Abstract
A strain of Eimeria necatrix isolated in Miyazaki Prefecture, Japan, was examined for its sensitivity to 13 anticoccidials. Nine out of the 13 drugs were not efficacious, but three quinolone derivatives and robenidine were efficacious at their recommended dose-levels. Minimal effective concentration levels of decoquinate (one of the quinolone derivatives) and robenidine were approximately one-tenth of the recommended dose levels of munufacturers: 4 ppm for decoquinate and 4.125ppm for robenidine. Therefore, we examined the reduction in sensitivity of this strain to these drugs by serially passaging in chickens fed the ration containing suboptimal dose-levels of these anticoccidials. The strain deteriorated the sensitivity to minimal effective concentration levels of decoquinate and robenidine after 50 and 10 passages, respectively. However, the strain was still highly sensitive to recommended-dose levels of decoquinate and robenidine after 50 and 40 passages, respectively.
