Drinking Water Outbreaks in the United States, 2015–2020: Legionella Leads

Safe drinking water is essential to public health. Each year in the United States, waterborne germs cause an estimated 7.15 million illnesses, 118,000 hospitalizations and 6,630 deaths, costing $3.33 billion in direct health care. Drinking water exposures account for 40% of those hospitalizations and 50% of the deaths — mostly from germs that live in biofilms, such as Legionella and nontuberculous mycobacteria (NTM), at a cost of $1.39 billion a year.

Biofilms are communities of microbes that cling to wet surfaces like the inside of pipes and shelter and feed many kinds of pathogens. They thrive when water sits still or disinfectant runs low, and they resist treatment such as disinfection. People can be exposed by touching, swallowing or breathing in mist from contaminated water — from showerheads, humidifiers and other fixtures and devices.

This CDC surveillance summary covers drinking water outbreaks that health departments voluntarily reported to the National Outbreak Reporting System (NORS) from 2015 to 2020, and for the first time sorts them into biofilm-associated and enteric (gut) illness outbreaks.

The totals

Twenty-eight states reported 214 outbreaks, causing at least 2,140 illnesses, 563 hospitalizations (26% of cases) and 88 deaths (4%).

Drinking water outbreaks by state of exposure, 2015–2020. CDC, MMWR.

• Cause: 187 biofilm-associated, 24 enteric, 2 unknown and 1 chemical or toxin.
• Water system: 172 (80%) public (community or noncommunity) systems, 17 (8%) individual or private systems, 22 unknown, 2 other and 1 not reported. Private systems accounted for 944 illnesses, 52 hospitalizations and 14 deaths.
• Water source: groundwater in 82 outbreaks (38%), surface water in 57 (27%), unknown in 61 (29%).
• Treatment: disinfection in 116 systems (54%), none in 17 (8%), unknown in 49 (23%); chlorine was named in 79 reports and chloramine in 12.

Legionella: fewer illnesses, most of the harm

Legionella caused 184 outbreaks; the other biofilm outbreaks were two NTM and one Pseudomonas. They produced 786 illnesses (37% of all), but 544 hospitalizations (97%) and 86 deaths (98%). Legionella outbreaks increased in number over the period.

• Public water systems: Legionella caused 160 of their outbreaks (92%), with 666 illnesses, 462 hospitalizations and 68 deaths.
• Private systems: 71 illnesses, 48 hospitalizations and 14 deaths.
• Health care settings — hospitals, long-term care, assisted living and rehabilitation — accounted for 113 of all outbreaks (53%), 456 illnesses, 372 hospitalizations and 75 deaths (87%). Legionella caused 111 of those outbreaks, with 364 hospitalizations and 73 deaths.
• Hotels, motels, lodges and inns: 35 outbreaks, 225 illnesses, 85 hospitalizations and 3 deaths — all Legionella.
• Private homes: for the first time, Legionella was reported to NORS as causing outbreaks in private residences — three, in 2015 and 2019, with 7 illnesses and 4 hospitalizations — an emerging concern.

Drinking water outbreaks by setting of exposure, 2015–2020. CDC, MMWR.

Why they happened. Of 393 contributing factors reported for biofilm outbreaks, 287 involved the building's own plumbing or the point of use. The most common were Legionella already present in the water system (67), water temperatures and chlorine levels in a building's plumbing that let Legionella grow (35), water at 86°F (30°C) or warmer (28) and aging pipes, tanks and valves (27).

Drinking water outbreak causes, Legionella compared with all others, 2007–2020. CDC, MMWR.

Enteric outbreaks: few, but large

Only 24 outbreaks (11%) were enteric, but they caused 1,299 illnesses (61%) — with 10 hospitalizations and no deaths. The agents were norovirus (7 outbreaks), Shigella (4), Giardia (3), Campylobacter (2), Cryptosporidium (2), E. coli (1) and multiple germs (5). Seventeen outbreaks involving norovirus, Shigella, Campylobacter or combinations including them caused 1,225 illnesses.

• The biggest outbreak, 693 illnesses, was caused by norovirus and enteropathogenic E. coli at an amusement park, traced to an over-pumped, poorly built well with chronically inadequate disinfection.
• Wells were the source in 13 of the 14 enteric outbreaks where the source was known, whatever the type of water system, and 11 of those had no disinfection where treatment was known.
• Contributing factors: the water source was the most cited (34 of 61 factors) — contamination through limestone or fissured rock (karst) (7), badly built or badly placed wells or springs (7), and flooding or heavy rain (5). Among treatment failures, no disinfection (9), no filtration (5) and chronically inadequate disinfection (2) led.

Settings ranged from mobile home parks and lodging to amusement parks, farms, camps and private homes.

What the patterns mean

Biofilm and enteric outbreaks start in different places: Legionella outbreaks mostly in a building's plumbing, enteric outbreaks upstream at the source or in treatment. That range is why prevention has to run from source to tap — proper well construction, siting, operation and maintenance for groundwater, and water management programs, biofilm control and sound regulation for buildings, including hospitals and homes. The Veterans Health Administration and the Centers for Medicare & Medicaid Services already require water management programs in certain health care facilities.

NORS was substantially updated in January 2023, adding a section on Legionella and other biofilm outbreaks, revised contributing factors and a new section on interventions. NTM infections are not nationally notifiable, so outbreaks may go undetected; the Council of State and Territorial Epidemiologists has a standard case definition for extrapulmonary NTM infections, and health departments could consider making them reportable.

Limitations

Reporting is voluntary and varies by jurisdiction, so NORS undercounts outbreaks and cannot give their true number. Legionella investigations can run for years before being reported. The COVID-19 pandemic may have limited reporting in 2019–2020. Contributing factors were missing for 70 outbreaks and water treatment for 49, and some factors may have been misclassified.

Sources

• Kunz JM, Lawinger H, Miko S, et al. "Surveillance of Waterborne Disease Outbreaks Associated with Drinking Water — United States, 2015–2020." MMWR Surveillance Summaries 73(1). CDC. The report's year-by-year counts of Legionella outbreaks add up to 160, not the 184 it reports in total, so they are not given here; it also gives private systems' share of illnesses as both 43% and 44%, so only the count is used.