Cryptosporidiosis is a diarrheal disease caused by several species of the genus Cryptosporidium, which are single-celled parasites. The disease can cause severe diarrhea. It tends to make the news when outbreaks occur associated with contaminated drinking water, but outbreaks only account for the minority of cases reported each year. Infection can be very serious (even fatal) in immunocompromised individuals, such as HIV/AIDS patients, but even immunocompetent (i.e. otherwise healthy) people can get sick. Different animals have their own species of Cryptosporidium. It is still unclear which of these species can normally infect people as well, and make them sick. Frequently cats and dogs with Cryptosporidium don’t look sick. Here are some facts:
- Cryptosporidium hominis primarily infects humans. Clearly it can make people sick, whether their immune systems are weakened or not.
- Cryptosporidium parvum primarily infects calves, and clearly makes people (and calves) sick. However, because it is relatively common in people as well, in many cases it is hard to say if a person with C. parvum was infected by contact with calf stool or human sewage.
- Both the dog-associated C. canis and cat-associated C. felis have been found in people, and C. felis can cause diarrhea even in immunocompetent individuals. Infection with these species in humans is very uncommon compared to C. hominis and C. parvum
- The largest outbreak of cryptosporidiosis ever reported in North America occurred in Milwaukee in 1993, when an estimated 1.6 million people were exposed to the parasite and over 400 000 people became sick as a result of the infection.
- In most studies, contact with pets is either not associated with the risk of cryptosporidiosis or may even have a slight protective effect. One study showed no significant association between pet ownership and cryptosporidiosis in HIV patients.
A scientific review of the transmission of Cryptosporidium (and Giardia, another common waterborne parasite) is available from the International Journal of Parasitology. For more information on Cryptosporidium in pets, check out the information sheets on the Worms & Germs Resources page.