Resistance and cross-resistance studies with narasin, a new polyether antibiotic anticoccidial drug.

The Wisconsin (Wis) strain and a field strain (FS-196) of Eimeria tenella were propagated in the presence of 80 ppm narasin in an attempt to select for narasin resistance. Comparisons of the narasin sensitivity of the selected strains (Wis-NR and FS-196-NR) and of the parent strains (Wis and FS-196) from which they were derived revealed no resistance development. These findings agree with the results of similar studies with the polyether antibiotic anticoccidial drugs monensin and lasalocid and support the conclusion that chicken coccidia do not readily develop resistance to this class of anticoccidial drugs. In a separate series of experiments, the efficacy of narasin was evaluated against five E. tenella field strains, one each resistant to amprolium, clopidol, decoquinate, nicarbazin, and robenidine. Narasin effectively controlled infections that were inadequately controlled by the anticoccidial drugs to which the strains were resistant. There was no evidence of cross-resistance to narasin.