Effect of MHC Haplotype on Mortality Due to Marek's Disease in Commercial Laying Hens.

Mortality from Marek's disease virus (MDV) infection results in economic loss for the poultry industry. It is controlled by vaccination, but the virus mutates and becomes more virulent. Variation within the MHC is well known to impact the outcomes following MDV infection from research performed utilizing the White Leghorn breed, with laboratory strains of the virus. The effect of the MHC haplotype following MDV challenge was determined from six lines of commercial elite (White Plymouth Rock (two), White Leghorn (three), and Rhode Island Red (one)) egg layer lines, challenged with vv+ virus. Mortality was recorded as sire daughter averages at 16-18 weeks of age from 19 generations of data. Sires were genotyped using a set of MHC-specific SNPs, encompassing 210,000 bp. Across all lines, there was a total of 23 unique MHC haplotypes, of which 15 were found at a frequency greater than 5% and used for further analysis. A significant impact on mortality was found for 16 of the haplotypes, with 9 haplotypes associated with decreased mortality and 7 haplotypes with increased mortality. There were three haplotypes identified in more than one line, allowing cross-line comparisons. The effect of these common haplotypes was consistent (either negative, positive or neutral) between lines.