Cystoisospora belli (formerly Isospora belli) is found worldwide, although it is endemic, mainly in tropical areas. It has been implicated in travelers' diarrhea, but no significant outbreaks have occurred in the U.S.
Oocysts passed in the feces typically require a few days to sporulate outside the body and become fully infective. Humans are the only hosts of this organism; it is acquired by consuming sporulated oocysts in contaminated food or water (see the life cycle to the right).
Symptoms include frequent watery or foamy diarrhea and eosinophilia. Some patients also show evidence of malabsorption. Intestinal infection is usually transient in healthy individuals; immunosuppressed patients may continue with symptoms and shedding cysts for months. Extraintestinal infections have even been reported in immunosuppressed patients.
Regarding diagnosis, Cystoisospora belli represents another exception to the rule, as direct wet prep examination is recommended (or wet prep of concentrate). Oocysts are in the diagnostic stage and are large, 20–30 μm. In permanent stained smears, they tend to take up too much dye and might look like helminth eggs because of their size. 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.
