The sensitivity of field isolates of Eimeria acervulina type to monensin.

The sensitivity of 32 field isolates of Eimeria acervulina to 100 ppm monensin was investigated in birds inoculated with 10(6) or 10(3) oocysts. Isolates were obtained from broiler farms where monensin had been used (broiler isolates) and from breeder farms where the drug had never been employed (breeder isolates). The drug was not completely effective against breeder isolates. When broiler and breeder isolates were compared, monensin was less effective against the former irrespective of whether weight gain or oocyst production was used to measure the infection. It is likely therefore that resistance to monensin may have been acquired as a result of its use in broiler chickens. The drug had activity against broiler isolates, however with more prolonged use, fully resistant strains may eventually emerge.