Sensitivity of field isolates of Eimeria to monensin following the use of a coccidiosis vaccine in broiler chickens.

The sensitivity of field isolates of Eimeria to monensin was investigated following the use of a commercial coccidiosis vaccine. Monensin was not effective against isolates from five commercial farms where ionophores had been used for many flocks. Monensin was more effective, however, against isolates obtained from the same farms following the use of a coccidiosis vaccine. No further improvement in drug efficacy was noted after five successive flocks in which the vaccine was used.