Several college swim teams learned a stomach-churning lesson the hard way during a recent series of meets: regardless of skill or speed, the real winner of the meet will always be the gastrointestinal pathogen that enters the pool.

In an outbreak investigation that is destined to become a nauseating cautionary tale, health officials traced the spread of the water-based intestinal parasite Cryptosporidium (aka crypto) through several pools of competitive swimmers with the squirts. The case involved a lucky break: Health officials fished out the crypto rather quickly. But, it wasn't fast enough to keep the parasite from butterflying its way to members of another college's team, showing how quickly and easily it could have spread without intervention

It also highlighted "an ongoing need to promote healthy swimming, including recommendations for persons not to swim if they have diarrhea and to avoid swallowing swimming pool water to prevent waterborne disease," the authors of a case report on the investigation wrote. The report, led by health officials in Massachusetts, was published Thursday in the Centers for Disease Control and Prevention's Morbidity and Mortality Weekly Report.

It all began earlier this year when competitive swimmers from a Massachusetts-based college went to Puerto Rico for a weeklong training session. While there, the swimmers plunged into a training pool, a waterfall, and the ocean. It's unclear where they picked up the gruesome gut crasher exactly, but three days after their return, the swimmers' health started dipping. By the end, 19 of 50 swim team members of the men's and women's teams would fall ill.

Amid the intestinal eruptions, the swim teams continued on with their training and meet schedule—including with the members experiencing tummy troubles. Within a week of their return, the swimmers had two separate meets: one against a New York-based team five days after their return and a subsequent meet with a Rhode Island-based team a day later.

Natural swimmers

With swimmers sprinting to bathrooms, officials at the Massachusetts college contacted the state's health department—nine days after they returned from the trip to Puerto Rico. By then, one of the swimmers had already tested positive for crypto. The health department didn't waste any time, contacting officials in Puerto Rico, New York, and Rhode Island the same day.

But, it was too late to halt all of the spread. Two swimmers on the team from Rhode Island team fell ill with crypto seven days after the meet with the Massachusetts team. Testing indicated that the subtypes of crypto in the Rhode Island swimmers matched some of those found in the Massachusetts team.

Crytpo is exceptionally infectious, particularly in pools. The parasite is shed in feces, and an infected person can spew 10 million to 100 million parasites in a single blowout, according to the CDC. Swallowing just 10 parasites is enough to ignite a new explosive infection. The symptoms of which include watery diarrhea, nausea, stomach pain and cramps, vomiting, fever, and dehydration. For otherwise healthy people, the infection and infectious period can last a few weeks. For those with compromised immune systems, the infection can be serious, longer-lasting, and potentially fatal.

While crypto can spread via anything with fecal contamination—dirty hands, contaminated food, etc.—recreational water is particularly problematic. The parasite has a tough outer shell, making it highly tolerant to chlorine. It can survive in a properly chlorinated pool for over seven days, the CDC says. It also is not effectively killed by hand sanitizers.

As the Massachusetts health department investigated the outbreak, the college closed its swimming pool and hired a contractor to sanitize it with a hyperchlorinated treatment.

In all, the health officials overseeing the investigation dodged a bullet. They caught the Rhode Island team fast enough before transmission continued. They noted that "because of the regular intercollegiate competition and subsequent championship schedule, the potential exists for sustained Cryptosporidium transmission among competitive swimmers," they wrote. And if they hadn't had an early detection of crypto in one of the swimmers, the Massachusetts college may not have shut down the pool, preventing further transmission on campus.