Analysis of disease resistance-associated parameters in broiler chickens challenged with Eimeria maxima

To determine an optimal dose for coccidial inoculation and to evaluate genetic resistance or susceptibility in individual chickens, broilers were inoculated with four different doses of Eimeria maxima oocysts. Body weight gain, fecal oocyst shedding, concentrations of plasma NO2- + NO3-, carotenoid, and interferon-gamma were measured at two different time periods postinfection. The results showed significant dose and sex effects on most parameters and interaction between dose and sex in some parameters. The dose effects were generally linear; however, some significant quadratic effects were also observed. The measurements from chickens inoculated with 10(4) oocysts displayed the highest correlation coefficients among oocyst shedding, body weight gain, and concentrations of carotenoid and NO2- + NO3-. An infection index, calculated from the correlated parameters, displayed high correlation coefficients with the parameters. The infection index may be a better parameter for evaluating individual genetic resistance against coccidial infection.