Monitoring Waterborne Disease Outbreaks in the US

Waterborne disease outbreaks in the United States are monitored through reports to the Centers for Disease Control and Prevention (CDC) via the National Outbreak Reporting System (NORS). CDC’s efforts to quantify the health and economic impacts of waterborne diseases in the US, show that communities of bacteria, biofilm-related pathogens, especially Legionella, are leading causes of hospitalization and death from drinking water-associated diseases.

Between 2015 and 2020 of the 214 reported outbreaks, a staggering 187 were associated with biofilms, showcasing the dominant role of these environments dominant role in disease transmission. 24 enteric illness-associated, 2 unknown, and 1 chemical or toxin outbreaks. Most outbreaks, 80%, were linked to public water systems. These outbreaks resulted in over 2,140 illnesses, 563 hospitalizations, and 88 deaths.

“During 2015–2020, Legionella-associated outbreaks were the leading cause of nationally reported drinking water-related outbreaks, hospitalizations, and deaths,” according to the CDC. “Outbreaks were linked with community and noncommunity water systems. In addition, Legionella was implicated in all lodging and nearly all (n = 111; 98%) healthcare-associated biofilm-related outbreaks. Legionella-associated outbreaks in health care settings resulted in approximately 2/3 (n = 364; 65%) of hospitalizations and 3/4 (n = 73; 85%) of deaths reported during this period.”

Main Takeaways

  1. The establishment of NORS by the CDC has been pivotal in monitoring waterborne disease outbreaks.
  2. Biofilm-associated pathogens, especially Legionella, in drinking water-associated diseases lead to hospitalizations and deaths.
  3. 80% of the outbreaks were linked to public water systems emphasizing the need for stringent regulatory standards, rigorous monitoring, and robust maintenance protocols for these systems.

The CDC introduced NORS in 2009 as an innovative web-based platform designed for public health departments to submit information on outbreaks voluntarily. NORS compiles data on outbreaks caused by the wide range of agents, including bacteria, viruses, parasites, chemicals, and toxins, focusing on enteric, foodborne, and waterborne diseases. The data highlights that enteric illness-associated pathogens, mainly norovirus, Shigella, and Campylobacter, were responsible for most illnesses, but Legionella was the primary biofilm-related pathogen. Outbreaks associated with Legionella showed an increasing trend over the years.

“Enteric illness outbreaks associated with norovirus, Shigella, Campylobacter, or multiple etiology outbreaks were primarily associated with individual or private and community water systems,” according to the CDC. “One outbreak of norovirus and enteropathogenic E coli that resulted in 693 (32%) cases occurred in an amusement park setting because of an overly pumped, improperly constructed well with chronically inadequate disinfection.”

NORS’s reliance on voluntary reporting and varying reporting effectiveness across areas likely leads to an underestimate of outbreaks, rendering NORS data unreliable for accurate outbreak counts. Additionally, only investigated and reported outbreaks are included. Legionella outbreak investigations can take years, delaying reporting to NORS and crucial information. Thirdly, the COVID-19 pandemic may have impacted reports of waterborne disease outbreaks in 2019–2020. Lastly, many reports on drinking water outbreaks lack specific details on contributing factors or water treatment, with a particular inconsistency in classifying factors for Legionella and biofilm-related outbreaks, potentially introducing bias. Citing Legionella frequently as a factor provides limited insight into its growth and spread reasons in water systems.

Overall, the findings from this report underscore the complexity of preventing diseases associated with drinking water and the critical need for comprehensive water source-to-tap prevention strategies. Efforts by the CDC to calculate the impact of waterborne disease on illness and health care costs in the US uncovered that pathogens associated with biofilms, along with NTM and Legionella, are now the leading reasons for hospitalizations and fatalities linked to diseases from waterborne sources and drinking water.


1. CDC. Surveillance of Waterborne Disease Outbreaks Associated with Drinking Water — United States, 2015–2020. Surveillance Summaries. Published March 14, 2024. Accessed March 14, 2024. https://www.cdc.gov/mmwr/volumes/73/ss/ss7301a1.htm?s_cid=ss7301a1_w

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