Drug resistance in coccidia: a robenidine-resistant strain of eimeria tenella.

Tests for resistance to a recently introduced anticoccidial drug, robenidine, were performed on a strain of Eimeria tenella recovered from broiler chickens infected with acute cecal coccidiosis. The strain was identified previously and again confirmed by the timing of mortality of the infected chickens (five to six days postinfection), the appearance of lesions and parasites in ceca only, and the measurements (21.6 +/- 1.9 micron X 19.8 +/- 2.4 micron) of the oocysts. The tests showed that the strain of E. tenella could establish infections in chickens medicated with fourfold the recommended level of robenidine in feed but no oocysts could be recovered from the feces of infected chickens when the level of drug was increased to eightfold. The decrease in mortality, in the severity of lesions and oocyst output and the increase in the average weight gain of infected chickens followed closely the increase in the level of robenidine in feed. In contrast, no infections were found in chickens infected with a sensitive strain of E. tenella and maintained on feed mixed with the recommended level of robenidine. This and other findings discussed here show that resistance to robenidine is developing in coccidia commonly found in broiler houses.