A genetic perspective on enzootic bovine leukosis resistance in Canadian Holstein cattle
Bovine leukemia virus (BLV) is the causative agent of enzootic bovine leukosis (leukosis). Infection with BLV can lead to persistent lymphocytosis and tumours and is associated with decreased production, and immune and fertility challenges. Reducing the prevalence of leukosis through management and culling has not yet been successful; therefore, this thesis aimed to examine leukosis resistance from a genetic perspective. Approximately 77% of herds tested in this study were positive for leukosis; within those herds, an average of 39% of cows tested were positive for leukosis. A heritability of 0.10 (SE=0.001) and 0.07 (SE=0.0006) was estimated for leukosis resistance using a linear animal model and single-step GBLUP methodology, respectively. Candidate genes associated with leukosis resistance were identified on chromosome 23 using a genome-wide association study. Overall, selection for leukosis resistance is possible and this thesis provides the foundations for the inclusion of leukosis resistance in Canadian national selection indices.