Monensin sensitivity of recent field isolates of turkey coccidia.

The efficacy of monensin at concentrations of 60 and 100 ppm was evaluated against 22 isolates not exposed to monensin and 16 monensin-exposed field isolates of coccidia obtained from US and Canadian turkey flocks, respectively. Isolates not previously exposed to monensin were effectively controlled by monensin. However, 7 monensin-resistant isolates (predominantly Eimeria meleagrimitis) were independently isolated from turkey flocks in Ontario, Canada where monensin was being used as an anticoccidial. Subsequent sensitivity evaluations of two of these isolates revealed cross-resistance to lasolocid, narasin, and salinomycin. Evaluation of the stability of monensin resistance in one isolate suggested that monensin sensitivity was not restored after 10 generations of relaxed selection. Although the extent of monensin resistance among field isolates of turkey coccidia is unknown, these results provide the first unequivocal characterization of monensin resistance in field isolates of coccidia. Contrarily, after more than eight years of intensive use of monensin as an anticoccidial in US broiler production facilities, this type of resistance to the polyether antibiotic anticoccidials has not been encountered in chicken coccidia.