Effects of pyrimethamine and sulfadiazine on the intestinal development of Toxoplasma gondii in cats.

Treatment with combined pyrimethamine and sulfadiazine prolonged the pre-patent period and shortened the patent period of cyst-induced toxoplasmosis in cats. Oocyst excretion was inhibited in 3 of 4 cats treated intramuscularly with 2 mg/kg body weight pyrimethamine and 100 mg/kg body weight sulfadiazine but not in 4 cats treated with one-half that dosage. Treatment with 2.2 mg/kg body weight pyrimethamine and 132 mg/kg body weight sulfadiazine mixed in cat food did not inhibit oocyst production but prevented antibody formation. No parasites were found upon histological examination of intramuscularly treated cats on days 3, 5, and 7 after infection. Infection with Toxoplasma gondii induced immunity to an oocyst-producing reinfection in both control and treated cats.