Physiological and pathological thresholds of somatic cell counts in ewe milk

In four flocks, 215 dairy ewes were monthly sampled, throughout lactation, in order to study the relationship between subclinical intramammary infections (IMI) and milk individual SCC (iSCC). A decision rule proposes to consider an udder as healthy (specificity = 75%) if every iSCC are lower than 0.500 × 106 cells/ml and infected (sensitivity = 82%) if at least two iSCC are higher than 1 or 1.2 million cells/ml. At a flock level (annual geometric mean of bulk SCC), this rule allows the estimation of the prevalence of subclinical IMI: an annual bulk SCC of 0.650 × 106 cells/ml corresponds to a prevalence of 15%. These results are discussed.