MSD Manual

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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.

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.

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 Endoscopy Endoscopy is an examination of internal structures using a flexible viewing tube (endoscope). Endoscopy can also be used to treat many disorders because doctors are able to pass instruments... read more ) 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.

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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Human Immunodeficiency Virus (HIV) Infection
Human immunodeficiency virus (HIV) infection is a viral infection that progressively destroys certain white blood cells. Untreated, it usually leads to acquired immunodeficiency syndrome (AIDS). Transmission of HIV requires contact with bodily fluid that contains the virus or cells infected with the virus. Which of the following is not a bodily fluid that commonly transmits HIV? 
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