Limited effect of trimethoprim-sulfamethoxazole prophylaxis on Pneumocystis carinii.

Trimethoprim-sulfamethoxazole has been proven effective in the treatment and prevention of Pneumocystis carinii pneumonitis in lower animals and humans. How effective the drug combination is in eradicating P. carinii from the host is not known. The immunosuppressed rat model was used to determine whether or not trimethoprim-sulfamethoxazole effectively eradicated the organism. Animals treated with trimethoprim-sulfamethoxazole for as long as 6 weeks were then placed in individual isolator cages, immunosuppressed with prednisone for 12 weeks, and sacrificed. P. carinii was found in the lungs of at least 90% of the drug-treated as well as untreated control groups. The data indicate that trimethoprim-sulfamethoxazole has a limited rather than a lethal effect on P. carinii and that protection is afforded only during the period of trimethoprim-sulfamethoxazole administration.