Pathogenesis of Sarcocystis falcatula in the budgerigar. II. Pulmonary pathology.

Sarcocystis falcatula is a protozoan parasite which obligatorily alternates its definitive host, the opossum (Didelphis virginiana), and a uniquely wide range of avian intermediate hosts. The disease produced by this coccidian parasite in intermediate hosts was studied in the budgerigar bird (Melopsittacus undulatus). Early schizogony occurs in pulmonary endothelial cells, resulting in endothelial cell hypertrophy, capillary obstruction preceding endothelial lysis, acute endophlebitis, and chronic periphlebitis. These impede outflow of the pulmonary vascular system resulting in interstitial, subpneumocytic, and then air-space edema. The edema is associated with retraction and degeneration of squamous pneumocytic processes, loss of the myelinoid "surfactant" covering over the pneumocytes, and atelectasis of the respiratory labyrinth. Epithelial repair is effected by migration of granular pneumocytes (normally confined to the parabronchi and their atria) down the infundibula and into the respiratory labyrinth. This pattern of injury and repair is comparable to diffuse alveolar damage (acute interstitital pneumonitis) of mammals despite the markedly different structure of the avian lung. The implications of this peculiar tissue tropism are discussed.