Drug sensitivity of coccidia from broiler breeder pullets and from broilers in the same integrated company.

Coccidia isolated from 12 breeder pullet farms and 18 broiler farms were tested for sensitivity to anticoccidial drugs. The species Eimeria acervulina, E. maxima, and E. tenella were common in the isolates and were fully pathogenic. Broiler coccidia were generally resistant to Zoalene but sensitive to nicarbazin. About half of the isolates had reduced sensitivity to amprolium and monensin. Most of the breeder pullet coccidia were resistant to Zoalene and amprolium, but almost all were highly sensitive to nicarbazin and monensin. There was some difference in responses of broiler isolates to various ionophores. Isolates responded best to salinomycin and lasalocid, which had not been previously used on the farms. Monensin controlled coccidia slightly better when given at 120 ppm than when given at 100 ppm. These results suggest that broiler coccidia have lost some responsiveness to ionophores as a result of long-term exposure, but that breeder pullet coccidia, which have never been exposed to those drugs, have retained their sensitivity. Resistance or sensitivity to other drugs was consistent with history of use: more isolates were sensitive to older drugs that had not been used for several years than was reported in previous surveys.