viernes, 6 de septiembre de 2013

Surveillance for Waterborne Disease Outbreaks Associated with Drinking Water and Other Nonrecreational Water — United States, 2009–2010

Surveillance for Waterborne Disease Outbreaks Associated with Drinking Water and Other Nonrecreational Water — United States, 2009–2010

HHS, CDC and MMWR Logos
MMWR Weekly
Volume 62, No. 35
September 6, 2013

Surveillance for Waterborne Disease Outbreaks Associated with Drinking Water and Other Nonrecreational Water — United States, 2009–2010


September 6, 2013 / 62(35);714-720

Despite advances in water management and sanitation, waterborne disease outbreaks continue to occur in the United States. CDC collects data on waterborne disease outbreaks submitted from all states and territories* through the Waterborne Disease and Outbreak Surveillance System. During 2009–2010, the most recent years for which finalized data are available, 33 drinking water–associated outbreaks were reported, comprising 1,040 cases of illness, 85 hospitalizations, and nine deaths. Legionella accounted for 58% of outbreaks and 7% of illnesses, and Campylobacter accounted for 12% of outbreaks and 78% of illnesses. The most commonly identified outbreak deficiencies§ in drinking water-associated outbreaks were Legionella in plumbing systems (57.6%), untreated ground water (24.2%), and distribution system deficiencies (12.1%), suggesting that efforts to identify and correct these deficiencies could prevent many outbreaks and illnesses associated with drinking water. In addition to the drinking water outbreaks, 12 outbreaks associated with other nonrecreational water** were reported, comprising 234 cases of illness, 51 hospitalizations, and six deaths. Legionella accounted for 58% of these outbreaks, 42% of illnesses, 96% of hospitalizations, and all deaths. Public health, regulatory, and industry professionals can use this information to target prevention efforts against pathogens, infrastructure problems, and water sources associated with waterborne disease outbreaks.
This report includes drinking water–associated outbreaks and other, nonrecreational waterborne disease outbreaks, in which the first illness occurred in 2009 or 2010. Outbreaks were reported to the Waterborne Disease and Outbreak Surveillance System through the electronic National Outbreak Reporting System†† as of October 3, 2012. Two criteria must be met for an event to be defined as a waterborne disease outbreak: 1) two or more persons must be linked epidemiologically by time, location of water exposure, and illness characteristics; and 2) the epidemiologic evidence must implicate water as the probable source of illness. Data requested for each outbreak include 1) the number of illnesses, hospitalizations, and deaths; 2) the etiologic agent (confirmed or suspected); 3) the implicated water system; 4) deficiencies contributing to the outbreak; and 5) the setting of exposure.
During 2009–2019, public health officials from 17 states reported 33 drinking water outbreaks (Table 1). The outbreaks resulted in 1,040 illnesses, 85 hospitalizations (8.2% of cases), and nine deaths. At least one etiologic agent was identified in all but one drinking water outbreak; Legionella was implicated in 19 outbreaks, 72 illnesses, 58 hospitalizations, and eight deaths, and Campylobacter was implicated in four single-etiology outbreaks involving 812 illnesses, 17 hospitalizations, and no deaths, as well as two multiple-etiology outbreaks resulting in 17 illnesses. The number and etiologies of drinking water outbreaks reported every year since 1971 were considered for comparison (Figure).
The etiologies, water systems, water sources, illnesses, and deficiencies identified for drinking water outbreaks and outbreak-associated cases were ranked in order of frequency (Table 2). Legionella caused the majority of outbreaks (57.6%); whereas non-Legionella bacteria caused the majority of illnesses (81.8%). The majority of outbreaks (75.8%) and outbreak-associated illnesses (79.4%) were linked to community water systems.§§ The majority of outbreaks (51.5%) and most illnesses (97.3%) occurred in systems that used ground water sources. The majority of outbreaks (57.6%) involved acute respiratory illness, whereas most outbreak-associated illnesses were acute gastrointestinal illness (92.6%). By deficiency categories, Legionella spp. in plumbing systems was present in the majority of outbreaks (19 [57.6%]); in three Legionella outbreaks, additional deficiencies in building-specific water treatment or plumbing systems were noted. Untreated ground water deficiency (i.e., contamination of ground water at the source) was identified in eight (24.2%) outbreaks, distribution system deficiency alone was identified in four (12.1%) outbreaks, and both deficiencies were identified in one outbreak (3.0%). Together, distribution system and untreated ground water deficiencies accounted for 965 (92.8%) of all outbreak-associated illnesses. All five outbreaks assigned a distribution system deficiency (i.e., distribution system or untreated ground water and distribution system) occurred in systems using ground water or mixed ground and surface water supplies; of these, three occurred in systems supplying unchlorinated ground water. Two of the distribution system-associated outbreaks (one in an unchlorinated supply) resulted from cross-connections (i.e., direct connections between piped water systems containing potable and nonpotable water).
In addition to the drinking water outbreaks, public health officials from 11 states reported 12 outbreaks associated with other nonrecreational water exposure (Table 1). The outbreaks included seven outbreaks of Legionella spp. resulting in 99 illnesses and six deaths. The water sources and settings for these outbreaks included cooling towers at a military facility and a hotel/motel setting, a mist/steam device in an industrial facility, an ornamental fountain in a health-care facility, and unidentified water exposures in long-term care, assisted-living, or rehabilitation facilities. The remaining outbreaks involved Campylobacter (two), Giardia (two), and acute gastrointestinal illness of unknown etiology (one) from ingesting water in various outdoor settings.

