Surveillance for Waterborne Disease Outbreaks Associated with Drinking Water — United States, 2011–2012

Karlyn D. Beer, PhD1,2; Julia W. Gargano, PhD2; Virginia A. Roberts, MSPH2; Vincent R. Hill, PhD2; Laurel E. Garrison, MPH3; Preeta K. Kutty, MD3; Elizabeth D. Hilborn, DVM4; Timothy J. Wade, PhD4; Kathleen E. Fullerton, MPH2; Jonathan S. Yoder, MPH, MSW2

Advances in water management and sanitation have substantially reduced waterborne disease in the United States, although outbreaks continue to occur (1). Public health agencies in the U.S. states and territories* report information on waterborne disease outbreaks to the CDC Waterborne Disease and Outbreak Surveillance System (http://www.cdc.gov/healthywater/surveillance/index.html). For 2011–2012, 32 drinking water–associated outbreaks were reported, accounting for at least 431 cases of illness, 102 hospitalizations, and 14 deaths. Legionella was responsible for 66% of outbreaks and 26% of illnesses, and viruses and non-Legionella bacteria together accounted for 16% of outbreaks and 53% of illnesses. The two most commonly identified deficiencies† leading to drinking water–associated outbreaks were Legionella in building plumbing§ systems (66%) and untreated groundwater (13%). Continued vigilance by public health, regulatory, and industry professionals to identify and correct deficiencies associated with building plumbing systems and groundwater systems could prevent most reported outbreaks and illnesses associated with drinking water systems.

This report provides information on drinking water–associated¶ waterborne disease outbreaks in which the first illness occurred in 2011 or 2012** (http://www.cdc.gov/healthywater/surveillance/drinking-surveillance-reports.html), and summarizes outbreaks reported to the Waterborne Disease and Outbreak Surveillance System through the electronic National Outbreak Reporting System (http://www.cdc.gov/nors/about.html) as of October 30, 2014. For an event to be defined as a waterborne disease outbreak, two or more persons must be linked epidemiologically by time, location of water exposure, and case illness characteristics; and the epidemiologic evidence must implicate water as the probable source of illness. Data submitted for each outbreak include 1) the number of cases, hospitalizations, and deaths; 2) the etiologic agent (confirmed or suspected); 3) the implicated water system; 4) contributing factors in the outbreak; and 5) the setting of exposure.

Public health officials from 14 states reported 32 outbreaks associated with drinking water during the time period (Table 1) (http://www.cdc.gov/healthywater/surveillance/drinking-water-tables-figures.html). These outbreaks resulted in at least 431 cases, 102 hospitalizations (24% of cases), and 14 deaths. At least one etiologic agent was identified in 30 (94%) outbreaks. Legionella was implicated in 21 (66%) outbreaks, 111 (26%) cases, 91 (89%) hospitalizations, and all 14 deaths. Norovirus was implicated in two single-etiology outbreaks involving 138 cases, with no hospitalizations or deaths. Three outbreaks caused by non-Legionella bacteria resulted in 90 (21%) cases, among which 56 (62%) were caused by Shiga toxin–producing Escherichia coli, 22 (24%) by Shigella sonnei, and 12 (13%) by Pantoea agglomerans (hospital-acquired bloodstream infection). Common exposure settings among drinking water–associated outbreaks were hospitals or health care facilities (n = 16, 50%), hotels (n = four, 13%), and camps/cabins (n = three, 9%). The number and etiological categories of drinking water–associated outbreaks reported every year since 1971 were reviewed for comparison (Figure).

