Cryptosporidium Outbreaks in the U.S., 2009–2017: Pools, Calves and Child Care

Cryptosporidium can survive more than seven days in a pool kept at the chlorine levels CDC recommends. That toughness — plus the enormous numbers of infectious parasites a sick person sheds — is why this parasite is the leading cause of U.S. outbreaks of diarrhea linked to water, and why its outbreaks keep growing.

The most common sources of Cryptosporidium outbreaks. Credit: CDC.

The illness

Cryptosporidiosis causes profuse, watery diarrhea that can last up to three weeks in healthy people and can lead to life-threatening malnutrition and wasting in people with weakened immune systems. It spreads by the fecal-oral route: swallowing contaminated recreational or drinking water or food, or contact with infected people or animals.

The numbers, 2009–2017

From 2009 to 2017, 40 states and Puerto Rico reported 444 outbreaks to CDC's National Outbreak Reporting System, with 7,465 cases, 287 hospitalizations and one death. The number of outbreaks reported each year rose an average of 12.8 percent per year.

Why it spreads so well

• An infected swimmer can release 10 million to 100 million oocysts (the parasite's infectious form) in a single diarrheal incident, while as few as 10 can cause infection.
• Oocysts are infectious as soon as they are shed and extremely tolerant of chlorine.
• Swimmers move between pools, and young children — often not yet toilet-trained, and prone to swallowing water — move between wading pools, water playgrounds and child care, so one outbreak can spread across a community.
• A calf can shed more than 10 billion oocysts a day, and outbreaks linked to cattle peak with spring calving.

Prevention

• Don't swim or go to child care while sick with diarrhea, and don't swim for two more weeks after it ends.
• Wash hands after contact with cattle, goats, sheep or their surroundings, and take off clothes and shoes worn around them before going into your home.
• Choose pasteurized milk and apple cider, especially for young children, pregnant women and people with weakened immune systems.
• After an outbreak, public pools need hyperchlorination, and child care surfaces need hydrogen peroxide, which inactivates the oocysts.

The reported outbreaks almost certainly understate the true number, and some of the increase may reflect wider use of multipathogen diagnostic tests.

Sources

Based on "Cryptosporidiosis Outbreaks — United States, 2009–2017," by Radhika Gharpure and colleagues, Morbidity and Mortality Weekly Report, Centers for Disease Control and Prevention; a work of the United States government in the public domain.