Cystoisosporiasis (Isosporiasis) - Symptoms and Treatment

Tropical Parasitic Infection Classified as an AIDS Defining Condition
Photo credit: U.S. Centers for Disease Control and Prevention (CDC)


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.

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 are 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 of Cystoisosporiasis

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.

Diagnosis of Cystoisosporiasis

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 diagnosis.

Treatment of Cystoisosporiasis

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 with 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 is 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 facilitate 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.

Pronunciation: sis-to-eye-so-spore-EYE-uh-sis

Also Known As: Isoporiasis


U.S. Centers for Disease Control and Prevention (CDC). "Appendix A – AIDS Defining Conditions." Atlanta, Georgia; last reviewed November 20,2008.

Heyworth, M. "Parasitic diseases in immunocompromised hosts. Cryptosporidiosis, isosporiasis, and strongyloidiasis." Gastroenterology Clinics of North America. 1996; 25(3):691-707.

Lagrange-Xélot, M.; Porcher, R.; Sarfati, C.; et al. "Isosporiasis in patients with HIV infection in the highly active antiretroviral therapy era in France." HIV Medicine. February 2008; 9(2):126-130.

Guiguet, M.; Furco, A.; Tattevin, P.; et al. "HIV-associated Isospora belli infection: incidence and risk factors in the French Hospital Database on HIV." HIV Medicine. March 2007; 8(2):124-130.

Sorvillo, F.; Lieb, L.; Seidel, J.; et al. "Epidemiology of isosporiasis among persons with acquired immunodeficiency syndrome in Los Angeles County.American Journal of Tropical Medicine and Hygiene. December 1995; 53(6):656-9.

Petchenik, M. "Doctors believe parasite causing mysterious illness in Atlanta neighborhood." Dayton Daily New; published July 15, 2015. 

DeHovitz, J.; Pape, J.; Boncy, M.; et al. "Clinical manifestations and therapy of Isospora belli infection in patients with the acquired immunodeficiency syndrome.New England Journal of Medicine. July 10, 1986; 315(2):87-90.