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

The relationship between oocyst dose and lesion score was evaluated in trials involving five field isolates each of Eimeria acervulina, Eimeria maxima, and Eimeria tenella. Each trial included an uninfected, unmedicated treatment, and at least three treatments of unmedicated birds given different doses of oocysts from a single isolate. In four trials each with E. acervulina and E. tenella, and all five trials with E. maxima, infected, salinomycin-medicated (60 ppm) treatments were included. Each treatment consisted of five cages with eight male broiler birds per cage using a randomized complete block design. The relationship between oocyst dose and lesion score was examined within each coccidial species using the linear model: Y = beta0 + beta1(log(n) oocyst dose + 1). The results demonstrated that in unmedicated birds, low oocyst doses caused mean lesion scores up to 2.0, but the numbers required to cause higher mean scores were many times greater. Second, the estimated oocyst dose in salinomycin-medicated birds for any given mean lesion score was substantially more than the corresponding estimate for unmedicated birds. These results indicated that there could be wide differences in levels of oocyst dose between unmedicated and medicated birds that lesion scores failed to measure. If lesion scores are used in trials comparing anticoccidial drugs, an alternative design may be to include three infected, unmedicated treatments each given a different level of inoculum (e.g., low, medium, and high). Medicated treatments, given the highest oocyst dose only, would then be compared to each of the infected, unmedicated treatments.