Efficacy of toltrazuril and clazuril against experimental infections with Eimeria labbeana and E. columbarum in racing pigeons

The efficacy of toltrazuril in comparison with clazuril on heavy experimental Eimeria labbeana and E. columbarum infections in pigeons was investigated. The minimum required dose of toltrazuril to completely suppress oocyst excretion is 20 mg/kg body weight for 1 day. After treatment with toltrazuril, there was a dose-dependent period during which pigeons remained negative; at a dosage of 35 mg/kg or higher, this period was at least 4 weeks. Clazuril at the recommended dose of 2.5 mg/pigeon resulted in a rapid suppression of oocyst excretion, but oocysts were again observed in the feces 20 days after treatment. The fact that oocysts appeared again in the feces 3 to 5 weeks after treatment can probably be explained only by a persistence of endogenous stages, which are not affected during treatment, rather than by a reinfection. Reinfection of the previously medicated and infected pigeons 30 days after the first Infection provided some data on possible induced immunity.