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Water-borne Pathogen Not Always What It Appears To Be

By Sharon Durham
April 23, 2002

The water-borne parasite Cryptosporidium parvum was thought to be a single species that infects humans and more than 150 animal species. Now Agricultural Research Service zoologist Ronald Fayer and his colleagues have described a unique species of this pathogen, C. canis, originally found in dogs.

C. canis can be transmitted by--and infect--dogs, humans and cattle. Scientists originally thought the new species was C. parvum. Identifying this and other Cryptosporidium species can help pinpoint potential sources of infection.

Cryptosporidium is a single-celled parasite that lives in the intestines of animals and people. This microscopic pathogen causes a disease called cryptosporidiosis, which is characterized by mild to life-threatening diarrhea. Disease is spread by a form of Cryptosporidium called an oocyst, which is excreted in the feces of infected humans and animals. The tough-walled oocysts survive under a wide range of environmental conditions.

Studies by Fayer and cooperators at the Animal Waste Pathogen Laboratory in Beltsville, Md., found that C. canis oocysts differ markedly at the molecular level from those in known species of Cryptosporidium.

Based on this and other research, scientists now believe C. parvum is not one woven cloth, but something of a quilt made of different blocks. Each block represents a biologically distinct and unique organism. Using powerful, new genetic tools, it is becoming clear that there are numerous Cryptosporidium species previously thought to be C. parvum. The slight genetic differences that distinguish one species from another have great implications for predicting which host species may become infected by the pathogen.

Other scientists have found, within the C. parvum classification, several unique genotypes associated with specific hosts such as humans, mice, pigs, marsupials, dogs and ferrets, based on genetic data.

ARS is the U.S. Department of Agriculture’s chief scientific research agency.

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