Isosporiasis is a disease caused by the protozoan Isospora belli. The organism infects the lining of the small intestine, and can cause severe diarrhea and malabsorption (an inability to absorb nutrients).

Isospora belli is spread by feces. Food or water contaminated with animal feces may carry this organism; it’s also possible that oral-anal sex may spread the infection. On occasion, there are outbreaks of isosporiasis that can be traced to a feces-contaminated water supply. Isosporiasis is quite rare in the United States; it is most common in tropical parts of the world and places where water contamination is a problem. In the U.S., isosporiasis is an initial AIDS-defining illness in approximately 0.2 percent of patients with AIDS.

People with compromised immune systems—usually people with a CD4 cell count below 150—may experience prolonged and severe bouts of diarrhea and malabsorption that can be difficult to treat. It’s also important to note that not all people exposed to Isospora belli, even if their immune systems are suppressed, experience symptoms of the infection.


What are the symptoms, and how is it diagnosed?

Watery diarrhea is a primary symptom of isosporiasis, along with abdominal pain, weight loss, loss of appetite, dehydration, and passing gas (flatulence). 

Most cases of isosporiasis can be diagnosed using a stool sample. To do so, the stool is stained with a dye and examined under a microscope. Another way to diagnose isosporiasis is by endoscopy or colonoscopy. These procedures use thin, long cameras inserted down the throat or through the anus to examine the small (and large) intestine. Samples of intestinal tissue are collected and examined in a lab.


How is it treated or prevented?

The most effective treatment for isosporiasis is a combination of two drugs: trimethoprim and sulfamethoxazole (TMP-SMX; Bactrim, Septra). To treat isosporiasis, two double-strength TMP-SMX pills are taken twice a day. An alternative is one double-strength pill three times a day. TMP-SMX treatment is usually continued for two to four weeks.

Unfortunately, between 25 percent and 50 percent of HIV-positive people are allergic to the sulfur in the SMX half of TMP-SMX. Two of the main symptoms seen in people with allergic reactions to SMX are fever and rash. Very often, the allergy can be so severe that people need to stop taking SMX.

For those who cannot tolerate SMX, the drug pyrimethamine (Daraprim), combined with folinic acid, can be taken. This combination of drugs is used for a month.


To help control the diarrhea, perhaps in combination with antibiotic therapy, a number of anti-diarrheal drugs can be taken. This include: diphenoxylate (Lomotil), loperamide (Imodium), paregoric, and Pepto-Bismol. And because diarrhea is the direct result of intestinal inflammation caused by the infection, some non-steroidal anti-inflammatory drugs (NSAIDS) may be helpful, such as ibuprofen (e.g., Advil). Another drug that has been shown to greatly reduce diarrhea, due to its anti-inflammatory activity, is thalidomide (Thalomid). Women who take this drug should avoid becoming pregnant; thalidomide can cause severe birth defects.

The most effective way to prevent isosporiasis is to avoid its sources —mainly potentially contaminated foods or human feces. This is particularly true for HIV-positive people with compromised immune systems raveling to tropical and subtropical countries where water and food could be contaminated. Drinking bottled water and making sure that food is cooked properly can help reduce the risk of isosporiasis while traveling to tropical and subtropical areas.

Drugs used to prevent isosporiasis (prophylaxis) are, for the most part, the same as those used to prevent Pneumocystis pneumonia (PCP). Trimethoprim-Sulfamethoxazole (TMP-SMX; Bactrim, Septra) is the most effective combination of drugs used to prevent PCP and to treat isosporiasis. And because PCP prophylaxis is generally started when a person’s CD4 cell count falls below 200, he or she should be well protected against Isospora belli in the event he or she is exposed to this protozoan.

Bringing or keeping CD4 counts above 200 with antiretroviral (ARV) therapy is also likely to help prevent isosporiasis. A syndrome—called immune reconstitution inflammatory syndrome (IRIS)—where antiretroviral treatment can actually exacerbate the symptoms of an opportunistic infection due to a strengthened immune response, has not been reported with isosporiasis.


Are there any experimental treatments in development for isosporiasis?

If you would like to find out if you are eligible for any clinical trials involving new treatments for isosporiasis, visit, a site run by the U.S. National Institutes of Health. The site has information about all HIV-related clinical studies in the United States. For more info, you can call their toll-free number at 1-800-HIV-0440 (1-800-448-0440) or email

Last Reviewed: January 18, 2016