Cystoisosporiasis FAQs

What is cystoisosporiasis?

Cystoisosporiasis is an intestinal disease caused by the microscopic parasite Cystoisospora belli. This is the same parasite that used to be called Isospora belli. The parasite can be spread by ingesting food or water that was contaminated with feces (stool) from an infected person.

In what areas of the world is Cystoisospora found?

Cystoisospora can be found worldwide. It is most common in tropical and subtropical areas.

How do people become infected with Cystoisospora?

People become infected by swallowing mature parasites, for example, by ingesting contaminated food or water. Infected people shed the immature form of the parasite in their feces. The parasite usually needs about 1 or 2 days in the environment (outside of people) to mature enough to infect someone else. In some settings, the parasite might mature in less than a day.

Can Cystoisospora be spread directly from one person to another?

Cystoisospora usually is spread indirectly, such as through contaminated food or water. This is because the parasite needs time to mature. However, oral-anal contact with an infected person might pose a risk for transmission.

What are the symptoms of Cystoisospora infection?

The most common symptom is watery diarrhea. Other symptoms can include abdominal pain, cramps, loss of appetite, nausea, vomiting, and fever. If untreated, people with weak immune systems, such as people with AIDS, may be at higher risk for severe or prolonged illness.

How is Cystoisospora infection diagnosed?

Cystoisospora is too small to be seen without a microscope. The infection is diagnosed by examining stool (fecal) specimens under a microscope. More than one specimen may need to be examined to find the parasite.

How is Cystoisospora infection treated?

The infection is treated with prescription antibiotics. The usual treatment is with trimethoprim-sulfamethoxazole, which is also known as Bactrim*, Septra*, or Cotrim*. People who have diarrhea should also rest and drink plenty of fluids.

How can Cystoisospora infection be prevented?

Avoiding food or water that might be contaminated with stool may help prevent infection. As always, good handwashing and personal-hygiene practices should be followed. Wash your hands with soap and warm water after using the toilet, changing diapers, and before handling food. Teach children the importance of washing hands to prevent infection.

More on: Handwashing

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This information is not meant to be used for self-diagnosis or as a substitute for consultation with a health care provider. If you have any questions about the parasites described above or think that you may have a parasitic infection, consult a health care provider.

*Use of trade names is for identification only and does not imply endorsement by the Public Health Service or by the U.S. Department of Health and Human Services.