Sensitivity of field isolates of Eimeria species to monensin and lasalocid in the chicken.

Isolates of Eimeria species obtained from broiler or from breeder farms were compared for their sensitivity to two ionophorous anticoccidial drugs, monensin and lasalocid. All of 25 isolates from broiler farms were resistant to 100 ppm monensin or 90 ppm lasalocid, while 14 of 16 isolates were resistant to monensin and seven of 16 to lasalocid from breeder farms (replacement layer and broiler breeder).