Retrospective evidence that the MHC (B haplotype) of chickens influences genetic resistance to attenuated infectious bronchitis vaccine strains in chickens.

Infectious bronchitis is a respiratory disease of chickens that is caused by the coronavirus infectious bronchitis virus (IBV). Virtually all broiler and layer breeder flocks are routinely vaccinated against IBV. Two hatches of 1-day-old chicks from four lines were mistakenly vaccinated for infectious bronchitis using a moderately attenuated vaccine designed for chicks of an older age. The vaccination resulted in high mortality, and chicks from three of four lines died with signs typical of infectious bronchitis. The mortality that occurred using this less-attenuated vaccine was significantly influenced by the genetic line, and the MHC (B) haplotype in chickens of three B congenic lines. B congenic chickens possessing the B*15 haplotype were resistant in contrast to chickens possessing the B*13 or B*21 haplotypes. Chicks from two further hatches of the four lines were vaccinated appropriately with a more attenuated IBV vaccine, and only limited chick mortality was seen. These retrospective data from two repeated hatches confirm earlier data indicating chicken genes influence resistance to IBV, and indicate for the first time that genes tightly linked to the B haplotype are relevant in resistance to IBV. Due to extenuating circumstances it was not possible to verify results with chicks from F2 matings. Factors that may enhance definition of the role of the B haplotype in immune response to IBV, and the desirability for further analysis of a B haplotype-linked influence on immunity to IBV are discussed.