Tolerance to imidocarb induced experimentally in tick-transmitted Babesia argentina.

Babesia argentina was repeatedly exposed to imidocarb by transmitting parasites from infected ticks in a series of 4 groups of nonsplenectomised calves that had been treated prophylactically at 2 mg/kg. As the number of exposures to imidocarb increased, the parasites more readily infected the treated calves, indicating increased tolerance to the drug. Tests comparing parasites not previously exposed to imidocarb with those exposed 4 times showed that a dose of 3 mg/kg imidocarb completely cured subclinical infections with non-exposed, but not exposed, parasites; a dose of 1 mg/kg controlled acute infections with either nonexposed or exposed parasites; and the virulence of the parasite was apparently unaffected by the exposures.