jirovecii

Pneumocystis jirovecii is an obligate fungal pathogen of the genus Pneumocystis. It is an opportunistic organism primarily affecting the human lung and is best known for causing Pneumocystis pneumonia (PCP). The species name jirovecii honors the Czech parasitologist Otto Jirovec; the organism was historically misclassified and has since been placed in the fungal kingdom, within the Pneumocystidaceae family.

Epidemiology and transmission: P. jirovecii is found worldwide and humans are considered the natural host. Many individuals harbor subclinical colonization, but disease occurs most often in people with impaired cellular immunity, such as those with HIV/AIDS, organ transplantation, or receiving high-dose corticosteroids or other immunosuppressants. Transmission is thought to occur by inhalation of airborne cysts; person-to-person spread is considered possible but not fully established.

Clinical presentation: PCP typically presents with subacute to progressive dyspnea, nonproductive cough, fever, and hypoxemia. Imaging often shows diffuse bilateral interstitial infiltrates; computed tomography may reveal ground-glass opacities. Severity ranges from mild to life-threatening respiratory failure, particularly in severely immune-suppressed patients.

Diagnosis: Diagnosis relies on identification of the organism in respiratory specimens, usually via bronchoalveolar lavage or induced sputum using special stains or immunofluorescence assays, or by PCR-based methods. Elevated serum lactate dehydrogenase can occur but is non-specific. The clinical context and imaging findings support the diagnosis.

Treatment and prevention: First-line therapy is high-dose trimethoprim-sulfamethoxazole for about 21 days, with adjunctive corticosteroids recommended for significant hypoxemia. Alternatives include pentamidine, atovaquone, or combinations such as primaquine with clindamycin for those with drug intolerance. Prophylaxis is advised for high-risk individuals (e.g., HIV patients with CD4 count <200 cells/µL) and may use TMP-SMX, dapsone, atovaquone, or aerosolized pentamidine.