Etiology, persistence, and risk factors of subclinical mastitis in a meat-producing sheep flock

Subclinical mastitis, the asymptomatic inflammation of the udder often caused by bacterial infection, is an important economic and welfare concern in meat-producing flocks. We conducted a longitudinal observational study to: (1) characterize the etiology, prevalence, incidence, and persistence of intramammary infections during the first 8 wk of lactation, and (2) investigate the potential risk factors of lactation stage, parity, and the number of lambs reared. Duplicate milk samples (n = 592) were collected aseptically from each udder half of 37 clinically healthy ewes (9 Hampshire, 22 Polypay, and 6 Targhee) weekly for their first 8 wk of lactation. A sample was considered a true infection if at least 100 CFU/mL of a bacterial species were isolated. Out of a total of 560 non-contaminated duplicate udder half samples, 186 (33.2%) were infected. The prevalence of subclinical mastitis in the first week of lactation was 26.0% at the udder half level and increased over lactation to 47.1% by 8 wk. Over the course of lactation, 34 of the 37 (91.9%) ewes had an intramammary infection in at least one udder half. The most common bacteria isolated were non-aureus Staphylococci (72.1%), Staphylococcus aureus (7.5%), and Mannheimia haemolytica (5.9%). The weekly incidence rate of new infections was 18.0% ± 10.2% (mean ± SD). Infections lasted 2.1 ± 2.1 wk, with 9.0% present the full 8 wk of observation. Spontaneous cures occurred in 36.0% of infection bouts but varied from 0% to 100% depending on the pathogen. The probability of subclinical mastitis infection increased as lactation progressed for ewes rearing single or multiple lambs and for ewes in their first or second parity, but not for ewes in later parities. In conclusion, intramammary infections in this flock were prevalent across lactation, parities, and litter sizes, and varied in their duration and spontaneous cure rate. Improved knowledge of the etiological agents and risk factors involved in subclinical mastitis is important in developing effective control measures in meat-producing flocks.