Genome Wide Association Study Identifies Loci Associated with Mastitis Antibiotic Therapy Phenotypes Following Experimental Challenge with Streptococcus Uberis

Mastitis is a detrimental disease in the dairy industry that costs upwards of $2 billion annually and decreases milk quality.  Often, mastitis results from bacteria entering the gland through the teat end.  A common mastitis causing pathogen is Streptococcus uberis (S. uberis), which is responsible for 14-26% of subclinical and clinical mastitis cases.  Following an intramammary experimental challenge with S. uberis, Holstein cows (n=40) received antibiotic therapy based on visual inflammatory signs (milk and mammary scores) or chronic subclinical infections extending >20 days after initial challenge.  Based on the cows’ need for antibiotics, three phenotypes emerged: 100%, 75%, and 33% need for antibiotic therapy.  Traditional GWAS have utilized somatic cell count (SCC) and/or sire pedigrees to identify loci of interest for mastitis.  We propose a novel approach using the antibiotic therapy phenotypes to identify loci.  A 50K SNP chip analysis was performed using the BovineSNP50 v2 DNA Analysis BeadChip from Illumina and antibiotic therapy phenotypes.  Associations were tested using Plink software.  Preliminary analyses revealed 7 highly significant (p<9.36x10^-5) SNPs.  Of these, 5 are part of significant clusters of genes on chromosome 2 and chromosome 16.  The identified loci should be further investigated to potentially identify causation behind the observed mastitis antibiotic therapy phenotypes.  Such investigations could lead to novel treatment and prevention compounds/protocols for mastitis or genetic selection methods for cows with greater potential to resist infection.