Evaluating somatic cell count, the California mastitis test, and infrared thermography for subclinical mastitis detection in meat ewes

Mastitis is a major concern in sheep farming. Detecting subclinical
mastitis, which manifests without apparent signs, is particularly
challenging. While somatic cell count (SCC) and the California Mastitis
Test (CMT) are widely used in dairy animals, and infrared thermography has
shown some promise, their diagnostic performance in meat sheep remains
under-evaluated. This study aimed to 1) characterize SCC and udder skin
surface temperature across lactation in subclinically infected and
uninfected udder halves; 2) determine diagnostic cutoffs for SCC, CMT, and
thermography; and 3) compare diagnostic performance using receiver
operating characteristic curves. Milk and thermal data were collected
weekly from 37 clinically healthy nursing ewes over the first 8 weeks
postpartum. Infection was defined by ≥100 CFU/mL of a bacterial species.
Infected udder halves had higher log SCCs than uninfected halves in weeks
1–4 and 6 postpartum. Infection status did not predict udder skin
temperature, which was more influenced by ambient temperature. SCC had the
highest diagnostic performance (AUC = 0.78), with an optimal cutoff of
148,500 cells/mL (sensitivity = 0.77; specificity = 0.66). The CMT yielded
an AUC of 0.70 with an optimal cutoff score above negative (i.e., trace or
positive reaction; sensitivity = 0.77; specificity = 0.51). Udder skin
temperature had the lowest AUC (0.56). SCC is the most effective of the
three tools for detecting subclinical mastitis, although CMT may serve as
a useful on-farm screening tool. Infrared thermography did not detect
subclinical infection, highlighting the need for further research into
non-invasive diagnostics.