The etiologies, water systems, water sources, predominant illness types, and deficiencies identified for drinking water–associated outbreaks and outbreak-associated cases were ranked in order of frequency (Table 2). Legionella was the most frequently reported outbreak etiology (65.6%), thus acute respiratory illness was the most commonly reported illness type. Outbreaks associated with community water systems†† (78.1%) outnumbered those associated with noncommunity systems and bottled water. Outbreaks associated with water systems that used surface water sources (56.3%) were more frequently reported than outbreaks associated with all other sources. The deficiency that led to most drinking water–associated outbreaks (n = 21, 65.6%) was the presence of Legionella in drinking water systems. The second most common deficiency was untreated groundwater (i.e., groundwater contamination at the source), both alone (n = four, 12.5%) and in combination with untreated surface water (n = one, 3.1%). All five drinking water–associated outbreaks with groundwater deficiencies (including one outbreak with multiple deficiencies) occurred in noncommunity water systems; four occurred in camps or outdoor workplaces and one occurred in a meeting facility. No reported outbreaks occurred in individual water systems (e.g., private wells).

Among 431 cases attributed to drinking water–associated outbreaks, the etiologies, illnesses, water sources and systems, and deficiencies were distributed differently than among the related outbreaks. Viruses caused 32.0% of cases, followed by Legionella (25.8%), and non-Legionella bacteria (20.9%). Over half of cases (51.5%) were linked to noncommunity water systems, and cases linked to groundwater (60.6%) were more frequently reported than all other reported sources. Most cases involved acute gastrointestinal illness (71.5%). Together, deficiencies of untreated groundwater and Legionella in drinking water systems accounted for 72.4% of all outbreak-associated cases.

Data were received concerning two previously unreported outbreaks with onset dates of first illness in 2009 (Table 1). These outbreaks were caused by Legionella pneumophila serogroup 1, and resulted in 14 cases, eight hospitalizations and one death. Data on these two outbreaks are presented (Table 1) (Figure) but are not included in the analysis of outbreaks that occurred in 2011 and 2012.

Discussion

Since the early 20th century, water treatment processes and regulations have greatly reduced the transmission of pathogens through public drinking water supplies in the United States (1). The outbreaks reported during this surveillance period highlight several emerging and persisting public health challenges associated with drinking water systems. First, Legionella is the most frequently reported etiology among drinking water outbreaks; it is typically acquired through inhalation of aerosolized water containing the organism. All 14 outbreak-associated deaths reported were caused by Legionella, including 12 (86%) cases associated with health care facilities. Therefore, improved Legionella control and mitigation are needed, especially in health care settings. Second, chlorine-sensitive, gastrointestinal pathogens (norovirus, non-Legionella bacteria, Giardia§§) accounted for more than half of drinking water outbreak-associated cases, even though they only caused eight outbreaks. The comparatively high morbidity that accompanied these outbreaks highlights the importance of source water monitoring, adequate initial disinfection, and maintaining sufficient levels of disinfectant throughout a system at all times when indicated by the results of monitoring and risk analyses (2). Finally, the increase in cases that accompanied drinking water–associated outbreaks in noncommunity water systems,¶¶ from 15% in 2009–2010 to 52% in 2011–2012, indicates that additional efforts are needed to prevent outbreaks associated with these small-scale, typically intermittently used systems; full implementation of the Environmental Protection Agency (EPA) Ground Water Rule and Revised Total Coliform Rule,*** might mitigate vulnerabilities in these systems in the future (2,3).

Although the total number of drinking water–associated outbreaks has remained nearly constant (36 in 2007–2008, 35 in 2009–2010, and 32 in 2011–2012), Legionella has caused increasing proportions of drinking water–associated outbreaks (33%, 60%, and 66% during each of these time periods, respectively) (4,5). This pattern has been driven by the increasing proportion of Legionella outbreaks among those in community water systems (60%, 76%, and 84% during each of these time periods, respectively) (4,5). In 2011–2012, among 21 Legionella outbreaks in community water systems, 14 (67%) occurred in hospitals or health care facilities, illustrating the disproportionate disease burden among hospitalized persons, who are more likely to be older or have underlying conditions that increase their risk of developing Legionnaire's disease (6). Legionella outbreaks are particularly challenging to prevent and control, in part because the organism lives and multiplies in building plumbing systems, which usually fall outside water utility and regulatory oversight (6,7). One Legionella outbreak occurred in a hotel that used point-of-entry water filters, which effectively dechlorinated all water entering the building, and illustrates the importance of maintaining sufficient residual disinfectant in plumbing systems.

