Cryptosporidiosis in College Swimmers After a Training Trip, 2023

Cryptosporidium is a parasite that is extremely tolerant of chlorine. Swallowing recreational water contaminated with feces carrying it can cause gastrointestinal illness. In early 2023, an outbreak among college swimmers showed how easily it can travel with a team.

The outbreak

A Massachusetts college told the Massachusetts Department of Public Health that 19 of the 50 members (38%) of its men's and women's swim teams had developed diarrhea starting three days after returning from a weeklong training trip to Puerto Rico. One ill swimmer had already tested positive for Cryptosporidium. Worryingly, sick swimmers had competed on days 5 and 6 after the trip in meets against teams from New York (meet 1) and Rhode Island (meet 2).

The response

Nine days after the team's return, when the department was notified:

• ill swimmers were encouraged to submit stool samples to the state public health laboratory, and positive samples were sent to CDC and New York's CryptoNet laboratories for genetic typing;
• the department alerted the Puerto Rico, New York and Rhode Island health departments, and New York and Rhode Island looked for cases on their teams; and
• the college closed its pool and hired a vendor to hyperchlorinate the water to kill the parasite.

Findings

• Massachusetts: of the 19 sick swimmers, 18 were tested and 13 were positive. They were aged 18 to 22, eight were male, and none was hospitalized. Their symptoms began 3 to 7 days after returning. In Puerto Rico, all 13 had swum in the training pool and a waterfall, and 10 in the ocean. Infected swimmers were kept out of the water until two weeks after their diarrhea ended.
• New York: no cases were found among swimmers exposed at meet 1.
• Rhode Island: two swimmers fell ill seven days after meet 2 and tested positive. Their symptoms began just after yet another meet, against a different out-of-state university, but no illness was linked to that meet.
• Genetic typing: Cryptosporidium parvum subtype IIaA16G3R1 was found in five Massachusetts swimmers (two others had different subtypes) — and in both Rhode Island swimmers, suggesting transmission at meet 2.

Lessons

• Competitive swimming can spread it. Although there was no sign the Rhode Island swimmers passed it on, frequent intercollegiate meets and championships give Cryptosporidium a chance to keep spreading among swimmers.
• Testing matters. Without the first laboratory diagnosis, the parasite might not have been suspected and the pool not closed and disinfected so quickly.
• Healthy swimming. People should not swim when they have diarrhea and should avoid swallowing pool water.

Sources

• Geena Chiumento, Anthony Osinski, Kelsey DeVoe, Amelia Houghton, Akita Joshi, Caryn Ivanof, Emma Creegan, Michael Gosciminski, Alexandra P. Newman, Susan Madison-Antenucci, Michele C. Hlavsa, Erin Imada, Colleen Lysen, Shanna Miko, Jordan Schultz, Emily Harvey, Johanna Vostok and Catherine M. Brown, "Notes from the Field: Outbreak of Cryptosporidiosis Among Collegiate Swimmers and Evidence of Secondary Transmission — Massachusetts and Rhode Island, 2023," Morbidity and Mortality Weekly Report, CDC: https://www.cdc.gov/mmwr/volumes/72/wr/mm7226a7.htm
• The report's figure, prepared with state health departments, is not reproduced.