By Kelly A. Reynolds, MSPH, Ph.D.

Researchers at the Centers for Disease Control and Prevention (CDC), the US EPA and others have published a review article chronicling drinking water outbreaks over the last 36 years (1971-2006) in the United States. This information gives industry professionals the opportunity to monitor past- and predict future- trends in waterborne disease transmission. In this summary, we will examine how historical outbreak data can be used to protect public health and the role of the POU industry in preventing future waterborne illnesses.

Building the waterborne disease database
Since 1971, CDC, US EPA and the Council of State and Territorial Epidemiologists (CSTE) have maintained the Waterborne Disease Surveillance System (WBDSS), a database of reported drinking water outbreaks and their causes. Information on related sources, etiological agent, water system deficiencies, persons involved, illnesses associated, and other trends, is collected and voluntarily reported by health departments in US states, territories and localities.

In addition to having the responsibility of reporting, state health departments also have the responsibility of investigating outbreaks. To be included in the WBDOSS database, some level of epidemiological data is needed implicating water as the source of cases. Clinical specimens and water quality samples aid in confirming the epidemiological data. Varying commitments from different regions to recognize, investigate and report water-related illnesses creates a bias in the data. Thus, caution must be utilized when identifying areas of increased concern, as these may in fact be areas of increased surveillance and resources.

In 2009, the CDC’s National Outbreak Reporting System (NORS) was made available to all states. This online, electronic reporting system is expected to improve data collection and reporting, enabling timelier submission of reports and analysis. Other modifications in the reporting system focus on deficiency classifications. Data from 1971-2002 were reevaluated by Craun et al. (2010) in consideration of more detail on the point of water contamination and responsible circumstances. The authors stressed that recognizing the distinction between contamination of source water, treatment facility or distribution system water, and POU or premise plumbing environments is important for identifying appropriate control measures for the future.

Historical trends
During the 36-year survey period, a total of 833 outbreaks associated with drinking water were reported, resulting in 577,991 cases of illness and 106 deaths. The majority (93.6 percent) of outbreaks were documented in drinking water but 0.1 percent (n=861) of cases and 12.3 percent of deaths were associated with water not intended for drinking (i.e., lakes, springs, or other non-potable water sources used for drinking water). All types of waterborne illnesses are eligible for inclusion in the database, i.e., acute gastrointestinal or respiratory illness, skin infections, neurological ailments and hepatitis. Chronic effects due to microbial infections are recognized as well (i.e., reactive arthritis, diabetes, heart disease) but are not utilized as an identified health endpoint in the WBDSS. Deaths from waterborne outbreaks were primarily due to bacterial pathogens although 50 deaths in 1993 were due to a single Cryptosporidium outbreak in Milwaukee, WI (see Table 1). In recent years, the category of unknown etiology has decreased from more than 50 percent to less than 25 percent as methods for investigation and identification have improved, while new hazards have been added to the list of surveillance categories. Even though Legionella has only been reported since 2001, 25 deaths have been documented.

Outbreaks are more likely to be reported between the months of June to August. Seasonal trends were significant in community (public water systems serving at least 15 service connections or 25 year-round residents), non-community (public system serving non-year-round residents such as schools, campgrounds, parks, etc.) and individual (privately owned systems typically serving a single family or farm but may serve up to 24 residents from 14 connections) water systems. Waterborne outbreaks peaked in 1980 (n=50) but there was a significant trend in the reduction of outbreaks after 1980 where no more than 30, and typically less than 20, events were documented in any given year. As the overall number of outbreaks was decreasing, a trend in increasing numbers of individual water systems was statistically significant. The majority of outbreaks in individual systems occurred in private residences (n=58; 70.7 percent) where the most likely deficiency was untreated groundwater (n=70; 83.3 percent). (Approximately 15 million households in the US are served by a private, individual water source.) No or improper treatment (i.e., no filtration or inadequate disinfection) were identified as the primary deficiencies linked to drinking water outbreaks. Untreated groundwater and treatment deficiencies caused 243 and 312 outbreaks, respectively. In addition, nearly 60 percent (n=203) of outbreaks from community water systems were due to untreated or insufficiently treated surface water.

Despite the outbreak surge in 1980, a trend analysis shows that the annual proportions of water system deficiencies under the jurisdiction of government regulatory requirements decreased over time during the 36-year period – at least with ground water sources. This trend provides evidence that regulations (i.e., the Safe Drinking Water Act, 1974; 1986 Amendments; Surface Water Treatment Rule [SWTR], 1989) improved drinking water quality. The 1989 SWTR, for example, increased requirements for filtration and disinfection, targeting the inactivation or removal of Giardia, viruses and bacteria. Regulations imposed during the survey period had little impact on the proportion of deficiencies associated with groundwater. Considering both private and public systems, nearly half of the US population is served by a groundwater source. Given the numerous studies and outbreak data indicating the presence of human viruses in groundwater wells, US EPA promulgated the groundwater disinfection rule in 2006. This regulation is expected to reduce cases of waterborne disease by 42,000 per year and will likely be evident in future trend analysis.

Prevention at the point of use
Future trend predictions indicate that private, individual water supplies and premise plumbing present the greatest risk for waterborne disease transmission. Historical data documents that the annual portion of deficiencies from premise plumbing increased in public water systems. Control of premise plumbing contamination presents a regulatory control problem, given that this environment is outside the jurisdiction of the US EPA. Legionella is a major concern in premise plumbing due to its ability to regrow and the lack of monitoring and treatment after the water meter or property line. The authors of the recent review recommend greater attention be placed on groundwater and unregulated drinking water systems in the future. Recognizing that premise plumbing and point-of-use environments are not well characterized, better understanding of water quality at the tap and public outreach, extension and education efforts are warranted. In addition, improved understanding of Legionella and its persistence in premise plumbing and biofilm is critical for risk reduction. Finally, recognizing that reported outbreaks are just the tip of the iceberg in terms of the overall disease burden, more research is needed to determine the true impact of waterborne disease and the cost-benefit of point-of-use water purification.


  1. Craun, Gunther F; Brunkard, Joan M; Yoder, Jonathan S; Roberts, Virginia A; Carpenter, Joe; Wade, Tim; Calderon, Rebecca L; Roberts, Jacquelin M; Beach, Michael J; Roy, Sharon L. 2010. Causes of outbreaks associated with drinking water in the United States from 1971 to 2006. Clinical Microbiology Reviews. 23(3):507-28.

About the author
Dr. Kelly A. Reynolds is an associate professor at the University of Arizona College of Public Health. She holds a Master of Science degree in public health (MSPH) from the University of South Florida and a doctorate in microbiology from the University of Arizona. Reynolds has been a member of the WC&P Technical Review Committee since 1997. She can be reached via email at [email protected].


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