Waterborne Disease Outbreaks Associated With Environmental and Undetermined Exposures to Water — United States, 2013–2014

Environmental Exposure to Water

Fifteen outbreaks associated with environmental exposures to water were reported from 10 states during the reporting period (Table 1). A total of 226 cases were identified in association with these outbreaks, with 69 hospitalizations and nine deaths reported. An etiologic agent was confirmed in 14 of 15 outbreaks. Giardia duodenalis was determined to be the etiology of seven outbreaks, and Legionella pneumophila was implicated in six. No hospitalizations or deaths were reported in association with outbreaks of giardiasis, whereas legionellosis outbreaks accounted for 90% (62 of 69) of hospitalizations and all nine deaths. The remaining seven (10%) hospitalizations were associated with an outbreak involving skin infections caused by Pseudomonas aeruginosa. The majority of the reported outbreaks involved either acute respiratory illness (six of 15, caused by Legionella) or acute gastrointestinal illness (eight of 15 [seven Giardia, one unknown]). Giardia was the etiology of six of seven outbreaks linked to a natural water system, whereas Legionella caused six of eight outbreaks linked to a human-made water system (Table 2).

Undetermined Exposure to Water

Twelve outbreaks associated with an undetermined exposure to water were reported from eight states (Table 1), involving 63 cases, 39 hospitalizations and eight deaths. Outbreaks of acute respiratory illness caused by Legionella accounted for 11 (92%) of these outbreaks, along with all reported hospitalizations and deaths. One outbreak of acute gastrointestinal illness caused by Cryptosporidium was also reported. In addition, one outbreak caused by Legionella with first illness onset in 2009 was reported during this reporting period, resulting in nine reported cases and six hospitalizations. Data from this outbreak are presented (Table 1) but not included in the analysis and discussion of outbreaks for 2013–2014.

Discussion

This summary of waterborne disease outbreaks associated with environmental or undetermined exposure to water features a range of etiologic agents and illustrates that human illness can result from interaction with contaminated water in numerous settings.

Similar numbers of waterborne disease outbreaks were reported in association with environmental exposures to water linked with human-made (eight outbreaks) and with natural water systems (seven). Six of the outbreaks associated with environmental exposure to water from human-made water systems were caused by Legionella. Water management programs can effectively control the growth and spread of Legionella in these water systems and are an important tool in outbreak prevention (6,7). All but one of the outbreaks associated with natural water systems were caused by Giardia and involved ingestion of water from a river, stream, or spring. To prevent illnesses and outbreaks in backcountry settings, it is important to appropriately treat water obtained from the natural environment before consuming it. (https://www.cdc.gov/healthywater/drinking/travel/backcountry_water_treatment.html).

Consistent with what has been reported previously (5), (11 of 12) outbreaks associated with an undetermined exposure to water in this report were caused by Legionella. These outbreaks could not be definitively linked to a single water exposure because they were associated with multiple suspected or confirmed water types (e.g., both spa and drinking water systems were implicated) or because insufficient epidemiologic, laboratory, or environmental data were available to identify a single exposure. Investigations of legionellosis outbreaks exemplify the challenges of determining a single water exposure associated with illness, as Legionella can colonize environmental, recreational, and drinking water systems, creating multiple opportunities for susceptible persons to be exposed to contaminated aerosols. All of the legionellosis outbreaks in this report were associated with human-made water systems. Although water management programs are broadly effective for reducing Legionella in a majority of these systems, identification of a specific exposure can help identify system control deficiencies and inform timely and targeted remediation to prevent future illness. Furthermore, these challenges underscore the importance of strong partnerships among epidemiology, laboratory, and environmental health practitioners in support of these complex investigations. In this report, Legionella was responsible for all nine outbreaks linked to institutional settings and was responsible for all 17 reported deaths and 94% of all reported hospitalizations. This corresponds with surveillance data from drinking water–associated outbreaks (8) and underscores the importance of water management programs for maintaining water quality, preventing illness, and saving lives.

The findings in this report are subject to at least one limitation. State and local health departments have varying capacities to detect, investigate, or report these outbreaks. As a result, these data likely underestimate the actual incidence of outbreaks and do not provide an appropriate estimate of the total number of cases or outbreaks for a specific time or location. Despite this limitation, more states reported outbreaks associated with environmental or undetermined exposures to water in this period than did in previous years (27 versus 18 reported for 2011 to 2012 [1]).

Outbreak surveillance data play a critical role in waterborne disease prevention for a broad range of pathogens across numerous water types. Legionella continues to challenge public health efforts to investigate and prevent outbreaks in human-made water systems and remains a key target for decreasing waterborne disease morbidity and mortality. Giardia remains a health risk for persons consuming water from natural water systems and should continue to be an area of focus for health education and prevention activities in these settings. Strong partnerships among epidemiology, laboratory, and environmental health practitioners at all levels are essential for effectively investigating and preventing waterborne disease outbreaks and protecting public health.