Identification of aerobic bacteria from palpably defective udder halves and their persistence over time in lactation in non-dairy (Romney) ewes

The aims of this study were to identify associations between udder half defect (hard or lump) and bacteria isolated from milk samples and to assess persistency of the bacterial species over time during lactation. Milk samples were aseptically collected from each udder half weekly for the first six weeks of lactation (day 7 to day 42), at weaning, and three weeks post-weaning from 46 non-dairy (Romney) ewes (92 udder halves). The ewes had a history of either having udder half defects (n = 70 udder halves), or not having udder half defects (n = 22 udder halves), over the previous two years. The milk samples were cultured, processed, and Matrix-assisted laser desorption/ionization time-of-flight (MALDI-ToF) mass spectrometry was used for bacterial identification. The Chi-square test of independency and multinomial logistic regression were applied for analysis. Almost half of the udder halves did not express milk during the study, ranging from 42.4% (39/92) on day 7 to 47.8% (44/92) three weeks post-weaning. In udder halves that did express milk, the frequency of udder halves that were bacterial culture positive decreased as lactation progressed, with the highest frequency being on day 7 of lactation while the lowest frequency was on the day of weaning. The relative risk ratio (±SE) of an udder half being bacterial culture positive at weaning was significantly less (RRR (SE) = 0.39 (0.45)) than at day 7 of lactation. Staphylococcus aureus, Streptococcus uberis, and some coagulase-negative staphylococcus (CNS) species were observed on both one occasion as well as more than two occasions in an individual udder half. However, Mannheimia haemolytica, S. xylosus, and S. pluranimalium were only isolated from an individual udder half on more than four occasions, apparently indicating persistence of these organisms over time. The association between udder half defect (hard/lump/normal) and bacterial culture positivity (positive/negative) or species involved at each time point was not significant (p > 0.05). But, descriptively Mannheimia haemolytica, Streptococcus uberis, and Staphylococcus aureus were more frequently isolated from milk from defective udder halves (hard/lump) whereas CNS were more frequently identified from normal udder halves. Overall, the results show that the isolation of bacteria from palpably defective udder halves was higher in the early weeks of lactation, and bacteria persistency over time was dependent on the species of bacteria involved.