Reported by

Elizabeth D. Hilborn, DVM, Timothy J. Wade, PhD, Environmental Protection Agency. Lauri Hicks, DO, Laurel Garrison, MPH, Div of Bacterial Diseases, National Center for Immunization and Respiratory Diseases; Joe Carpenter, MS, Div of Healthcare Quality Promotion; Elizabeth Adam, MPH, Bonnie Mull, MPH, Jonathan Yoder, MPH, Virginia Roberts, MSPH, Julia W. Gargano, PhD, Div of Foodborne, Waterborne, and Environmental Diseases, National Center for Emerging and Zoonotic Infectious Diseases, CDC. Corresponding contributor: Julia W. Gargano,, 404-718-4893.

Editorial Note

Since the early 20th century, water treatment processes and regulations have vastly reduced the transmission of illnesses 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 and other nonrecreational outbreaks. Fourteen of the 15 deaths reported were caused by Legionella, underscoring the need for improved Legionella control and mitigation methods. Second, the large proportion of outbreaks associated with untreated ground water (e.g., well water) indicates that additional efforts are needed to monitor ground water sources and protect them from contamination and to ensure that adequate, continuous disinfection is used when indicated by the results of monitoring and risk analyses (2). Finally, the large proportion (78%) of illnesses observed in outbreaks involving distribution system deficiencies emphasizes the importance of protecting, maintaining, and improving the public drinking water distribution system infrastructure (3) because these deficiencies can lead to widespread illness.
The total number of drinking water outbreaks reported during 2009–2010 (33) is similar to the number in previous 2-year intervals (e.g., 36 outbreaks during 2007–2008) (4). Although Legionella historically has been the most frequently reported etiology among drinking water outbreaks, during 2009–2010 Legionella comprised over half of reported drinking water outbreaks for the first time. In addition, Legionella also caused the majority of other nonrecreational water outbreaks (seven of 12). Legionella outbreaks are particularly challenging to prevent and control, in part because the organism multiplies in plumbing systems within buildings, which usually fall outside of regulatory oversight (5,6). Two of the 19 reported Legionella outbreaks occurred at health-care facilities where treatment systems to control Legionella growth had been installed, underscoring the limited effectiveness of engineering controls in complex plumbing systems.
In contrast to the emerging issue of Legionella, the problem of untreated ground water deficiencies in public and individual water systems persists. Full implementation of the Ground Water Rule, a federal regulation that aims to provide increased protection against microbial pathogens in public water systems that use ground water sources, might reduce the number of ground water outbreaks in public systems (2). However, this regulation does not address private wells, which the Environmental Protection Agency lacks the authority to regulate, emphasizing the continued need for education and outreach to private well owners to prevent outbreaks (7,8).
Distribution system deficiencies continue to be a major contributor to drinking water outbreaks and outbreak-associated illnesses. Three outbreaks occurred in systems supplying unchlorinated water; if a disinfectant residual had been present, pathogens introduced by the distribution system deficiency might have been inactivated before the water reached consumers. Two outbreaks resulted from cross-connections between potable and nonpotable water pipes. The piecemeal nature of some infrastructure development might contribute to the occurrence of these cross-connections, highlighting the importance of distribution system monitoring and adherence to guidelines for the prevention of backflow of nonpotable water into the potable water supply (5,9).
The findings in this report are subject to at least two limitations. First, detection, investigation, and reporting of outbreaks are incomplete, and the level of surveillance and reporting activity varies across states and localities. 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). Environmental investigations provide information on deficiencies that contribute to outbreaks and strengthen evidence implicating drinking water as a common source of infection; however, capacity to conduct these investigations and report the results also differs by state and locality, and might change over time. For these reasons, outbreak surveillance should not be used to estimate the total number of illnesses from waterborne disease because most cases of waterborne disease are believed to occur sporadically or as part of outbreaks that are never recognized. Second, changes in the surveillance system occurred during this cycle, namely implementation of electronic reporting of waterborne disease outbreaks and the assignment of multiple deficiency categories for Legionella outbreaks. These changes do not affect the internal validity of the data in this report but might limit the ability to interpret trends in the number of outbreaks and deficiencies across reporting periods.
As observed in recent years, the proportion of outbreaks in the federally regulated portions of public water systems has declined, although these still contribute the majority of outbreak-associated illnesses. Deficiencies at points not under the jurisdiction of water utilities (e.g., private wells and plumbing systems) continue to cause illness. In addition, challenges with aging water infrastructure are ongoing, and efforts to understand the number of illnesses associated with drinking water distribution system deficiencies are needed. Partnerships between state and local public health agencies, as well as cooperation and coordination among epidemiologists, laboratorians, and environmental health specialists within agencies, are needed to optimize investigation and reporting of waterborne disease outbreaks. Additional information about the waterborne disease outbreaks reported during 2009–2010 is available at


