ESSENTIALS OF DIAGNOSIS
Fever, dyspnea, dry cough, hypoxia with exertion; often only slight lung physical findings.
Chest radiograph: diffuse interstitial disease or normal.
Detection of P jirovecii in sputum, bronchoalveolar lavage fluid, or lung tissue; PCR of bronchoalveolar lavage; (1,3)-beta-D-glucan in blood.
Pneumocystis jirovecii, the Pneumocystis species that affects humans, is found worldwide. Although symptomatic P jirovecii disease is rare in the general population, serologic data indicate that asymptomatic infections have occurred in most persons by a young age; evidence suggests airborne transmission. Following asymptomatic primary infection, latent and presumably inactive organisms are sparsely distributed in the alveoli. De novo infection and reactivation of latent disease likely contribute to the mechanism of symptomatic disease in older children and adults.
Overt infection is characterized by a subacute interstitial pneumonia that occurs among older children and adults with abnormal or altered cellular immunity, either due to an underlying condition (eg, AIDS, cancer, malnutrition, hematopoietic stem cell or solid organ transplantation, autoimmune disease) or treatment with immunosuppressive medications (eg, corticosteroids or cytotoxic agents).
Pneumocystis pneumonia occurs in up to 80% of patients with AIDS not receiving prophylaxis and is a major cause of morbidity and mortality. Its incidence increases in direct proportion to the fall in CD4 cells, with most cases occurring at CD4 cell counts less than 200/mcL. In patients without AIDS receiving immunosuppressive therapy, symptoms frequently begin after corticosteroids have been tapered or discontinued.
Findings are usually limited to the pulmonary parenchyma; extrapulmonary disease is reported rarely and usually occurs in individuals who are receiving aerosolized pentamidine prophylaxis. Onset may be subacute, characterized by dyspnea on exertion and nonproductive cough. Pulmonary physical findings may be slight and disproportionate to the degree of illness and radiologic findings; some patients have bibasilar crackles. Without treatment, the course is usually one of rapid deterioration and death. Patients may present with spontaneous pneumothorax, usually in patients with previous episodes or those receiving aerosolized pentamidine prophylaxis. Patients with AIDS will typically have other evidence of HIV-associated disease, including fever, fatigue, and weight loss, for weeks or months preceding the respiratory illness.
Arterial blood gas determinations usually show hypoxemia with hypocapnia but may be normal; however, rapid desaturation occurs if patients exercise before samples are drawn. Serum (1,3)-beta-D-glucan levels have reasonable sensitivity but lack specificity as elevated levels occur in other fungal infections. The organism cannot be cultured, and definitive diagnosis depends on morphologic demonstration of the organisms in respiratory specimens using specific stains, such as immunofluorescence. PCR of bronchoalveolar lavage is overly sensitive in that the test can be positive in colonized, uninfected persons; quantitative values may help with identifying infected patients, although precise cutoffs have not ...