Waterborne Disease Outbreaks

A recently released CDC study (Surveillance of Waterborne Disease Outbreaks Associated with Drinking Water — United States, 2015–2020) found that out of 214 outbreaks associated with drinking water over the study period, 87% were associated with biofilms. Although 214 outbreaks over 5 years may appear to be a small number, it is important to consider the impact. 

In epidemiology, an outbreak is defined as two or more cases of similar illness associated with a common exposure. To break this down, the number of persons directly impacted by these outbreaks included 2,140 cases of illness, 563 hospitalizations, and 88 deaths. Overall, waterborne pathogens cause approximately 7.15 million illnesses, 118,000 hospitalizations, and 6,630 deaths annually in the United States, which results in $3.33 billion in direct healthcare costs.

CDC Study
The CDC study covers the data obtained over a 5-year period, with a focus on Legionella and biofilm-related outbreaks. Public health officials from 28 states voluntarily reported 214 outbreaks associated with drinking water from 2015 to 2020. Legionella bacteria were implicated in most (98%) of the biofilm-related outbreaks.

Legionella is a genus of bacteria that causes a severe form of pneumonia called Legionnaire’s diseases, which is fatal in 10% of all cases. Thus, biofilms and Legionella remain significant threats to public health and must be controlled within these water systems. The remaining 2% of biofilm-related outbreaks were attributed to NTM and Pseudomonas bacteria.

Of the 214 outbreaks, 187 (87%) were associated with biofilm and 172 (80%) were linked to water from public water systems. These drinking water-associated outbreaks resulted in 2,140 cases of illness, 563 hospitalizations, and 88 deaths. This CDC report included outbreaks where two or more cases were epidemiologically linked by time, location of water exposures, and illness characteristics.

Biofilms are communities of microbes that attach themselves to moist surfaces, such as water pipes within building water systems. These biofilms benefit several types of pathogens, including Legionella and nontuberculous mycobacteria (NTM), by providing them with protection and nutrients. Biofilms thrive in environments of stagnation within water pipes, particularly where disinfectant residuals are depleted, and are resistant to water treatment processes, such as disinfection. Biofilm-related pathogens not only impact health but cost the United States approximately $1.39 billion annually.

During the 5-year period, Legionella-associated outbreaks generally increased and resulted in 786 illnesses, 544 hospitalizations, and 86 deaths. In particular, Legionella bacteria were the most implicated etiology in public water system outbreaks (160 outbreaks, 666 cases, 462 hospitalizations, and 68 deaths). The most cited contributing factors included:

  • Premise or point-of-use plumbing,
  • Legionella growth-promoting water temperatures and permissive chlorine levels within the building potable water system,
  • Water temperatures above 85 °F, and
  • Aging plumbing components, such as pipes, tanks, and valves. Chlorine was the disinfectant in 79 (37%) of the outbreaks, followed by unknown or no treatment (n = 99, 46%) and monochloramine treatment (n = 12, 6%).

In healthcare settings, Legionella was associated with 111 (52%) outbreaks, 444 (21%) cases, 364 (65%) hospitalizations, and 73 (85%) deaths. Hotels, motels, lodges, or inns were identified as the exposure settings in 35 (16%) outbreaks, 225 (11%) cases, 85 (15%) hospitalizations, and 3 (3%) deaths, all of which were caused by Legionella. In private residences, Legionella causes 3 outbreaks, resulting in 7 cases, 4 hospitalizations, and 0 deaths.

Drinking water-related disease prevention is extremely complex and requires various mitigation strategies. When water service is disrupted by drinking water contamination, public health is negatively impacted. Therefore, drinking water treatment, regulations, and public health programs reduce the risk for exposures to drinking water pathogens. Effective regulations, water management programs, and public health prevention programs are necessary.

If you have any questions, please contact the Water Risk Management Group at 866-209-3373.


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