Susceptibility of broilers to colibacillosis : Opportunities of Challenge Testing and Indicator Traits

Abstract

This dissertation aimed to evaluate broiler susceptibility to colibacillosis and the potential of genetic selection to reduce broiler susceptibility to colibacillosis. A challenge experiment with E. coli at 7 days of age was carried out on eight broiler genotypes: five pure broiler lines, a slow­growing line, and two 2­way crosses of the pure­lines. Based on the results from this experiment, a sensible definition of the susceptibility to colibacillosis was defined, and the indication of genetic variation in the susceptibility was found, suggesting that selection for reduced susceptibility is possible. Maternal antibodies did not have an effect on susceptibility, but there were indications of genetic variation in the changes in thyroid hormones in response to challenge as well as in the antibody response to challenge. Difficulties in evaluating immunological variables hinder attempts to improve animal health through selection on immunological variables. A model was developed that describes immunocompetence development as well as kinetics of immunoresponsiveness to a pathogenic challenge in an individual chick. This model provides a useful tool in the definition of appropriate challenge and measurement strategies when evaluating immunocompetence and immunoresponsiveness. The model was expanded into a stochastic model that describes a population of individual chicks with variation among them as well as stochastic variation within individuals across age. The model predicts that heteroscedasticity in variance across age decreases with increasing challenge age, and that minimum probability to detect a given difference in immunocompetence or responsiveness at another age than the selection age increases with increasing selection age. Therefore, a high challenge age, at which maternal immunity no longer has influence, is preferable. Selection against susceptibility to colibacillosis should aim at reducing the incidence of colibacillosis at commercial broiler level and be based on a combination of information on indicator and clinical traits. Because the economic importance of bacterial diseases as a whole is much higher than that of colibacillosis alone, changing the breeding goal to reducing the incidence of bacterial diseases as a whole is sensible.