Pneumocystis carinii infection in germ-free rats: implications for human patients.

Pneumocystis carinii infections have been detected both serologically and histologically in untreated, germ-free and conventional rats killed immediately upon arrival from two commercial sources. Pneumocystis antigenemia was detected by counterimmunoelectrophoresis (CIE) and antibody was titered by indirect immunofluorescence. The data suggest that 1) P. carinii is enzootic in certain rat colonies; 2) in utero transmission is a distinct possibility; and 3) paucity of cysts does not rule out P. carinii, as trophozoites predominate in early or sub-clinical infections. Histologic data support the validity of CIE for noninvasive detection of P. carinii antigen. Infection with this agent may be missed by basing diagnosis upon the presence of cyst forms alone, and it may be more common than previously supposed. These data have significant implications for the natural history, diagnosis, and epidemiology of P. carinii with regard to the human host.