Cryptosporidium parvum - Parasite
A Protozoan that Gets Into Drinking Water and Swimming Pools
Cryptosporidium parvum is a tiny parasite that causes outbreaks of diarrhea. Drinking water is one source of this parasitic disease, but there are others.
Cryptosporidium parvum, a protozoan parasite, is becoming a familiar cause of outbreaks of diarrhea. When it contaminates a municipal water supply, it can make many people sick at once— it does this quite often because of its small size and its ability to survive chlorination.
A Cryptosporidium parvum oocyst (pronounced oo-oo-cyst), 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.
Municipal water filtration systems famously fail—more than 300,000 people got cryptosporidiosis in Milwaukee in 1993 due to inadequate treatment and filtration. More recently, thousands of people in Galway, Ireland got the parasitic disease from their water supply. 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?
Livestock and cryptosporidium
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 after heavy rains is an important source of cryptosporidium in surface waters. Ranched elk and bison also spread the parasite.
Sewage and cryptosporidium
Untreated sewage from human communities often contains cryptosporidium oocysts. When sewage effluent is discharged into bodies of water without proper treatment, as it frequently is after rainfall when treatment plants are overwhelmed, oocysts are discharged with it.
Wild animals and cryptosporidium
Many species of wild animals can be infected with Cryptosporidium parvum, the same 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
Cryptosporidium oocysts are infective as soon as they are passed in stool. Thus, an infected person can pass on the parasite on dirty hands or objects contaminated with feces. Likewise, infected animals can pass the infection directly to other animals or to humans.
Swimming pools and cryptosporidium
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. Chlorine doesn’t kill the oocysts and pool filtration systems cannot remove the oocysts—or at least not fast enough to prevent swimmers from swallowing some with a mouthful of water.
Shellfish and cryptosporidium
Oysters feed by filtering nutrients from the water around them. Cryptosporidium oocysts 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. Eating raw oysters or other raw shellfish is a potential source of cryptosporidium.
Though it is more common in warm climates, Cryptosporidium parvum is found in surface water everywhere—never drink untreated water and heed any boil water advisory issued by your local water utility.
E. coli 0157 and Giardia lamblia have also contaminated municipal water supplies.
Sources
Giardia sp. Cysts and Infectious Cryptosporidium parvum Oocysts in the Feces of Migratory Canada Geese (Branta canadensis). Graczyk, T K, R Fayer, J M Trout et al. Applied and Environmental Microbiology 1998 Jul; 64(7), pp. 2736-8.
Foundations of Parasitology 6th Ed. Roberts, Larry S. and John Janovy Jr. Boston: McGraw Hill, 2000.
"Relationship Between Beef Production and Waterborne Parasites (Cryptosporidium spp. and Giardia spp.) in the North Saskatchewan River Basin." Agriculture, Food and Rural Development. Alberta Government Jul 2002.
