Surveillance for Waterborne Disease Outbreaks Associated With Drinking Water — United States, 2013–2014

Katharine M. Benedict, DVM, PhD; Hannah Reses, MPH; Marissa Vigar, MPH; David M. Roth, MSPH; Virginia A. Roberts, MSPH; Mia Mattioli, PhD; Laura A. Cooley, MD; Elizabeth D. Hilborn, DVM; Timothy J. Wade, PhD; Kathleen E. Fullerton, MPH; Jonathan S. Yoder, MPH, MSW; Vincent R. Hill, PhD


Morbidity and Mortality Weekly Report. 2017;66(44):1216-1221. 

In This Article


Water treatment processes, regulations, and rapid response to illness outbreaks continue to reduce the transmission of pathogens, reduce exposure to chemicals and toxins, and protect the public drinking water supplies in the United States. Outbreaks reported during this surveillance period include the first reports of drinking water–associated outbreaks caused by harmful algal blooms as well as the continued challenges of preventing and controlling illnesses and outbreaks caused by Legionella and Cryptosporidium. Outbreaks in community water systems caused by chemical spills (West Virginia),[2] harmful algal blooms (Ohio), Cryptosporidium (Oregon),[3] and Legionella (Michigan) demonstrated that diverse contaminants can cause interruptions in water service, illnesses, and persistent community concern about drinking water quality. Outbreaks in community water systems can trigger large and complex public health responses because of their potential for causing communitywide illness and decreasing the availability of safe water for community members, businesses, and critical services (e.g., hospitals). These outbreaks highlight the importance of public health and water utility preparedness for emergencies related to contamination from pathogens, chemicals, and toxins.

Legionella continues to be the most frequently reported etiology among drinking water–associated outbreaks.[4] All of the outbreak-associated deaths reported during this surveillance period as well as all of the outbreaks reported in hospital/health care settings or long-term care facilities, were caused by Legionella. A review of 27 Legionnaires' disease outbreak investigations in which CDC participated during 2000–2014 identified at least one water system maintenance deficiency in all 23 investigations for which this information was available, indicating that effective water management programs in buildings at increased risk for Legionella growth and transmission (e.g., those with more than 10 stories or that house susceptible populations) can reduce the risk for Legionnaires' disease.[5,6] Although Legionella was detected in drinking water, multiple routes of transmission beyond ingestion of contaminated water more likely contributed to these outbreaks, such as aerosolization from domestic or environmental sources. Cryptosporidium was the second most common cause of both outbreaks and illnesses, demonstrating the continued threat from this chlorine-tolerant pathogen when drinking water supplies are contaminated. Existing drinking water regulations and filtration systems targeted to control Cryptosporidium help protect public health in community water systems that are primarily served by surface water sources or groundwater sources under the influence of surface water.[7] Through the Epidemiology and Laboratory Capacity for Infectious Diseases (ELC) Cooperative Agreement, CDC has recently begun a laboratory-based cryptosporidiosis surveillance system in the United States, CryptoNet, to better track Cryptosporidium transmission and rapidly identify outbreak sources through molecular typing.[8] The cyanobacterial toxin microcystin caused the largest reported toxin contamination of community drinking water in August 2013 and September 2014 and was responsible for extensive community and water disruptions. In June 2015, the Environmental Protection Agency released specific health advisory guidance for microcystin concentrations in drinking water.[9] The contamination of a community drinking water supply with 4-metholcyclohexanementanol (MCHM) also illustrates the importance of source water protection from chemicals and toxins.[2]

The findings in this report are subject to at least three limitations. First, 17% of drinking water–associated outbreak reports could not be assigned a specific deficiency classification other than "unknown or insufficient information," because of a lack of information. Furthermore, the deficiency classification most frequently reported ("presence of Legionella in drinking water systems") does not provide insight into the specific factors contributing to Legionella amplification and transmission. Second, the detection and investigation of outbreaks might be incomplete. Because of universal exposure to water, linking illness to drinking water is inherently difficult through traditional outbreak investigation methods (e.g., case-control and cohort studies).[10] Finally, reporting capabilities and requirements vary among states and localities. Therefore, outbreak surveillance data likely underestimate actual occurrence of outbreaks and should not be used to estimate the actual number of outbreaks or cases of waterborne disease.

Public health surveillance is necessary to detect waterborne disease and outbreaks, and to continue to monitor health trends associated with drinking water exposure. Despite resource constraints, 19 states reported drinking water–associated outbreaks for 2013–2014 compared with 14 for the previous reporting period.[4] In this reporting cycle, more reported outbreaks and cases were caused by parasites and chemicals than by non-Legionella bacteria, and more cases were reported from community systems than from individual systems. Most of the outbreaks and illnesses reported in this period were in community systems, which serve larger numbers of persons; outbreaks in these systems can sicken entire communities. Although individual, private water systems likely serve fewer persons than community systems, they can still result in relatively large numbers of illnesses. One outbreak reported during 2013–2014 in an individual system led to 100 estimated illnesses associated with a wedding. The public health challenges highlighted here underscore the need for rapid detection, identification of the cause, and response when drinking water is contaminated by infectious pathogens, chemicals, or toxins to prevent and control waterborne illness and outbreaks.