Influence of the Chicken Major Histocompatibility Complex on the Pathogenesis of Bacterial Skeletal Disease, Chicken Infectious Anemia and Infectious Bronchitis

Abstract

Genetic resistance to several poultry pathogens has been unequivocally linked to the major histocompatibility B complex (MHC). Relatively few MHC association studies have been conducted during natural disease outbreaks, or in commercial chickens. Specific objectives during this three-part investigation were to identify potential MHC influence on the development of naturally occurring bacterial skeletal disease in a commercial flock of broiler breeders, characterize the pathogenesis of experimental chicken infectious anemia in commercial broiler breeder chickens selected for specific MHC genotypes, and evaluate potential MHC association with response to infectious bronchitis virus (IBV) in commercial leghorn chickens possessing select MHC genotypes. Bacterial osteomyelitis and tenosynovitis, primarily caused by Staphylococcus spp., were observed with increased frequency in a line of commercial broilers expressing B A4/A4 (equivalent to standard B21) and B A12/A12 MHC genotypes by comparison with 14 other genotypes in the flock. Previous studies have shown that the B A4 haplotype is relatively susceptible to Escherichia coli induced cellulitis. These findings are suggestive that the B A4 haplotype may be linked to increased susceptibility to bacterial disease. The pathogenesis of chicken infectious anemia virus (CAV) was examined in the same line of commercial broilers using chickens with select MHC genotypes. Differences in clinical and pathologic lesions were not observed in 2 week old, orally inoculated chickens or between MHC genotypes. However, differences in seroconversion and CAV genomes per cell were detected in birds expressing B A9/A9 and B A9/A4 MHC genotypes. These findings suggest that certain MHC genotypes may influence CAV infection; however additional studies are needed to better define the interaction of CAV with the broiler MHC. Infectious bronchitis disease progression was evaluated following challenge in vaccinated commercial white leghorn chickens with the B 2/15 and B 2/21 MHC genotypes. The incidence of clinical disease and the frequency of IBV-specific IgA detection in tears were higher in B 2/21 genotype birds. Differences between genotypes were discrete, suggesting that IB susceptibility might be associated with MHC genotype. Collectively, the findings presented in this dissertation indicate that identifying candidate genes and characterizing MHC associations with other diseases may elucidate specific immune mechanisms involved in responses to certain pathogens. Further insight may be gained into the immunopathogenesis of several human diseases caused by related pathogens.