Pathological changes in epidemic porcine Pneumocystis carinii pneumonia.

An outbreak of epidemic porcine Pneumocystis carinii pneumonia on a "farrow-to-finish" pig farm was examined. The clinical disease was observed in weanling piglets, mostly in the seventh to eleventh weeks of life. Lesions of P. carinii pneumonia were detected histologically in the pigs at 36 to 75 days of age. These included pulmonary septal thickening, with mononuclear cell infiltration, swelling of the alveolar lining, an intra-alveolar exudate and the presence of P. carinii organisms. Focal pneumonia was seen at 36-43 days of age and diffuse pneumonia at 46-75 days. This outbreak resembled the infantile epidemic form of P. carinii pneumonia in man, in being age-related and in causing a non-suppurative intense inflammatory response in the lungs.