Investigations on worm expulsion, resistance and tolerance to mixed-nematode infections in chicken genotypes with different performance directions
The main objective was to investigate whether selection for divergent performance directions resulted in a different ability to cope with nematode infections. Therefore male and female birds of divergent genotypes developed either for meat or egg production or for both purposes were separately investigated. The worm expulsion and associated immune responses have been studied in the chicken host, too. It was found that tolerance to nematode infections is dependent on both growth rate and laying performance level in male and female chickens, respectively. In males the infection reduced the body weight in the fast-growing chickens only, whereas infection had no effect on slower growing genotypes. Differences found between male birds may be associated with a mismatch between the genotype-specific nutrient requirements and the nutrient supply during infectious challenges. In females, the laying rate (LR) of the high performing genotype was reduced by infection in the early infection period, whereas the LR of the lower performing genotype was reduced in the advanced infection period only. The time dependent impairment in laying performance was likely due to differences in genetic programming for production peak and persistency of the two genotypes. Both the host individual performance level as well as the nutrient supply appears to play a key role in tolerance against nematode infections in chickens. Resistance to infections, however, is dependent on the type of infection (i.e., experimentally vs. naturally infections). While overall resistance against infections likely depend on the host immunity and the intestinal conditions of the host, susceptibility to re-infections, which finally determines the risk for an infection, may be associated with host behaviour. Worm expulsion was found to take place non-linearly in three distinct phases, in which juvenile worms were especially targeted. Expulsion of the worms was associated with humoral and cell-mediated immune responses.