Anticoccidial drug resistance in the southeastern United States: polyether, ionophorous drugs.

Coccidia were isolated from poultry farms in the U.S. and tested for sensitivity to the polyether, ionophorous drugs. Of 52 isolates, 24 were individual; Eimeria spp. and 28 were mixed species. Eimeria tenella, E. maxima, and E. acervulina were commonly present in mixed isolates. The isolates differed considerably in response to individual drugs and to the drugs as a group. All isolates responded to some extent, so none was judged completely resistant to the ionophores, but control of some isolates was poor, with up to 47% of medicated birds dying in one instance. Based on lesion score reduction, the best indicator of effectiveness, salinomycin provided the best overall control (64%), followed by lasalocid (56%) and monensin (52%). When weight gain was used as evidence of resistance, lasalocid provided the best control (97%), followed by salinomycin (90%) and monensin (81%). Examination of testing methods revealed that the arbitrarily selected endpoint should significantly affect th number of isolates designated resistant or sensitive.