Pneumocystis carinii, recently classified as a fungus, inhabits the respiratory tracts of laboratory mice and rats. It is a pathogen only under conditions of induced or inherent immunodeficiency. Transmission is via inhalation of infective cysts. Placental transmission does not occur. Recent studies have demonstrated differences in host specificity and susceptibility.
Clinical signs are absent in immunocompetent animals.
Infection has been detected and clinical signs have been induced following several weeks of corticosteroid administration. Clinical signs in immunosuppressed or immunodeficient mice and rats include wasting, rough coat, dyspnea, cyanosis, and death. The lungs are enlarged, dark, and rubbery. Microscopic changes include alveolar septal thickening and alveolar filling with foamy, eosinophilic material consisting of organisms, dead host cells, serum protein, and pulmonary surfactant. Pneumonia may be exacerbated by the presence of coinfecting pneumotropic pathogens.