Of 67 infants enrolled in a prospective study of infant pneumonia ten (14%) had evidence of Pneumocystis carinii infection. Diagnosis was achieved by demonstrating circulating P carinii antigens by counterimmunoelectrophoresis in all ten cases and by histopathology in the only infant who underwent an open lung biopsy. Antigenemia did not occur in 64 control infants ( P = .003), nor in 57 patients of similar age who were hospitalized with pneumonitis due to Chlamydia trachomatis , respiratory syncytial virus, cytomegalovirus, adenovirus, and influenza A and influenza B viruses. None of the ten infants with P carinii pneumonitis had evidence of a primary immunodeficiency nor had any received immunosuppressive medication. These patients were hospitalized at a mean age of 6 weeks (range 2 to 12) and their illness was characterized by its afebrile course, presentation in crisis with severe respiratory distress, apnea, tachypnea, cough, increased IgM, and bilateral pulmonary infiltrates with hyperaeration. The clinical features of P carinii pneumonitis were indistinguishable from those of C trachomatis and cytomegalovirus pneumonia. Treatment with trimethoprim-sulfamethoxazole was associated with rapid disappearance of circulating antigens; however, the small number of patients studied did not permit an analysis of its clinical efficacy. These results indicate that P carinii singly or in combination with other infectious agents may be an important cause of pneumonitis in young, immunocompetent infants with no underlying illnesses.
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