Pneumocystis carinii infection in corticosteroid-treated cats

Corticosteroids were administered to produce Pneumocystis carinii infection in cats. Six of 10 cats, injected intramuscularly for 97-141 days with 2 mg/cat twice weekly of betamethasone sodium phosphate, developed a light infection with P. carinii. Six of 7 cats, injected intramuscularly for 11-168 days with 10-25 mg/cat weekly of prednisolone acetate, also developed a light infection with P. carinii. There was no significant difference in the infection rate between the sexes and ages of the cats. Using Giemsa staining and Gomori's methenamine silver nitrate stain, P. carinii organisms were indistinguishable morphologically from human and rat P. carinii. The cysts and trophozoites were usually present singly or in small groups, and they always were adhering to the periphery of alveoli. The inflammatory changes were inconspicuous except for the fact that alveolar macrophages often were seen. Corticosteroid-treated cats should be useful in the study of experimental P. carinii infection. This is the first reported case of experimentally induced P. carinii infection in cats.