With the summer swimming season in full swing, a new Centers for Disease Control and Prevention report is urging the public to safeguard against a fecal parasite that can survive in chlorinated water for up to seven days and is a leading cause of water-linked diarrhea.

The CDC said the annual number of reported outbreaks of cryptosporidiosis, a watery diarrhea that can last up to three weeks in some patients, has climbed an average of about 13 per cent each year. A total of 444 outbreaks, defined as two or more cases linked to a common source, were reported between 2009 and 2017, totalling 7,465 cases voluntarily reported by public health officials from 40 states and Puerto Rico.

In Canada, there have been a total of 13,451 reported cases between 2000, when cryptosporidiosis was added to the list of notifiable diseases, and 2016. There has been a near-steady increase since 2011, when 586 cases were reported, and 2016, when 968 cases were reported. More recent data was not immediately available.

The cryptosporidium parasite lives in the intestine of infected people and animals, and is found all over the world. It can be life-threatening for those with weak immune systems, such as the elderly or cancer patients. A bowel movement from an infected person or animal can release millions of crypto parasites.

Infection occurs through ingestion and is most commonly contracted through swallowing contaminated water, such as from swimming pools or splash pads. This accounts for more than a third of the outbreaks and more than half the cases documented by the CDC. Children between the ages of one and four are disproportionately affected. Contact with infected animals such as cattle, or someone infected in a daycare environment, were other major sources of outbreaks and cases, according to the CDC.

The CDC says good hygiene habits are an important measure in preventing a crypto infection:

• Wash hands before preparing or eating food, after using the toilet, changing diapers, helping a child use the toilet, aiding someone sick with diarrhea, touching an animal, and even after gardening

• Children with diarrhea should not attend daycare and should be kept home

• At the pool, do not swim for at least two weeks after diarrhea stops if you are diagnosed with crypto, do not swallow pool water, take children on bathroom breaks every hour, or check their diapers every 30-60 minutes

The CDC noted that alcohol-based hand sanitizers were not effective against cryptosporidium.

While the overall numbers in Canada have mostly remained under 900 cases per year, more than 1,700 were reported in 2001. That year, thousands of residents in North Battleford, Sask., became sick with flu-like symptoms after the crypto parasite contaminated the city’s drinking water. No deaths were reported, however.

The worst documented outbreak of its kind in U.S. history occurred in Milwaukee in 1993, when an estimate 403,000 people in the region were reportedly infected and at least 69 died after the parasite contaminated the city’s water.