Polymorphisms at β-defensin genes and mastitis in Valle del Belice dairy sheep: a case control study

Mastitis is the most common and costly pathology affecting dairy animals. The identification of genetic markers that allow the inclusion of mastitis resistance in selection programs would help to reduce the costs due to the disease and the use of antibiotics. In recent years, β-defensin genes have been studied in several domestic species due to their important role in the innate immune response. The aim of this study was to determine whether β-defensin genes variability, β-defensin 1 (SBD1) and β-defensin 2 (SBD2), could distinguish between infected cases and healthy controls in Valle del Belice dairy sheep. Somatic cell count (SCC) is a powerful tool to identify animals with intramammary infection. Ewes were classified as affected by mastitis due to major (MAJP) and minor pathogens (MINP) within a lactation, based on having more than 645x103 or 2138x103 somatic cells, respectively, as reported in a previous study. Two single nucleotide polymorphisms (SNPs) in the SBD1 and five SNPs in the SBD2 genes were genotyped in 300 individuals by PCR and Single Nucleotide Primer Extension assay. All ewes had SCC test day registration. Association analysis were performed using the Case Control procedure in SAS 9.2, with the overall association with genotype based on the Armitage trend test and odd ratios (OR) based on allele counts, reflecting additive effects. Frequency differences indicated an association of the SNPs with the disease. On the basis of minor allele frequency for control group, both SNPs of SBD1 (A/G 1747 and T/C 1757) were associated with MAJP (P<0.05, OR=1.26, CI=1.08-1.48 and P<0.05, OR=0.79, CI=0.68-0.92, respectively) and MINP (P<0.001, OR=1.45, CI=1.28-1.64 and P<0.001, OR=0.69, CI=0.61-0.78, respectively). Only one SNP in SBD2 (G/A 1659) was simultaneously associated with MAJP (P<0.001, OR=0.79, CI=0.71-0.88) and MINP (P<0.001, OR=0.81, CI=0.74-0.89). Moreover, one SNP in SBD2 (G/A 1761) was associated with MINP (P<0.05, OR=0.81, CI=0.69-0.95). Association analysis between SBD1 and SBD2 genotypes and SCC showed that the investigated SNPs were associated with increased risk or resistance to intramammary infection.