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Pneumocystis jirovecii (pronounced “yee-row-vet-zee”), formerly known as Pneumocystis carinii, is an opportunistic pathogen that causes pneumonia in the immunocompromised individual. The initials “PCP” stood for Pneumocystis carinii pneumonia but were kept for ease of use after the organism was renamed. Disease occurs when both cellular and humoral immunity are impaired. Serologic studies have shown that Pneumocystis has a worldwide distribution but the prevalence of antibodies to specific antigens varies among different geographic regions. PCP first came to attention when it caused interstitial pneumonia in severely malnourished and premature infants in Central and Eastern Europe during World War II. Prior to the acquired immunodeficiency syndrome (AIDS) epidemic in the 1980s, fewer than 100 cases were reported annually in the United States. PCP is one of several life-threatening opportunistic infections in patients with human immunodeficiency virus (HIV) infection worldwide and is still the most common AIDS-defining illness in patients with advanced HIV infection. The decline in the number of PCP cases in the United States occurred after the introduction of anti-pneumocystis prophylaxis in 1989 and highly active antiretroviral therapy (HAART) in 1992. In patients without HIV infection, the incidence of PCP has increased in those being treated with immunosuppressive and chemotherapeutic agents and in hematopoietic stem cell (HSCT) and solid organ transplant recipients.
The taxonomic classification of the Pneumocystis genus and the organism’s name has changed throughout the years. In the 1980s, biochemical analysis identified the organism as a unicellular fungus. Pneumocystis jirovecii is found in three distinct morphologic stages: the trophozoite, in which it often exists in clusters, the sporozoite (precystic form), and the cyst, which contains several intracystic bodies (spores). The cyst is the diagnostic form of P. jirovecii and stains with Giemsa, Papanicolau, and Grocott methenamine silver nitrate (GMS) and immunocytochemical techniques using monoclonal antibodies. Giemsa- and Papanicolau-stained smears show indirect evidence of P. jirovecii infection by the demonstration of foamy exudates in the form of alveolar casts.