RESPONSE OF 3 CLASS-IV MAJOR HISTOCOMPATABILITY COMPLEX HAPLOTYPES TO EIMERIA-ACERVULINA IN MEAT-TYPE CHICKENS

1. The importance of MHC genes and background genes in controlling disease resistance, including resistance to avian coccidiosis, has not been clarified in meat-type chickens. 2. The role of class IV MHC genes in resistance to Eimeria acervulina was assessed in F2 progeny of a cross between 2 meat-type lines, selected divergently for immune response to Escherichia coli. 3. Disease susceptibility was assessed by lesion score, body weight, packed cell volume and carotene absorption. 4. Chickens with the ''K'' class IV MHC haplotype had lower lesion scores than chickens with ''F'' and ''A'' haplotypes. 5. Plasma carotene concentrations were higher in chickens with ''K'' haplotype and lower in chickens with ''F'' and ''A'' haplotypes whereas body weight and packed cell volume were less sensitive measures of Eimeria infection. 6. Eimeria acervulina resistance appears to be associated with MHC class IV genes; information about MHC haplotypes may be useful in selecting for increased resistance of meat-type chickens to coccidiosis.