GWAS for Differential Manifestation of Clinical Signs and Symptoms Related to Bovine Respiratory Disease Complex in Holstein Calves

Bovine respiratory disease complex (BRDc) is one of the most common causes of morbidity and mortality among beef and dairy cattle, with bacterial and viral pathogens as well as stress and the environment contributing to the onset of disease.  The primary objective of this study was to investigate whether host loci were associated with differences in the manifestation of BRDc clinical signs in Holstein calves. A standardized calf-health scoring system (Scores 0-12; McGuirk 2008) was used to characterize 2,682 calves from two locations (CA, n=1978; NM, n=705).  Blood samples and diagnostic swabs (mid-nasal and 2 deep pharyngeal) were obtained for each calf to facilitate bovine DNA isolation, bacterial culture, viral isolations, and qPCR detection of pathogens.  DNA from all calves was SNP genotyped using the Illumina BovineHD Assay (777K), and resulting SNP were filtered for UMD3.1 chromosomal assignment, low minor allele frequency, genotype call rate, and Hardy-Weinberg Equilibrium; with removal of Kleinfelter males and individuals with a genotype call rate <90%, or heterozygosity >40%. Genome wide association analysis was performed using a mixed-model approach (EMMAX) that accounted for pedigree relationship structure. Additive SNP effects explained 17% of the variation in total clinical signs in NM, 20% in CA, and 11% in the combined cohort, with no moderate or large-effect QTL in common among calves from NM and CA.  These results represent an initial inquiry to determine whether host loci can be used to predict the overall severity of BRDc clinical signs in Holstein calves from different dairy facilities.