The five drinking water–associated outbreaks and 222 outbreak-associated cases from noncommunity water systems reported for 2011–2012 represented an increase since 2009–2010, illustrating two additional public health challenges beyond Legionella. First, the etiologies in these outbreaks were varied but were predominantly norovirus, non-Legionella bacteria and Giardia. Moreover, the majority of cases caused by these pathogens occurred during the five outbreaks associated with noncommunity systems. Second, all five noncommunity outbreaks originated from groundwater sources. Specifically, four occurred in outdoor camp or work settings where a source spring was contaminated directly or by inflow from a stream, and the fifth occurred at a meeting facility where a well was contaminated with septic tank overflow. Because these outbreaks share common settings, water system types, and chlorine-sensitive pathogens, a large potential reduction in gastrointestinal illnesses is possible when noncommunity groundwater systems are properly maintained and operated to reduce or inactivate microbial contamination. In addition, these outbreaks underscore the importance of protecting groundwater sources from fecal contamination. Groundwater source protection will be enhanced by improved awareness of and full compliance with protective regulations, such as EPA's Ground Water Rule and Revised Total Coliforms Rule (2,3). However, EPA lacks authority to regulate private wells or onsite wastewater systems (i.e., septic systems) not connected to public water or wastewater systems. Septic systems are used in 20% of U.S. homes, and each year 10%–20% of septic systems malfunction (8). Improper design, maintenance, or location of private wells and septic systems contributed to 67% of reported outbreaks from groundwater contamination from 1971–2008 (9), but these outbreaks can be avoided with proper design and regular service and maintenance as recommended by EPA (8).

The findings in this report are subject to at least two limitations. First, the detection and investigation of outbreaks might be incomplete, for several reasons. Linking illness to drinking water is inherently difficult through outbreak investigation methods (e.g., case-control and cohort studies) because most persons have daily exposure to tap water (10). The capacity to conduct environmental investigations that can provide information on water system deficiencies contributing to outbreaks, and strengthen evidence implicating drinking water as a common source of infection, might vary by state and locality. Second, the level of surveillance and reporting activity, as well as reporting requirements, vary across states and localities. For these reasons, outbreak surveillance data underestimate actual values, and should not be used to estimate the total number of outbreaks or cases of waterborne disease.

Compared with the previous 2-year reporting period (2009–2010), the proportion of outbreaks with deficiencies in the federally regulated portions of public water systems (i.e., up to the water meter or property line) during 2011–2012 has declined from 46% to 20%. Nonetheless, challenges with noncommunity water systems are ongoing, and efforts to prevent illnesses associated with untreated groundwater are needed. Furthermore, deficiencies at non-federally (i.e., not under jurisdiction of water utilities or EPA) regulated points, such as private wells and building plumbing systems, are also increasingly reported to cause illness, especially legionellosis. Of additional concern is the likelihood that, as older age is a risk factor for Legionnaire's disease (6), an aging U.S. population will result in an increased proportion of individuals at higher risk. Expanded partnerships between public health, regulatory, and industry professionals to develop and use both regulatory and nonregulatory approaches to identify and address groundwater and building plumbing system deficiencies could prevent most reported outbreaks associated with drinking water systems.

Acknowledgments

State, territorial, local, and freely associated state waterborne disease coordinators, epidemiologists, and environmental health personnel; Hannah Reses, Michael J. Beach, PhD, Division of Foodborne, Waterborne, and Environmental Diseases, National Center for Emerging and Zoonotic Infectious Diseases, CDC.

1Epidemic Intelligence Service, CDC; 2Division of Foodborne, Waterborne, and Environmental Diseases, National Center for Emerging and Zoonotic Infectious Diseases, CDC; 3Division of Bacterial Diseases, National Center for Immunization and Respiratory Diseases, CDC; 4U.S. Environmental Protection Agency.

Corresponding author: Karlyn Beer, kbeer@cdc.gov, 404-718-1151.

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