PSV-17 - Prevalence, etiology, and risk factors of subclinical mastitis in meat-breed ewes

Abstract: Mastitis, the inflammation of the mammary gland, is an important welfare and economic concern in meat-breed ewes. Subclinical mastitis is particularly difficult to diagnose, and therefore to treat, due to a lack of clinical signs. Understanding its prevalence, etiology, and associated risk factors will facilitate proactive disease management and improvements in ewe health. Our objectives were to characterize the prevalence and bacterial etiology of subclinical mastitis in meat-breed ewes and to investigate the potential risk factors of lactation stage, parity, and the number of lambs reared. We enrolled 39 meat-breed ewes between 2-9 years of age at the University of Wisconsin-Madison sheep unit. All procedures were approved by the UW-Madison Institutional Animal Care and Use Committee. Duplicate milk samples were collected aseptically from each udder half once a week beginning 2-4 days postpartum until 8 weeks postpartum. Samples were sent to the Wisconsin Veterinary Diagnostics Laboratory for culture and speciation within 5 days of collection. Predominant organisms were identified using matrix-assisted laser desorption ionization time-of-flight mass spectroscopy Biotyper 3.1 (MALDI-TOF MS). A sample was considered a true infection if at least 100 CFU/mL were isolated. A true infection was identified in 30.8% of udder halves. The most common pathogens isolated were non-aureus staphylococci (70.8%), Staphylococcus aureus (8.9%) and Mannheimia haemolytica (5.7%). To assess risk factors for subclinical mastitis, we fit a mixed logistic regression, including fixed effects of parity, weeks since lambing, and number of lambs reared, and their two-way interactions and udder half nested within ewe ID as the random effect. The risk of subclinical mastitis increased throughout lactation in primiparous ewes (Week*Parity: p < 0.001) and in ewes that reared 1 or 2 lambs (Week*Lambs reared: p = 0.01). However, within each week, there were no differences in subclinical mastitis risk among parities, nor among ewes rearing singletons, twins, or triplets. These results indicate that the risk of subclinical mastitis varies during lactation depending on parity and litter size, which has implications for efficient mastitis management. Research is needed to test whether changes in management practices in early lactation could prevent mastitis from developing in the later stages, leading to improvements in ewe health.