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
weeks of lactation, and (2) to 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 weeks 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 weeks. Over the course of lactation, 34 of 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 weeks, with 9.0% present the full 8 weeks 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.