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Richard D. Pearson

, MD, University of Virginia School of Medicine

Last full review/revision May 2020| Content last modified May 2020
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Cystoisosporiasis is infection with the parasite Cystoisospora (Isospora) belli. The main symptoms are watery diarrhea with abdominal cramping and nausea.

  • People can become infected with Cystoisospora by consuming food or water contaminated with the parasite.

  • Cystoisosporiasis symptoms are more severe and prolonged in people with a weakened immune system, such as people with AIDS.

  • Symptoms vary but include watery diarrhea, abdominal cramping, fever, and weight loss.

  • Doctors usually diagnose the infection by identifying Cystoisospora in a sample of stool.

  • Cystoisosporiasis can be treated with trimethoprim/sulfamethoxazole (TMP/SMX).

Cystoisosporiasis is most common in tropical and subtropical climates. When encountered in the United States, it is usually in immigrants or travelers with exposure in endemic areas.

Symptoms of Cystoisosporiasis

The primary symptom of cystoisosporiasis is sudden, nonbloody, watery diarrhea. Other symptoms include fever, abdominal cramps, nausea, fatigue, and weight loss. Symptoms in people with a normal immune system usually last weeks, then subside.

In people with a weakened immune system, including people with AIDS, cystoisosporiasis may cause severe diarrhea that may persist for a long time.

Diagnosis of Cystoisosporiasis

  • Stool tests

To diagnose cystoisosporiasis, a stool sample is examined under a microscope for Cystoisospora eggs. Specialized techniques can be used to increase the chances of identifying the eggs.

When stool analysis does not identify a cause for persistent diarrhea, doctors may use a flexible viewing tube (endoscope) to examine the upper part of digestive tract. Doctors may use this procedure to obtain a sample of tissue to be examined and analyzed (biopsied). Cystoisospora, if present, can be seen in a biopsy sample from the intestine.

Prevention of Cystoisosporiasis

When traveling to tropical and subtropical areas where the infection is common, people should avoid eating uncooked foods, including salads and vegetables, and should avoid consuming potentially contaminated water and ice. Hand washing with soap and water is important. Drinking water that has been boiled is safe. Filtering water through a 0.1 or 0.4 micron filter can remove cysts of Cystoisospora and other parasites, as well as bacteria that cause diseases.

Treatment of Cystoisosporiasis

  • Trimethoprim/sulfamethoxazole (TMP/SMX)

Infected people who have symptoms can be treated with trimethoprim/sulfamethoxazole (TMP/SMX) taken by mouth.

In people with AIDS, it is very important that the HIV infection is treated as effectively as possible with antiretroviral drugs. Such treatment can strengthen the weakened immune system, which usually helps control the diarrhea and other symptoms. People with AIDS may need a higher dose of TMP/SMX and a longer course of treatment. Sometimes long-term TMP/SMX maintenance therapy is needed in people with HIV/AIDS.

For cystoisosporiasis, people who are allergic to (or intolerant of) TMP/SMX can be treated with pyrimethamine. Leucovorin is given concurrently to prevent anemia and low white blood count, which are side effects of pyrimethamine.

Ciprofloxacin has been used to treat cystoisosporiasis, but it is less effective than TMP/SMX.

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