Cystoisosporiasis (Isosporiasis) Symptoms and Treatment

Cystoisosporiasis (formerly known as isosporiasis) is an uncommon parasitic infection of the intestines classified by the U.S. Centers for Disease Control and Prevention (CDC) as an AIDS-defining condition. Worldwide prevalence is variable, with infections occurring most frequently in tropical or subtropical regions (particularly the Caribbean, Central America, and South Africa).

With the advent of combination antiretroviral therapy (ART), cystoisosporiasis is considered rare among people living with HIV in the developed world. However, occasional outbreaks have been reported in recent years, generally due to returning travelers or migrants from tropical regions.

Doctor examining abdomen of patient
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Causal Agent

Cystoisosporiasis is caused by Cystoisospora belli (C. belli), an intestinal parasite closely related to Toxoplasma gondii (T. gondii) and Cryptosporidium.

(T. gondii and Cryptosporidium is the causal agents for two other AIDS-defining conditions, toxoplasmosis of the brain and cryptosporidiosis, respectively.)

Mode of Transmission

Humans are the only known hosts for C. belli, the disease of which is spread through food or water that has been contaminated with feces from infected humans. Transmission through oral-anal sex ("rimming") is also possible.


Symptoms can last for weeks and include crampy abdominal pain and profuse watery diarrhea, accompanied by weakness and low-grade fever. For immune-compromised individuals, these symptoms can progress to dehydration, malnutrition or cachexia if left untreated.

In immune-competent individuals, C. belli infection is often asymptomatic.


The clinical presentation is indistinguishable from cryptosporidiosis and requires microscopic examination of the patient's stool sample (or, occasionally, a biopsy of the intestinal wall) to confirm the diagnosis.


Cystoisosporiasis is most often treated with the sulfa-based antibiotic, trimethoprim-sulfamethoxazole (TMP-SMZ).

In immune-competent individuals, cystoisosporiasis is generally a self-limiting illness and usually resolves within a few days of treatment. Immune-compromised persons with CD4 counts under 150 cells/µL generally respond less well and are prone to relapse once therapy is stopped. In such cases, life-long prophylaxis of TMP-SMZ may be indicated.


Cystoisosporiasis is endemic in Africa, Australia, the Caribbean, Latin America, and Southeast Asia. In some countries, such as Haiti, as many as 15% of people are infected with C. belli. Among people with advanced HIV (CD4 count under 200 cells/mL), the rate is even higher, hovering at around 40%.

International travel has facilitated the spread of the disease in other regions, with one outbreak noted in the Los Angeles area from 1985 to 1992. In this instance, infections were confirmed primarily in Hispanic neighborhoods and almost entirely among individuals who were classified as having AIDS. Prevalence ranged from between 5-7%.

More recently, residents of an Atlanta neighborhood were believed to have been infected with C. belli in and around July 2015, with one person reported to have returned from a trip to Kenya.

Rates in low-income, high-prevalence countries have been dramatically cut in recent years due to the widespread use of TMP-SMZ, a drug given as prophylaxis to prevent pneumocystis pneumonia (PCP) in people with HIV.

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By James Myhre & Dennis Sifris, MD
Dennis Sifris, MD, is an HIV specialist and Medical Director of LifeSense Disease Management. James Myhre is an American journalist and HIV educator.