Table 1). The outbreak was associated with less than 1% (2/511) of cases, 2% (2/104) of hospitalizations, and no deaths. This outbreak was also associated with an ornamental fountain in a hotel/motel setting (Table 15).

Unknown Exposures to Water

Unknown water exposure was associated with 8% (4/52) of reported waterborne disease outbreaks (Table 1). The four outbreaks were associated with 2% (11/511) of cases, 9% (9/104) of hospitalizations, and two deaths. Legionella was confirmed to cause 75% (3) of the outbreaks, two of which were associated with a hospital/healthcare facility (Table 16).

Limitations

The findings in this summary are subject to at least three limitations. First, only a proportion of waterborne outbreaks are detected, investigated, and reported to the National Outbreak Reporting System (NORS). The counts here are an underestimate of the true burden of waterborne disease outbreaks in the United States and should not be used to estimate the actual number of outbreaks or cases of waterborne disease. Second, these numbers are largely dependent on public health capacity and reporting requirements, which vary across jurisdictions, and therefore do not necessarily indicate the actual occurrence in each jurisdiction. Furthermore, the COVID-19 pandemic likely contributed to changes in reporting from states due to limited resources and other factors. Third, data on outbreaks with a chemical/toxin etiology might be limited because of differences in how these outbreaks are detected and investigated compared with infectious disease outbreaks or characteristics of the contaminants (e.g., persistence in the environment).

Conclusions

Public health surveillance is key to understanding the epidemiology of waterborne disease and outbreaks. Legionella was the most frequent outbreak etiology of reported waterborne disease outbreaks for 2021. Of the 26 Legionella outbreaks, 35% (9) were associated with a hospital/healthcare facility (seven outbreaks associated with drinking water exposure and two with unknown water exposure) and 23% (6) were associated with a hotel/motel hot tub. In contrast, only three reported outbreaks were caused by Cryptosporidium, the historic leading etiology of reported waterborne disease outbreaks in past years. The findings could be due to a change in reporting by jurisdictions during the COVID-19 pandemic. Jurisdictions might have prioritized detecting, investigating, and reporting outbreaks of diseases (e.g., Legionnaires’ disease) of increased severity (e.g., associated with hospitalization and death) because of resources being diverted to the pandemic response. Additionally, the pandemic might have led to conditions that promoted replication and transmission of Legionella (e.g., water quality not being maintained in hot tubs during low to no hotel/motel occupancy). For 2021, eight outbreaks were confirmed or suspected to be associated with exposures to cyanobacterial species or toxins. This too could be pandemic-related and reflect the public choosing to participate in outdoor activities, such as using untreated recreational water venues. Alternatively, reporting of outbreaks of illness caused by Legionella or by harmful algal blooms could reflect increased occurrence or CDC support for jurisdictions’ surveillance activities for the respective illnesses.

Over the past 100 years, filtration and disinfection of water have reduced (though not eliminated) the burden of waterborne disease outbreaks in the United States caused by disinfectant-susceptible pathogens. Much of the remaining burden is driven by pathogens that are resistant to disinfection (e.g., Cryptosporidium, whose oocysts are chlorine tolerant due to their hard outer shell) or challenging to control due to their growth in biofilm (e.g., Legionella, nontuberculous mycobacteria, and Pseudomonas). Effective prevention strategies beyond traditional filtration and disinfection of water include developing and implementing water management programs at the state or local level by building owners and managers and implementing treated recreational water venue guidelines identified in CDC’s Model Aquatic Health Code.

Acknowledgements

The authors thank local, state, territorial, and freely associated state public health officials and CDC staff for collecting and submitting outbreak data that made this summary possible.

References

1. Collier SA, Deng L, Adam EA, et al. Estimate of Burden and Direct Healthcare Cost of Infectious Waterborne Disease in the United States. Emerging Infectious Diseases. 2021;27(1):140-149. doi:10.3201/eid2701.190676.
2. Information about Public Water Systems. 2021 11/10/2021 [cited 2021 11/22]; Available from: https://www.epa.gov/dwreginfo/information-about-public-water-systems.

Suggested Citation

Lawinger H, Hlavsa M, Miko S, Kunz J, Thuneibat M, Gerdes, M, Gleason M, and Roberts, V. Waterborne Disease and Outbreak Surveillance System (WBDOSS) Summary Report, United States, 2021. Atlanta, Georgia: U.S. Department of Health and Human Services, CDC, 2023.