Pneumocystis

Pneumocystis is a genus of obligate intracellular fungi that cause Pneumocystis pneumonia (PCP) in immunocompromised hosts. The most important human pathogen is Pneumocystis jirovecii, historically referred to as Pneumocystis carinii f. sp. jirovecii; related species inhabit animals, such as P. carinii in rats.

Taxonomy and biology: Pneumocystis belongs to the phylum Ascomycota, class Pneumocystidomycetes, order Pneumocystidales, family Pneumocystidaceae. It cannot be cultured in routine fungal media. The organism exists as trophic forms and cysts, and is thought to be transmitted via airborne routes. Most people harbor some level of colonization, but disease arises primarily in people with cellular immune deficiency, especially low CD4+ T-cell counts.

Clinical features and diagnosis: PCP typically presents with subacute dyspnea, nonproductive cough, and fever. Chest imaging characteristically shows diffuse bilateral interstitial infiltrates. Diagnosis relies on visualization or detection of the organism in respiratory specimens, using special stains such as Gomori methenamine silver or toluidine blue O, or by PCR on induced sputum or bronchoalveolar lavage. Beta-D-glucan may be elevated; histology shows foamy, eosinophilic alveolar exudate containing cysts and trophic forms.

Treatment and prevention: First-line therapy is high-dose trimethoprim-sulfamethoxazole for 21 days, with adjunctive corticosteroids in patients with significant hypoxemia. Alternatives include pentamidine, atovaquone, clindamycin–primaquine, or dapsone–trimethoprim. Prophylaxis with TMP-SMX is recommended for high-risk groups, including HIV-infected individuals with CD4 counts below 200 cells/µL, and certain transplant or chemotherapy recipients; alternatives include dapsone, atovaquone, or aerosolized pentamidine.

Outlook and epidemiology: PCP remains a major opportunistic infection in AIDS and other immunocompromised populations worldwide. It does not occur in healthy hosts. The organism was first described in animals; the human-infecting species P. jirovecii was named in the late 20th century, reflecting its host specificity. Ongoing surveillance and preventive strategies have reduced PCP incidence in many regions, but the disease persists in settings with advanced immunosuppression.