The effects of monensin, narasin, salinomycin and nicarbazin, on field strains of chicken coccidia.

The sensitivity was established in vivo of 47 isolates of coccidia obtained from commercial broiler sites, to monensin, narasin, salinomycin and nicarbazin. Seventeen samples were obtained from the UK, 15 from the Netherlands, six from West Germany, six from Spain, and one each from France, Italy and Israel. Fourteen (30%) samples were predominantly Eimeria acervulina, 18 (38%) predominantly E. tenella. The remaining 15 (32%) were mixed species, predominantly E. acervulina and E. tenella with some samples showing lesions characteristic of E. maxima and E. brunetti. All isolates were ionophore-tolerant, as judged by lesions, weight gain, feed conversion and mortality. Most isolates were sensitive to nicarbazin. Cross-resistance was evident between all ionophores but not between ionophores and nicarabazin.