Case 99
Seen here are Pneumocystis carinii organisms stained with the Gomori methenamine silver (GMS) stain. P.carinii is the cause for P. carinii pneumonia (PCP) which is a frequent opportunistic infection of AIDS patients, typically when the CD4 count is <200/µl. Carlos Chagas first saw the cysts of P. carinii in 1909 but thought they were a cystic form of Trypanosoma cruzi. A year later Dr. Antonio Carini observed the cysts in rats with experimental trypanosomiasis, but he suspected they were from an unknown organism and sent samples to a colleague, Dr. Charles Laveran, who named the new organism Pneumocystis carinii. P. carinii is most closely related to fungi, based upon molecular studies, but its appearance and pathogenicity are more like that of a protozoan. The symptoms of PCP are typically fever, cough, and dyspnea.
Until the 1980s, PCP was sporadically seen in immunocompromised patients undergoing chemotherapy, transplantation, or with primary immunodeficiencies. PCP became commonplace in immunocompromised patients with AIDS. The organism exists in the alveoli of the lung as trophozoites and as cysts. As the trophozoite matures, it becomes a cyst with 8 intracystic bodies that become trophozoites when the cyst ruptures. A Giemsa stain may demonstrate these intracystic bodies in cytologic preparations. However, the organism is best diagnosed from samples obtained via bronchoalveolar lavage with the GMS stain that demonstrates the cysts. Sputum samples and biopsies may also be employed for diagnosis. Direct fluorescent antibody (DFA) and immunohistochemical methods are also available to demonstrate the organism in tissue or cytologic samples. Culture cannot be performed.
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