Effects of intramammary infections on somatic cell score and milk yield in Sarda sheep.

Aim: To evaluate the effects of intramammary infections (IMI) on somatic cell score (SCS) and milk yield in dairy ewes.

Methods: Monthly milk samples were collected from a flock of 202 Sarda sheep, over a period of 7 months, for bacteriological culture and measurement of somatic cell counts (SCC). During the same period, milk yield was measured daily using electronic milk meters connected to each half-udder cluster of the milking machine. SCC was transformed to SCS using a base-2 log transformation. One SCS is equivalent to a SCC of 25,000 cells/ml, and each increase of 1 in SCS is associated with doubling of the SCC. IMI was defined by the presence of five or more colonies of similar morphology isolated from a milk sample (≥500 cfu/ml). The effect of IMI on SCS and milk yield was assessed using a generalised estimating equation (GEE).

Results: There were 1,186 udder halves with IMI from 2,828 milk samples, a prevalence of 41.9%. The distribution of bacterial species within the 1,186 culture-positive samples was comprised of 476 (40.1%) Staphylococcus epidermidis, 172 (14.5%) Staph. chromogenes, 38 (3.2%) Staph. caprae, 134 (11.3%) Staph. simulans, 114 (9.6%) Streptococcus uberis, 123 (10.4%) Strep. dysgalactiae, and 129 (10.9%) Strep. equi subsp. zooepidemicus. SCS was greater in udder halves with IMI (mean 7.71; SD 0.82) than in udder halves without IMI (mean 5.53; SD 1.02) (p<0.01). IMI due to streptococcal species were associated with greater SCS (mean 8.24; SD 0.62) than those due to staphylococcal species (mean 7.48; SD 0.79) (p<0.01). Milk yield from udder halves with IMI was lower (mean 439 (SD 162) ml/half udder/day) than from udder halves without IMI (mean 602 (SD 170) ml/half udder/day) (p<0.01). IMI due to staphylococcal species was associated with a lower milk yield (mean 399 (SD 167) ml/half udder/day) than IMI due to streptococcal species (mean 427 (SD 156) ml/half udder/day) (p<0.01).

Conclusions: These findings provide sheep milk producers with information on the losses associated with subclinical mastitis, which can be used to evaluate the economics of prevention and treatment protocols concerning udder health in ovine dairy flocks.