Experimental evaluation of combined prophylaxis against murine pneumocystosis and toxoplasmosis.

Prophylactic efficacy of antimicrobial agents against pneumocystosis and toxoplasmosis was examined in a model of concurrent Pneumocystis carinii and Toxoplasma gondii infections in rats. Corticosteroid-treated rats naturally infected by P. carinii were challenged with the RH strain of T. gondii. Infection was assessed by counting P. carinii cysts in lung and by titration of T. gondii in tissues by tissue culture. Untreated rats died after challenge, with P. carinii infection in lungs and T. gondii infection in liver, spleen, lungs, and brain. In rats that received trimethoprim-sulfamethoxazole or pyrimethamine plus dapsone, T. gondii was eradicated and P. carinii pneumonia prevented. Roxithromycin, 200 or 400 mg/kg, provided significant protection against toxoplasmosis but had no efficacy against P. carinii. Atovaquone, 100 or 200 mg/kg, had only partial efficacy against pneumocystosis and toxoplasmosis. These results definitively confirm use of trimethoprim-sulfamethoxazole and pyrimethamine plus dapsone for prophylaxis against combined infection in immunocompromised hosts.