Outbreaks From Swimming in Lakes, Rivers and Oceans, United States, 2000–2014

At a glance

• Known before: outbreaks tied to untreated recreational water — fresh water such as lakes and rivers, or marine water such as the ocean — can come from germs, toxins or chemicals.
• Found here: in 2000–2014, 140 outbreaks caused at least 4,958 illnesses and two deaths; 80 were caused by enteric (gut) germs.
• What to do: obey posted beach closures; don't swim in discolored, smelly, foamy or scummy water; don't swim with diarrhea; and keep water out of your nose in warm fresh water.

What was counted

An outbreak is two or more people with similar illness, linked by place and time to recreational water — or to germs, toxins or chemicals from it carried into the air. Untreated water is water not disinfected or treated to keep it microbiologically safe. Public health officials report outbreaks to CDC voluntarily; officials from 35 states and Guam reported these 140.

The causes

The cause was confirmed for 103 outbreaks (74%): 95 were caused by pathogens (five of them by more than one), causing at least 3,125 cases, and eight by toxins or chemicals — seven of those by algal toxins from harmful algal blooms.

Enteric germs caused 80 of the 95 pathogen outbreaks (84%) and 2,704 of their cases (87%). The two deaths came from one outbreak of Naegleria fowleri. Of the unidentified cases, about 1,341 came from one outbreak of mostly skin illness at an endurance race, where dirt was mixed with water to make an obstacle — suspected to be poison ivy.

Where and when

• Settings: public parks (50, 36%) and beaches (45, 32%).
• Venues: mostly lakes, reservoirs and ponds (117, 84%).
• Season: 113 (81%) began in June–August, 65 in July; none began in December–February.
• Trend: the yearly count didn't change significantly over the 15 years.

Outbreaks by cause and month (A) and by year (B), 2000–2014. CDC.

Why these germs

The germs behind these outbreaks — enteric bacteria, parasites and viruses — are all killed by chlorine, and many rarely cause outbreaks in treated water: pools with proper chlorine or bromine levels carry less risk.

• Gut germs spread when people swallow water with feces in it. Swimmers can be the source, through a fecal accident or what washes off their bodies; so can storm runoff, flooding, sewage overflows and treatment-plant discharges, septic systems, boat waste and animal waste. E. coli and Cryptosporidium can come from people or animals; norovirus and Shigella point to human feces. Risk rises in water that is shallow, poorly circulating or crowded; used by children under 5 not yet toilet-trained; without good toilets and diaper-changing stations; or soon after heavy rain.
• Avian schistosomes cause swimmer's itch (cercarial dermatitis): in fresh or brackish water where infected birds and the host snails live, larvae burrow into the skin and trigger an allergic rash. They can't mature in people, who are accidental hosts.
• Algal toxins from harmful blooms, in fresh or marine water, cause anything from skin or eye irritation to breathing, stomach or nerve symptoms. Blooms are being seen more often and in more places — perhaps from nutrient pollution and warming water, or better surveillance. In 2016 CDC launched the One Health Harmful Algal Bloom System for reporting illness in people and animals and environmental data.
• Naegleria fowleri causes primary amebic meningoencephalitis: water carrying the ameba goes up the nose, and it travels to the brain along the olfactory nerve. Infection, usually fatal, typically follows swimming or diving in warm, untreated fresh water. Two U.S. patients have recently survived, suggesting early diagnosis and treatment may help.

Limitations

• The counts likely underestimate illness: reporting differs between places, and people who travel far to swim may fall ill back home in scattered places, so their illnesses are never linked.
• Outbreaks without laboratory confirmation were counted as unidentified, even when other evidence pointed to a cause.
• Reporting and review changed over time, limiting comparison across years.

Prevention takes One Health

Swimmer health, animal health and the environment are connected. Epidemiologists, environmental health staff, veterinarians and ecologists should work together to prevent and clean up fecal contamination (stopping sewage overflows, improving circulation), manage wildlife such as birds on the beach, monitor bacteria and nutrients, and run strong monitoring and notification. Under the BEACH Act of 2000, the Environmental Protection Agency funds coastal and Great Lakes beach monitoring, and its BEACON database posts water conditions — but most outbreaks happen at smaller inland lakes, reservoirs and ponds, so swimmers should check local advisories too. Healthy-swimming messages matter most before and during June–August.

Protect yourself

Stay out of the water if:

• the beach is closed, or an advisory warns of high bacteria, sewage spills or algal blooms;
• there's been heavy rain recently;
• you can see a discharge pipe on the beach;
• fish or other animals in or near the water are dead;
• the water is discolored, smelly, foamy or scummy.

Diarrhea-causing germs: don't swim, or let children swim, with diarrhea — after Cryptosporidium, wait 2 weeks after it stops — and don't swallow the water. Healthy swimming.

Swimmer's itch: don't swim or wade in marshy areas where snails are common, and towel dry or shower right after. More.

Harmful algal blooms: when in doubt, stay out; keep children and pets from drinking discolored, smelly, foamy or scummy water; rinse off with clean water as soon as possible after a swim that may have exposed you; rinse dogs at once, and don't let them lick algae off their fur. More.

Naegleria fowleri: the only sure protection is to stay out of warm fresh water. Otherwise, use nose clips, hold your nose or keep your head above water; don't put your head under in hot springs; and avoid warm fresh water when temperatures are high. More.

Sources

Based on "Outbreaks Associated with Untreated Recreational Water — United States, 2000–2014," by Daniel S. Graciaa and colleagues of CDC, the Environmental Protection Agency and Emory University, Morbidity and Mortality Weekly Report, CDC; a work of the United States government in the public domain.