Sick After a Swim: Outbreaks from Lakes and Rivers in California, Maine and Minnesota

Lakes, rivers and oceans are untreated recreational water: unlike swimming pools, they aren't treated. Germs or chemicals, including toxins, in that water can make swimmers sick.

CDC's National Outbreak Reporting System has taken voluntary reports of these outbreaks since 2009. From 2009 through 2019 (as of May 13, 2020), health officials in 31 states reported 119 outbreaks and at least 5,240 illnesses. Most — 103 (87%) — began in June, July or August. Of the 88 outbreaks with a confirmed cause, the leading culprits were germs that infect the gut:

A CDC MMWR report described three outbreaks from 2018–2019, each caused by one of these leading germs, that show how widespread such outbreaks are.

California: Shigella in the Santa Ana River, 2019

On July 22, 2019, the California Department of Public Health learned of three shigellosis cases in people who had played in the Santa Ana River, which runs 100 miles through southern California. Investigators found 24 cases with closely related Shigella sonnei. Of 19 sick people with exposure details, 16 had played between July 6 and August 5 in a shallow swim area of the river where water quality wasn't regularly monitored; two said they had swallowed river water. No other shared risk turned up.

• The 16 were young — median age 7 (range 1–20).
• Two of 15 with clinical details were hospitalized; no one died.
• Officials closed public access to the swim area from August 8 to 15.

Water samples upstream, downstream and at the swim area all held E. coli, a bacterial sign of fecal contamination, at 350 to 1,600 per 100 mL by the most-probable-number method, all above EPA's beach action values of 190–235. No source was pinned down, and no new cases appeared after the area reopened.

Maine: norovirus at Woods Pond Beach, 2018

On July 6, 2018, the Maine Center for Disease Control and Prevention heard that several people had fallen ill with stomach symptoms after visiting Woods Pond Beach in Bridgton. The town closed the beach from July 6 to 10. Using social media, the agency found people who had visited July 1–6 and surveyed 148 household members.

• Of 139 visitors, 97 (70%) got sick; their median age was 12 (range 1–73).
• Illness began a median 38 hours after exposure and lasted a median 24 hours; 80% vomited.
• People who swallowed pond water or went under the surface were about three times as likely to get sick.
• Two of four stool samples tested positive for norovirus, which, with the symptoms, pointed to norovirus as the cause.

The source of the contamination was never found, and no new cases were reported after the beach reopened on July 11.

Minnesota: E. coli at a public lake, 2019

On August 13, 2019, Minnesota Department of Health epidemiologists identified three STEC infections in people who had swum at a public lake, and advised officials to close it to swimmers. A survey spread on social media found 69 cases, four of them lab-confirmed STEC O145:H28 with closely related strains. Symptoms began between July 18 and August 16.

• The median age was 29 (range 1–65), and 80% were female.
• Swallowing lake water and being 10 or younger were both tied to illness.
• No one was hospitalized or developed hemolytic uremic syndrome.

The beach had been tested weekly for E. coli all summer, and no result exceeded the state's standard. No single source of contamination was found — but 15 visitors and four lifeguards said they had kept swimming or working in the lake while sick. There were no new cases after the beach reopened on September 5.

How the water gets contaminated

These germs spread when people swallow water contaminated with feces or vomit. Swimmers themselves can contaminate it; so can stormwater runoff, sewage overflows and discharges, leaking septic or municipal systems, dumped boat waste, and animal waste near swim areas. Shigella and norovirus point to human sources, but STEC may not.

Testing has limits. Beach managers measure E. coli and enterococci, as EPA's 2012 criteria recommend, to track changes in fecal contamination — not to detect germs that cause illness. So test results alone can't prove the water caused an outbreak or say where the contamination came from, and some harmful strains, such as E. coli O157:H7, can linger in sediment and go undetected by most routine tests. Microbial source tracking, which identifies which kind of animal the contamination came from — horses, for example — can guide targeted fixes such as redesigning horse trails near swimming spots.

Outbreaks are also hard to spot: swimmers often travel more than 100 miles to a lake, ocean or river and fall ill after returning home to different health jurisdictions; not every jurisdiction asks sick people about swimming; and agencies may struggle to coordinate sampling and closures.

What helps

• Beach managers can let sick lifeguards take duties out of the water — as CDC already recommends for pools — and make it safe for staff to report diarrhea without fear of losing pay or their job.
• Swimmers can:
• • obey beach closures and water-quality advisories;
  • stay out of water that heavy rain has made cloudier;
  • never swim while sick with diarrhea;
  • avoid swallowing the water;
  • keep sand out of their mouths.

Sources

• Vanden Esschert KL, Mattioli MC, Hilborn ED, et al. "Outbreaks Associated with Untreated Recreational Water — California, Maine, and Minnesota, 2018–2019," MMWR Vol. 69, No. 25, CDC; a work of the United States government in the public domain.