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
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
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
Agricultures chief scientific research agency.