Natural mode of acquisition for de novo infection with Pneumocystis carinii.

Axenic rats maintained in germfree isolators were found to be free of Pneumocystis carinii after three months of immunosuppression with dexamethasone. This P. carinii-free rat model was used to identify the mode of acquisition of P. carinii from the natural environment. Germfree were exposed in selective manner to potential sources of P.carinii, including air, water, and food. Animals exposed in the isolator with filtered (sterile) air and to regular (unsterile) water and food did not acquire P. carinii. Rats exposed in open cages to room air but maintained on sterile water and food acquired the infection. Animals fed lung tissue infected with P. carinii did not acquire the infection. These findings show that P. carinii is naturally acquired as an airborne organism in a de novo infection.