Selection for increased resistance to Salmonella carrier-state.

Background. A recent regulation has been implemented by the European Commission to reduce Salmonella prevalence in poultry flocks at the long term at less than 2%, in order to insure food safety. Some animals may be colonized without exhibiting any clinical signs and, therefore, become a vector to horizontal or even vertical transmission of Salmonella. This silent carrier state should be accounted for in any efficient campaign against Salmonella. Improving fowl’s natural ability to clear their organism from Salmonella is a promising way to reinforce the classical prophylactic measures (elimination of positive flocks, vaccination and food treatment). Once experimental inoculation protocols have been established, genetic parameters could be estimated and a selection experiment was initiated to test the feasibility of a genetic improvement of fowl’s ability to eliminate Salmonella. In parallel, genes controlling variations to Salmonella resistance were researched and a mathematical modelling of Salmonella propagation in laying hens flocks initiated. Results. Two series of divergent lines were selected, for increased or decreased resistance, after inoculation at one week of age (chick resistance) or at the peak of lay (adult resistance). Selection for the adult resistance proved successful, as the mean percentages of global contamination were respectively 41% and 60% in the resistant and sensible lines in the 5 generation. Heritability coefficients were 0.16 in chicks and 0.18 for global contamination of hens. The genetic correlation between these two traits was highly negative (-0.50). This genetic correlation can partially be explained by the pleiotropic effects of some loci involved in the genetic resistance. For instance SLC11A1 was shown to be involved in both chick and adult resistance. Large scale genotyping should soon result in the identification of new genome regions and in a shorter term, SNP-assisted or even genomic selection. The modelling of Salmonella propagation within a flock was derived, and three steps were distinguished in the contamination: digestive contamination, then systemic contamination, followed by the bacterial clearance leading to recovery. The ability of hens to eliminate Salmonella is a key factor which influences both the maximal prevalence and the duration of the epizooty. In our simulations, the combination of vaccination and genetic selection resulted in very low percentage of contamination, very similar to the level the E.U is asking for. Conclusion. Selection for a reduced propensity to carrier state is possible and synergize with other means of prevention. It will no doubt benefit very soon from Marker Assisted or Genomic Selection.