Cystoisospora belli (formerly Isospora belli)

Cystoisospora belli (formerly Isospora belli) is found worldwide, although is endemic mainly in tropical areas. It has been implicated in travelers' diarrhea but no big outbreaks have occurred in the U.S.

Oocysts that are passed in the feces typically require a few days to fully sporulate outside the body and become infective. Humans are apparently the only hosts of this organism; it is acquired by consumption of sporulated oocysts in contaminated food or water (see the life cycle to the right).

Symptoms include frequent diarrhea, which is watery or foamy, and eosinophilia. There is also evidence of malabsorption in some patients. Intestinal infection is usually transient in healthy individuals; immunosuppressed patients may continue with symptoms and shedding cysts for months. Even extraintestinal infections have been reported in immunosuppressed patients.

As far as diagnosis, Cystoisospora belli represents another exception to the rule as direct wet prep examination is actually recommended (or wet prep of concentrate). Oocysts are the diagnostic stage and are quite large - 20-30 μm. In permanent stained smears they tend to take up too much dye, and because of their large size, might look like helminth eggs. Oocysts can also be stained with acid-fast stains including auramine-rhodamine stains. They will also auto-fluoresce (see top, right image with an oocyst containing one sporocyst, and an oocyst containing two sporocysts).

Molecular methods of diagnosis have been developed but are not widely used.