Waterborne Disease Outbreaks Associated with Drinking Water

On November 10, 2017, the CDC published a report that found that, from 2013 to 2014, 42 drinking water–associated outbreaks were reported, accounting for at least 1,006 cases of illness, 124 hospitalizations, and 13 deaths. Legionella was associated with 57 percent of these outbreaks and all of the deaths. Sixty-nine percent of the reported illnesses occurred in four outbreaks in which the etiology was determined to be either a chemical or toxin or the parasite Cryptosporidium.


Drinking water contamination events can cause disruptions in water service, large impacts on public health, and persistent community concern about drinking water quality. Effective water treatment and regulations can protect public drinking water supplies in the United States, and rapid detection, identification of the cause, and response to illness reports can reduce the transmission of infectious pathogens and harmful chemicals and toxins.


Public health officials from 19 states reported 42 outbreaks associated with drinking water during the surveillance period. These outbreaks resulted in at least 1,006 cases of illness, 124 hospitalizations (12 percent of cases), and 13 deaths. At least one etiologic agent was identified in 41 (98 percent) outbreaks. Legionella was implicated in 24 (57 percent) outbreaks, 130 (13 percent) cases, 109 (88 percent) hospitalizations, and all 13 deaths. Eight outbreaks caused by two parasites resulted in 289 (29 percent) cases, among which 279 (97 percent) were caused by Cryptosporidium, and 10 (3 percent) were caused by Giardia duodenalis. Chemicals or toxins were implicated in four outbreaks involving 499 cases, with 13 hospitalizations, including the first reported outbreaks associated with algal toxins in drinking water.


The most commonly reported outbreak etiology was Legionella (57 percent), making acute respiratory illness the most common predominant illness type reported in outbreaks. Thirty-five (83 percent) outbreaks were associated with public (i.e., regulated), community, or noncommunity water systems, and three (7 percent) were associated with unregulated, individual systems. Fourteen outbreaks occurred in drinking water systems with groundwater sources and an additional 14 occurred in drinking water systems with surface water sources. The most commonly cited deficiency, which led to 24 (57 percent) of the 42 drinking water–associated outbreaks, was the presence of Legionella in drinking water systems. In addition, 143 (14 percent) cases were associated with seven (17 percent) outbreak reports that had a deficiency classification indicating “unknown or insufficient information.”


Among 1,006 cases attributed to drinking water–associated outbreaks, 50 percent of the reported cases were associated with chemical or toxin exposure, 29 percent were caused by parasitic infection (either Cryptosporidium or Giardia), and 13 percent were caused by Legionella infection. Seventy-five percent of cases were linked to community water systems. Outbreaks in water systems supplied solely by surface water accounted for most cases (79 percent). Of the 1,006 cases, 86 percent originated from outbreaks in which the predominant illness was acute gastrointestinal illness. Three (7 percent) outbreaks in which treatment was not expected to remove the contaminant were associated with a chemical or toxin and resulted in 48 percent of all outbreak-associated cases.


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 continued to be the most frequently reported etiology among drinking water–associated outbreaks. 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 the CDC participated between 2000 and 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. 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.


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.


The contamination of a community drinking water supply with 4-metholcyclohexanementanol (MCHM) also illustrates the importance of source water protection from chemicals and toxins.


The findings in this report are subject to at least three limitations. First, 17 percent 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). 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 between 2013 and 2014 compared with 14 for the previous reporting period. 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 the 2013 to 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 to prevent and control waterborne illness and outbreaks when drinking water is contaminated by infectious pathogens, chemicals, or toxins.


See the CDC Report


See also Medical Law Perspectives Report: Foodborne Illness: When Grabbing a Bite Can Be Deadly