Restoration of anticoccidial sensitivity to a commercial broiler chicken facility in Canada

ABSTRACT Increasing resistance of Eimeria species to anticoccidial medications is an issue in the broiler chicken industry. Using drug-sensitive strains in live-coccidiosis vaccines has been shown to improve anticoccidial effectiveness in US-based broiler production. In Canada, litter is removed between flocks, which differs from the US industry practice. Thus, we investigated the use of drug-sensitive vaccine strains in a Canadian broiler production facility with suspected anticoccidial resistance. Weekly oocyst shedding patterns from flocks before, during, and after vaccine seeding were collected; following the vaccine seeding, OPG counts from similar aged birds were lower than flocks prior to live coccidiosis vaccine use. Eimeria species isolates, collected before and after vaccine seeding, were used in two Anticoccidial Sensitivity Tests to evaluate their susceptibility to commercially available anticoccidial medications; a low-dose challenge to define parasite replication, and a high-dose challenge to monitor broiler performance. In both experiments, isolates collected after seeding were more susceptible to almost every anticoccidial evaluated compared to the isolates collected prior to seeding. These results demonstrate an improvement in sensitivity to many anticoccidials following the use of live-coccidiosis vaccines at this facility. However, the regulated removal of litter at the end of each flock required under Canadian management rules could limit the establishment of vaccine-strain Eimeria species in broiler facilities and could shorten the longevity of improved drug sensitivity observed in this study.