Eimeria tenella, E. acervulina and E. maxima: studies on the development of resistance to diclazuril and other anticoccidial drugs in the chicken.

Resistance to diclazuril was induced by 10 passages of the Houghton strains of Eimeria acervulina and E. tenella in chickens given progressively greater concentrations of the drug. This resistance was, however, not complete since the drug retained some efficacy against the drug-passaged lines. Attempts to passage the Houghton strain of E. maxima in birds medicated with concentrations of diclazuril greater than 0.016 parts per million (ppm) were unsuccessful and after 10 passages at this concentration resistance had not developed. Resistance to methyl benzoquate developed after 6 passages of E. tenella in medicated chickens but resistance to amprolium was only partial after 10 passages. The pathogenicity of diclazuril- and amprolium-resistant lines of E. tenella was lower than that of the parental line. Diclazuril was effective against lines of E. tenella resistant to amprolium, arprinocid, clopidol, dinitolmide, halofuginone, methyl benzoquate, monensin and robenidine.