Before dipping into a swimming pool or lake this long weekend, take precautions against stomach-turning parasites that may lurk in the water, public health officials say.

Swallowing just a mouthful of water contaminated with the parasite Cryptosporidium, or Crypto, can make otherwise healthy people sick for up to three weeks with watery diarrhea, stomach cramps, nausea, or vomiting and can lead to dehydration, the U.S. Centers for Disease Control and Prevention (CDC) says.

The infection risk comes mainly from fecal contamination of water.

It can be found at a pool, hot tub, water park and splash pad or in the natural beauty of lakes or rivers.

Crypto has emerged as the most common cause of diarrhea outbreaks linked to swimming pools or water playgrounds in the U.S.

Filtering pool water and adding chlorine, salt and other disinfectants will usually control harmful germs. But hardy Crypto can survive more than a week in properly treated water, the CDC says.

Cryptosporidiosis became a notifiable disease in Canada in 2000. Since then, the number of reported cases have ranged from 587 per year to more than 1,700 in 2001. 

In 2001, hundreds of people in North Battleford, Sask., became sick after their drinking water was contaminated with the parasite Cryptosporidium. 

In Canada, serious pool-related outbreaks have been rare, researchers said in 2014 review. But there have been outbreaks, linked to factors such as pool fouling, poor education and training and lack of disinfection equipment, they said.

How to protect yourself 

That's why the CDC recommends closing pools and treating the water with high levels of chlorine when responding to a diarrheal incident in the water or a Crypto outbreak.

"Diarrhea and Swimming Don't Mix" is the agency's theme for National Healthy and Safe Swimming Week, which runs May 22–28 in the U.S.

The CDC offered a few tips to protect yourself, including:

  • Don't go swimming or let your kids swim if sick with diarrhea.
  • "Protect yourself from getting sick by not swallowing the water in which you swim," Michele Hlavsa, chief of the CDC's healthy swimming program said.
  • Rinse off in the shower before getting into the water.
  • Take kids on frequent bathroom breaks and only check diapers in a diaper-changing area — not right next to the pool.

Other germs such as norovirus, E. coli bacteria and the parasite giardia can also end up in water that's been fouled. 

It's often difficult to directly link outbreaks of illness with pool water because the evidence is usually circumstantial, the World Health Organization says. The detective work involves looking for a pattern between swimmers' symptoms and where they last swam.

The U.S. public health authority said at least 32 Crypto outbreaks were linked to pools or water playparks in 2016 compared with 16 in 2014, based on preliminary data.

"Not swimming when ill with diarrhea is key to preventing and controlling aquatic facility–associated cryptosporidiosis outbreaks," the study's authors said. 

It's not clear whether the numbers of outbreaks has really increased of if better surveillance and the introduction of DNA-based tools are increasing detection, the CDC noted in a report. 

The DNA-based tool helped to identify to Crypto species involved in outbreaks last year in Alabama, Ohio and Arizona. Investigators hope to better identify chains of transmission of Crypto when outbreaks occur to help people to continue going for a healthy dip. 

Cryptosporidium cases in Canada

(Public Health Agency of Canada)