Tuesday, 27 August 2013

Cryptosporidium: A Parasite That Gets Into Drinking Water and Swimming Pools

Swimming pools become contaminated with infective
oocysts of Cryptosporidium when ill people have
accidents in the water.
Image courtesy of the US Dept of State.
Cryptosporidium spp. are tiny parasites that cause outbreaks of diarrhea. Contaminated drinking water is a common source of cryptosporidiosis, but there are others, including swimming pools and food.

At least two species of Cryptosporidium infect humans, and they are increasingly familiar causes of outbreaks. In recent years, online searches have constantly turned up swimming pool closures and boil water orders attributed to Cryptosporidium. Such outbreaks may indeed be more common; we may be getting better at identifying the parasites in outbreaks; and it may be that Internet reports mean that more people hear of it – all three factors likely play a part in the raised profile of these parasites.

When Cryptosporidium contaminates a municipal water supply, it can make many people sick at once, and this happens relatively easily because of the parasite’s small size and its ability to survive chlorination.

An oocyst (pronounced oo-oo-cyst) of  Cryptosporidium sp., the infective stage of the organism, is spherical and only about three to five one-thousandths of a millimetre wide. Environmentally resistant, it survives cold, chlorination, and salt water. It’s found in surface waters all over the globe - municipalities that use surface water supplies must do more than chlorinate water to avoid an outbreak. Most rely on filtration.

In the summer of 2013, an outbreak of cryprosporidiosis in Baker City, Oregon highlighted the risks of unfiltered water supplies, even when the watershed appears pristine. Even municipal water filtration systems can famously fail - more than 300,000 people got cryptosporidiosis in Milwaukee in 1993 due to inadequate treatment and filtration. In terms of numbers, a contaminated water supply is the most common source of human infections, but how does Cryptosporidium get into the water, and how else can we catch it?

Cryptosporidium in Livestock

Dairy and beef cattle suffer from and spread Cryptosporidium parvum. Young calves catch it and suffer severe diarrhea, while older cattle continue to carry the parasite and spread it. Runoff from pastures into rivers and wells after heavy rain is an important source of Cryptosporidium in surface waters. Ranched elk and bison also spread the parasite.

Cryptosporidium in Human Sewage

Untreated sewage from human communities often contains oocysts of Cryptosporidium. When sewage effluent is discharged into bodies of water without proper treatment, as it frequently is, especially after rainfall when treatment plants are overwhelmed, oocysts are discharged with it.

Canada Geese and other water birds could potentially
spread Cryptosporidium from cattle pastures to
distant surface waters.
Image by Robert Lawton; CC BY-SA 2.5.

Cryptosporidium in Wild Animals

Many species of wild animals can be infected with Cryptosporidium parvum, one of the species that infects humans. Dogs, cats, goats and mice are among them. Although this does not appear to be a significant source of water contamination, migratory birds may be a different story.

Cryptosporidium parvum is known to pass unharmed through the gut of a Canada goose without making the bird sick. Thus a goose can ingest millions of oocysts while pecking corn kernels from cow dung in Maryland, and discharge them into a watershed in Pennsylvania. It’s not clear how much geese and other migratory birds contribute to the spread of Cryptosporidium.

Direct Person to Person Spread of Cryptosporidium

Oocysts of Cryptosporidium are infective as soon as they are passed in stool. Thus, an infected person can pass on the parasite with dirty hands or objects contaminated with feces. Likewise, infected animals can pass the infection directly to other animals or to humans.

Cryptosporidium in Swimming Pools

It’s fairly common for swimming pools to become contaminated with Cryptosporidium - sometimes people go swimming and have minor “accidents” in the water, or feces work their way out of leaky diapers. Chemical treatment of swimming pools must reach high concentrations in order to kill the oocysts and pool filtration systems cannot remove them —or at least not fast enough to prevent some swimmers from swallowing some with a mouthful of water.

Food and Cryptosporidium


Food items can potentially be contaminated with oocysts of Cryptosporidium, particularly produce that has been irrigated with contaminated water. Because of this, and other disease-causing organisms that may be present, produce that will be eaten raw should be thoroughly washed.

Oocysts of Cryptosporidium have been found in oysters along the eastern seaboard of North America where human sewage effluent and runoff from agricultural lands flows into the ocean, probably because . oysters feed by filtering nutrients from the water around them. Eating raw oysters or other raw shellfish is, therefore, a potential source of cryptosporidiosis.

Though it is more common in warm climates,  Cryptosporidium is found in surface water everywhere - never drink untreated water and heed any boil water advisory issued by your local water utility.


Alberta Government. “Relationship Between Beef Production and Waterborne Parasites (Cryptosporidium spp. and Giardia spp.) in the North Saskatchewan River Basin.” Agriculture, Food and Rural Development. Apr 2006.

Graczyk, T. K. et al. “Giardia sp. Cysts and Infectious Cryptosporidium parvum Oocysts in the Feces of Migratory Canada Geese (Branta canadensis).” Applied and Environmental Microbiology 1998 Jul; 64(7), pp. 2736-8.

Roberts, Larry S. and John Janovy Jr. Foundations of Parasitology 8th Ed. Boston: McGraw Hill, 2009.

Terrey. Lynn. “Goats Not Behind Baker City Parasite Suspected of Sickening Thousands, Officials Say.” Oregon Live: The Oregonian; Aug 21, 2013 Accessed Aug 21, 2013.

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