Pneumocystosis

Pneumocystosis is a pulmonary infection in mammals caused by fungi belonging to the genus Pneumocystis. Members of the Pneumocystis are so named because of their affinity to the lungs and ability to form cysts (asci) in the lungs. While considered as zoonoses in the past, cross infection and molecular studies have demonstrated that these organisms have an extremely narrow host range, making them species-specific pathogens. Animal studies are limited to case reports from dogs, rabbits, monkeys, pigs, and cats. The disease itself is self-limiting or asymptomatic in healthy individuals. However, in immunocompromised hosts, Pneumocystis causes interstitial pneumonia with alveolar exudate described as honeycomb on histological examination. HIV, steroid therapy, immunosuppression following an organ transplant, hematological malignancies, solid tumors, and autoimmune disorders are major risk factors for disease. Although its prevalence is declining in developed countries following the introduction of antiretroviral therapy, it is expected to be high in developing countries without access to diagnostics and antiretroviral treatments. P. jirovecii is acquired by inhalation and may cause outbreaks in centers catering to patients with deficient immunity. Diagnosis is achieved using microscopic methods like immunofluorescence and silver staining. PCR is also widely recognized as a valuable tool in diagnosis and can be applied for typing and antimicrobial resistance detection. Trimethoprim/sulfamethoxazole is the drug of choice for prevention and curative therapy. Despite the best care, mortality rates are high, with a case fatality rate of 50% and in non-HIV patients and 15% in HIV-infected patients. The inability to culture the organism in a reliable manner has hindered our ability to understand the pathogenesis and develop diagnostics and effective vaccines.