State, territorial, local, and freely associated state waterborne disease coordinators, epidemiologists and environmental health personnel; LaTonia C. Richardson, MS, Div of Foodborne, Waterborne, and Environmental Diseases, National Center for Emerging and Zoonotic Infectious Diseases, CDC.


  1. Cutler D, Miller G. The role of public health improvements in health advances: the twentieth-century United States. Demography 2005;42:1–22.
  2. US Environmental Protection Agency. National primary drinking water regulations: ground water rule, final rule. Fed Regist 2006;(71):65574–660.
  3. US Environmental Protection Agency. Drinking water infrastructure needs survey and assessment, fifth report to Congress. Washington, DC: US Environmental Protection Agency; 2013. Available at Adobe PDF fileExternal Web Site Icon.
  4. CDC. Surveillance for waterborne disease outbreaks associated with drinking water—United States, 2007–2008. MMWR 2011;60(No. SS-12):38–68.
  5. Committee on Public Water Supply Distribution Systems: Assessing and Reducing Risks, National Research Council. Drinking water distribution systems: assessing and reducing risks. Washington, DC: National Academies Press; 2006.
  6. Marciano-Cabral F, Jamerson M, Kaneshiro ES. Free-living amoebae, Legionella and Mycobacterium in tap water supplied by a municipal drinking water utility in the USA. J Water Health 2010;8:71–82.
  7. Knobeloch L, Gorski P, Christenson M, Anderson H. Private drinking water quality in rural Wisconsin. J Environ Health 2013;75:16–20.
  8. Swistock BR, Clemens S, Sharpe WE, Rummel S. Water quality and management of private drinking water wells in Pennsylvania. J Environ Health 2013;75:60–6.
  9. US Environmental Protection Agency. Cross-connection control manual. Washington, DC: US Environmental Protection Agency; 2003. Available at Adobe PDF fileExternal Web Site Icon.
  10. Tostmann A, Bousema T, Oliver I. Investigation of outbreaks complicated by universal exposure. Emerg Infect Dis 2012;18:1717–22.

* Outbreak reports can be submitted by public health agencies in U.S states, the District of Columbia, Guam, Puerto Rico, the Marshall Islands, the Federated States of Micronesia, the Commonwealth of the Northern Mariana Islands, Palau, and the U.S. Virgin Islands.
A description of the Waterborne Disease and Outbreak Surveillance System is available at
§ Outbreaks are assigned one or more deficiency classifications based on available data. The classifications provide information about how the water became contaminated, water system characteristics, and factors leading to waterborne disease outbreaks. A full description of CDC deficiency classification is available at
"Plumbing" refers to the pipes that are within a building or within a service line leading into a building, distinguished from the distribution system of pipes that comprise the water supply.
** Nonrecreational category includes outbreaks involving water not intended for drinking and water of unknown intended use, but does not include recreational water exposures, which are reported separately.
†† A description of the National Outbreak Reporting System is available at This reporting platform allows states to submit reports of waterborne, foodborne, animal-to-person, person-to-person, and unknown outbreaks to CDC. The Waterborne Disease and Outbreak Surveillance System transitioned from a paper-based system to electronic reporting through the National Outbreak Reporting System in 2009; this is the first report of data collected in the electronic system.
§§ Public water systems include community and noncommunity water systems, which have ≥15 service connections or serve an average of ≥26 residents for ≥60 days a year. A community water system serves year-round residents of a community, subdivision, or mobile home park. A noncommunity water system serves an institution, industry, camp, park, hotel, or business.

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