Genome-Wide Association Analysis Identifies QTL Associated with Clinical and Sub-Clinical Bovine Respiratory Disease

Bovine respiratory disease (BRD) adversely effects the beef and dairy cattle industries through acute and chronic loss of production, morbidity, death and reduced profitability. Our long-term goal is to utilize genomic approaches to identify and select cattle by genomic selection that are less susceptible to BRD in an effort to reduce the prevalence of the disease. Our short-term objective was to identify quantitative trait loci (QTL) associated with susceptibility to clinical and subclinical BRD using genome-wide association analyses. Cattle with McGuirk health scores ≤ 4 were classified as healthy animals, whereas those with health scores ≥ 5 were classified as clinically affected BRD cases. Subclinical BRD was defined by the presence of lesions in the lung and healthy cattle were characterized by the absence of lung lesions. The identification of loci associated with subclinical BRD allows selection against cattle that are affected with BRD but that fail to show clinical symptoms of disease. Dairy analyses investigating clinical BRD were conducted on Holstein dairy populations from California (CA; n = 2,014), New Mexico (NM; n = 767) and Wisconsin (WI; n = 140) and beef feedlot populations from Colorado (CO; n = 999) and Washington (WA; n = 1,005). The subclinical BRD phenotypes were established for the CO population post-harvest. All genes proximal to QTL identified (p < 1 x 10^-5) in the clinical BRD analyses for dairy (CA - 126 QTL, NM - 138 QTL, WI - 78 QTL) and beef (CO - 5 QTL, WA - 5 QTL, 2 gene sets) were functionally related to innate and adaptive immunity, but varied according to the pathogen prevalence distribution detected for each population. A robust innate immune response to infection is critical for host defense prior to the activation of the adaptive immune response. The analysis of subclinical BRD in the CO population identified 7 additional QTL, near genes that have functional roles related to the maintenance of lung health. The identification and validation of QTL associated with susceptibility to both clinical and subclinical BRD will provide producers the ability to select cattle that are less susceptible to the disease, ultimately reducing the prevalence of the disease and diminishing its economic impact.