Pathogenesis of acute toxoplasmosis in specific-pathogen-free cats.

Systemic toxoplasmosis was produced in specific-pathogen-free cats by intravenous inoculation with Toxoplasma gondii tachyzoites. Infectious organisms were recovered from all tissues studied, but the number of organisms recovered from liver, lungs and spleen was 10-fold to 10,000-fold higher than from heart and brain. The occurrence and severity of Toxoplasma-induced lesions correlated with the number of infectious organisms recovered from the various tissues. In nonlymphoid tissues, the Toxoplasma-associated lesions consisted of multifocal necrosis, usually with demonstrable organisms. Lesions in the spleen and mesenteric lymph nodes consisted of reticuloendothelial and lymphoid hyperplasia, with few demonstrable organisms. Pneumonitis was severe and sometimes fatal in the early stages of systemic toxoplasmosis. Light- and electron-microscopic studies showed that the earliest lung lesions were randomly distributed infiltrates of neutrophils, eosinophils, and mononuclear cells into alveolar walls. Later lesions were diffuse alveolar necrosis, pneumocytic hyperplasia, and extensive fibrinocellular exudates in alveoli. Tachyzoites were present in cytoplasmic vacuoles of fibroblasts, macrophages, type I and II pneumocytes, bronchiolar epithelial cells, bronchiolar smooth muscle cells, endothelial cells, neutrophils, and eosinophils, and circulating monocytes. Replication of organisms was found in all parasitized cell types except neutrophils and eosinophils.