Selection for resistance to monensin, nicarbazin, and the monensin plus nicarbazin combination.

Two series of experiments were conducted to assess the relative ability of strains of Eimeria acervulina and Eimeria tenella to develop resistance to monensin (MON), nicarbazin (NIC), and the monensin plus nicarbazin combination (MON plus NIC). The studies were designed so that drug concentrations in the selection experiments were increased whenever possible. During selection, E. acervulina increased its reproductive index in the presence of NIC or MON plus NIC, equivalent selection in the presence of MON resulted in only a slight increase in reproductive ability. Eimeria tenella, however, was unable to increase its reproductive capacity to the respective drugs. Sensitivity tests after 60 generations of selection revealed that patterns of resistance development for E. acervulina and E. tenella corresponded with the changes in reproductive indices established in the selection experiments. Thus, results of these tests indicate that E. acervulina possesses the ability to develop resistance to NIC and MON plus NIC. Under essentially the same conditions of selection, E. tenella developed only partial resistance to the respective drugs.