Comparative testing of anticoccidials in broiler chickens: The role of coccidial lesion scores

The relationship between oocyst dose and lesion score was evaluated in tria ls involving five field isolates each of Eimeria acervulina, Eimeria maxima , and Eimeria tenella. Each trial included an uninfected, unmedicated treat ment, and at least three treatments of unmedicated birds given different do ses of oocysts from a single isolate. In four trials each with E. acervulin a and E. tenella, and all five trials with E. maxima, infected, salinomycin -medicated (60 ppm) treatments were included. Each treatment consisted of f ive cages with eight male broiler birds per cage using a randomized complet e block design. The relationship between oocyst dose and lesion score was e xamined within each coccidial species using the linear model: Y = beta(0) beta(1)(log(n) oocyst dose + 1). The results demonstrated that in unmedica ted birds, low oocyst doses caused mean lesion scores up to 2.0, but the nu mbers required to cause higher mean scores were many times greater. Second, the estimated oocyst dose in salinomycin-medicated birds for any given mea n lesion score was substantially more than the corresponding estimate for u nmedicated birds. These results indicated that there could be wide differen ces in levels of oocyst dose between unmedicated and medicated birds that l esion scores failed to measure. If lesion scores are used in trials compari ng anticoccidial drugs, an alternative design may be to include three infec ted, unmedicated treatments each given a different level of inoculum (e.g., low, medium, and high). Medicated treatments, given the highest oocyst dos e only, would then be compared to each of the infected, unmedicated treatme nts.