Chronic fatal pneumocystosis in nude mice.

A chronic pulmonary disease was encountered in nude mice of a barrier sustained colony, and Pneumocystis carinii was identified as the causative agent histopathologically as well as on impression smear preparations in the affected lungs. Fatal infection was seen only in old nude mice aged more than 6 months, while focal pulmonary lesions were developed without clinical signs in young adult nudes 2 to 3 months of age. The lesions produced in aged nude mice were characterized by propagation of mononuclear cells with the presence of foamy masses of P. carinii. Heterozygous littermates were much less susceptible to the infection but pneumocystic lesions could be produced readily by multiple treatment with immunosuppressants. The infection could be transmitted without immunosuppressant to non-infected nudes but not to heterozygous littermates after intranasal inoculation of affected tissue emulsion or by cage mating with severely affected nudes.