I. Monensin sensitivity of recent field isolates of turkey coccidia. II. Experimental development of monensin resistance in Eimeria meleagrimitis

Jeffers, T.K.; Bentley, E.J.

Poultry Science 59(8): 1722-1735


ISSN/ISBN: 0032-5791
Accession: 000904612

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The efficacy of monensin at concentrations at 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 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 lasalocid, 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. These results provide the first unequivocal demonstration of monensin resistance in field isolates. After more than eight years of intensive use of monensin as an anticoccidial in US broiler flocks, this type of resistance has not been encountered in chicken coccidia. A strain of E. meleagrimitis from a monensin-sensitive parent strain was selected for monensin sensitivity by propagation in monensin medicated turkeys in isolation, while the parent strain was maintained as a control. Monensin resistance was demonstrated by the fourth generation. This is the first reported experimental development of monensin-resistant coccidia. Unlike fowl coccidia, turkey E. meleagrimitis possessed the ability to develop resistance to polyether anticoccidials.