The chicken major histocompatibility complex (MHC-B) and alloantigen systems A, D, E, and I impact resistance to coccidiosis.

Coccidiosis, a major poultry protozoal disease caused by several Eimeria species, compromises gut health causing significant losses. This study assessed the association of haplotypes of the major histocompatibility complex (MHC) and other blood alloantigens found in commercial egg production chickens with resistance to coccidiosis. Pedigreed White Leghorn offspring segregating for the MHC-B region, plus four additional alloantigen systems A (C4BPM), D (CD99), E (FCAMR), and I (RHCE) were tested for differential resistance to coccidiosis in five 26-day (d) trials (n= 235 birds in total). On d 19, all birds were inoculated with a cocktail of E. acervulina, E. maxima, and E. tenella oocysts and allocated to individual cages. Phenotypes evaluated included body weight gain (BWG), feed intake (FI), feed conversion ratio (FCR), gross and microscopic lesion scores (GLS and MLS), and oocyst shedding (oocysts per gram, OPG). Haplotypes of the five blood systems were determined by SNP genotyping. A positive and negative association means an increase and decrease in a phenotypic trait, respectively, with each additional copy (0, 1 or 2) of a given haplotype. Results were considered statistically significant at P ≤ 0.05. The CD99-H01 haplotype association was positive with BWG but negative with FCR. Genotype B21B21 had the highest GLS in the jejunum establishing a positive association between MHC B21 and jejunal GLS. Further, the B12B15 genotype had a lower E. maxima OPG compared with the B12B21 genotype. The I system I-H01 haplotype had a negative association with jejunal and cecal GLS. Duodenal GLS was lower in E-H02/H02 compared to the E-H07/H07 genotype of the E system. Haplotypes B21, blood systems D-H01, E-H02, and I-H01 were associated with improved resistance to coccidiosis. The association of specific haplotypes of the MHC-B and other alloantigen systems D, E, and I with resistance and susceptibility traits during mixed Eimeria infection underscores the need for further investigations of these haplotypes' effects on coccidiosis resistance in commercial lines, validating the inclusion of different blood systems in selection programs.