Eimeria tenella: experimental studies on the development of resistance to amprolium, clopidol and methyl benzoquate

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Abstract
The development of resistance by the Houghton strain of E. tenella to the anticoccidial drugs amprolium, clopidol and methyl benzoquate was studied. Resistance to amprolium and clopidol developed more readily in experiments where a large number of coccidia were exposed to the drug by increasing the number of oocysts in the inoculum or by increasing the number of birds [chickens] in the group. When 45 birds were given 2.0 .times. 106 oocysts, resistance to amprolium and clopidol appeared after 6 and 7 passages respectively. In previous experiments, under similar conditions, resistance to robenidine developed after 6 passages, suggesting little difference between these 3 drugs. Resistance to amprolium and clopidol arose gradually as the concentration of drug was increased, but resistance to methyl benzoquate appeared in a single step from sensitivity to high-level resistance. Amprolium and clopidol-resistant lines showed an 8-fold reduction in drug sensitivity. Attempts to measure the degree of resistance by calculation of the ED50 were unsuccessful.