Immunization of Cats against Shedding of Toxoplasma Oocysts

Abstract

Development of immunity to the shedding of oocysts was examined in 75 kittens that survived infection with the three stages of Toxoplasma gondii. Of 16 kittens fed bradyzoites in cysts, 94% were immune and did not shed oocysts. Of seven injected with tachyzoites 86% were immune. Of 18 fed sporozoites only 11% were immune, but following injection, 54% of 12 were immune. After the administration of either bradyzoites or tachyzoites from nonoocyst-producing strains, only 9% of 22 were immune. Considering all inocula, immunity was present in 93% of kittens that had previously shed oocysts, 25% of those that only develop antibody, and none that had neither shed nor developed an antibody titer. After a second challenge with a different isolate, a similar percentage of immunity was observed. Infection with killed tachyzoites, alone, or together with Freund's complete or incomplete adjuvants was followed by immunity in only one of 24 kittens. Eighty-five percent of 13 kittens were immune, after they had been treated prophylactically with 200 mg/kg monensin, or 60 mg/kg cat sulfadiazine combined with 1 mg/kg cat of pyrimethamine; oocyst shedding had been suppressed in all. It is concluded that cats can be immunized against oocyst shedding by infections where oocysts are produced, or where developmental stages are suppressed by chemoprophylaxis, but not if enteroepithelial stages are absent, as in the oocyst-less strain examined.