ESSENTIALS OF DIAGNOSIS
Fever, dyspnea, dry cough, hypoxia.
Often only slight lung physical findings.
Chest radiograph: diffuse interstitial disease or normal.
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 distributed worldwide. Although symptomatic P jirovecii disease is rare in the general population, serologic evidence indicates that asymptomatic infections have occurred in most persons by a young age. Accumulating 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.
The overt infection is a subacute interstitial pneumonia that occurs among older children and adults who have an abnormal or altered cellular immunity, either due to an underlying disease process (eg, cancer, malnutrition, stem cell or organ transplantation or, most commonly, AIDS) or due to treatment with immunosuppressive medications (eg, corticosteroids or cytotoxic agents).
Pneumocystis pneumonia occurs in up to 80% of AIDS patients 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 non-AIDS patients 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 the radiologic findings; some patients have bibasilar crackles. Without treatment, the course is usually one of rapid deterioration and death. Adult patients may present with spontaneous pneumothorax, usually in patients with previous episodes or those receiving aerosolized pentamidine prophylaxis. Patients with AIDS will usually have other evidence of HIV-associated disease, including fever, fatigue, and weight loss, for weeks or months preceding the 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. Serologic tests are not helpful in diagnosis; measurement of serum (1,3)-beta-D-glucan levels has good sensitivity, although specificity is compromised by being positive 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. Polymerase chain reaction (PCR) of bronchoalveolar lavage is overly sensitive in that the test can be positive in colonized, noninfected persons; quantitative values may help with identifying infected patients, although precise cutoffs have not been ...