Phenotypic and genotypic aspects of Staphylococcus aureus isolated from chronic subclinical infections in dairy goats.

The objectives of this study were to identify goats with chronic infections by Staphylococcus aureus (SAUR), isolate bacteria from these animals, genotype the bacteria to identify different strains, and perform tests of antimicrobial sensitivity. A herd of 75 Alpine goats in Northern Italy was monitored for a complete production season. Bacterial cultures were taken from each udder half during eight monthly visits. Goats with at least 2 consecutive positive tests for SAUR (n = 28) in the same udder half were identified as chronically infected. Goats with no infections in either udder half during any visit were considered healthy (n = 26). The bacteria isolated from one sample from each infected goat were genotyped based on variable numbers of tandem repeats in 6 genomic regions. One sample from each animal was also subject to a test for betalactamase production and to Minimum Inhibitory Concentration tests for 11 antimicrobial agents: benzylpenicillin, ampicillin, amoxicillin, amoxicillin plus clavulanic acid, cloxacillin, cephalonium, cephoperazone, oxytetracycline, doxicycline, kanamycin, and lincomicin. A linear mixed model was used to examine the relationship between chronic infection by SAUR and somatic cell score. This analysis involved 841 records. Factors in the statistical model were sample day, parity, infection status, goat, and udder half nested within goat. No genetic variability was observed among the bacteria isolated, suggesting that all were from the same strain. All SAUR isolates were negative for the betalactamase production test and no isolate was resistant to any of the antimicrobial agents used. Penicillins were the most effective drugs tested, however. As expected, SCS was significantly higher in infected goats (least-square mean = 7.55) than in healthy goats (LSM = 5.50). With regard to specific udder halves, mean SCS from infected udder halves (LSM = 8.02) was greater than in uninfected udder halves from the same goats (LSM = 6.44). No differences were observed in milk yield or fat and protein percentages between infected and healthy goats.