Merck Manual

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Chelsea Marie

, PhD, University of Virginia;

William A. Petri, Jr

, MD, PhD, University of Virginia School of Medicine

Reviewed/Revised Oct 2022 | Modified Jan 2023
Topic Resources

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). In addition to examinations, doctors can use endoscopy to do biopsies and give treatment. Endoscopes... 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.

In people with AIDS More severe symptoms Human immunodeficiency virus (HIV) infection is a viral infection that progressively destroys certain white blood cells and is treated with antiretroviral medications. If untreated, it can cause... read more More severe symptoms , it is very important to treat HIV infection Antiretroviral Treatment of Human Immunodeficiency Virus (HIV) Infection Antiretroviral medications used to treat human immunodeficiency virus (HIV) infection aim to do the following: Reduce the amount of HIV RNA (viral load) in the blood to an undetectable amount... read more 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.

Drugs Mentioned In This Article

Generic Name Select Brand Names
Primsol, Proloprim, TRIMPEX
No brand name available
Cetraxal , Ciloxan, Cipro, Cipro XR, OTIPRIO, Proquin XR
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