Pneumocystis carinii infection in human immunodeficiency virus-positive patients

Pneumocystis carinii is a ubiquitous, atypical unicellular fungus. P. carinii pneumonia (PCP) is responsible for considerable morbidity and mortality in acquired immune deficiency syndrome (AIDS) patients, and is the leading complication in advanced human immunodeficiency virus (HIV) infection. Many different host (mammal)-specific species of Pneumocystis exist, but the life-cycle is not understood fully. Human strains are designated as P. carinii f. sp. (special form) hominis (at least 59 different types). P. carinii is spread via the airborne route. Disease is most frequently caused by fresh exposure to a source of P. carinii, rather than by reactivation of latent infection. Asymptomatic carriage among healthy persons may occur. PCP occurs in HIV-infected patients when the CD4+ count falls below a certain threshold; organisms multiply and gradually fill the alveoli. Symptoms, which include a mildly productive cough, progressive dyspnoea and fever, may persist for months prior to diagnosis. Without treatment, progressive respiratory insufficiency invariably ends in death. Pulmonary specimens may be obtained by procedures of varying sensitivity and risk. Diagnosis is usually confirmed by detection of stained organisms; however, staining procedures vary in sensitivity and ease of use. Robust polymerase chain reaction (PCR) protocols with good predictive results may be useful in the future. Therapy falls into two categories: for acute primary infections and for prophylaxis. A confirmed diagnosis ensures that patients do not receive potentially toxic medication (adverse drug reactions can occur). Prophylaxis can dramatically reduce the frequency of PCP in HIV patients, and its more widespread use should lead to a decline in the incidence of PCP in the future.