The diffuse bilateral pneumonitis caused by Pneumocystis jirovecii (also called Pneumocystis carinii) occurs almost exclusively in immunocompromised patients with cancer, organ transplantation, congenital immunodeficiency disorders, and acquired immunodeficiency syndrome (AIDS). The organism is known to cause pneumonitis occasionally in patients without underlying immunodeficiency. CLINICAL FEATURES: The clinical manifestations of Pneumocystis pneumonitis are fever, cough, tachypnea, and dyspnea progressing to cyanosis. The onset may be abrupt or subtle. With an abrupt onset, fever, marked increase in respiratory rate, and severe dyspnea occur within 24 to 48 hours. The disease progresses rapidly with marked decrease in arterial oxygen tension (PaO2) and increase in alveolar-arterial oxygen gradient. The chest radiograph shows bilateral diffuse alveolar disease with an air bronchogram. Without treatment, the disease worsens and within a month all patients will have died. Even in fatal cases and even in the most severely compromised host, the organism and the disease remain localized to the lung, with rare exception. When specific and supportive treatment is introduced early in the disease, the mortality rate can be reduced to around 10% in most medical centers. Abrupt onset tends to occur in patients with cancer, organ transplantation, and AIDS.
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