Coccidian infections in AIDS. Toxoplasmosis, cryptosporidiosis, and isosporiasis.

Cryptosporidium sp. and Isospora belli are coccidian protozoan parasites that were long recognized as pathogens for many animal species. The medical community became acquainted with these organisms with the advent of AIDS. Both parasites are associated with persistent, debilitating enteritis and, in the case of Cryptosporidium, biliary tract involvement in patients with AIDS. For the immunocompetent host, infection with these two pathogens usually results in self-limited diarrhea. Cryptosporidiosis appears to occur more often than isosporiasis, but the true prevalence of both infections for various populations of humans is unknown. Clinically, cryptosporidiosis is indistinguishable from isosporiasis. Diagnosis is based on finding the acid-fast (red staining oocyst in stained fecal specimens). There is no known effective therapy for cryptosporidiosis, whereas patients with isosporiasis respond promptly to treatment with trimethoprim-sulfamethoxazole. Patients with AIDS and isosporiasis have a high relapse rate after achieving complete remission and therefore need to be maintained on suppressive therapy. Much more needs to be learned about these two fascinating, "newly recognized" parasites.