Relationship of somatic cell counts in goat milk to mastitis and productivity

Abstract

This review covers research since the Symposium at Bella, Italy, in 1994, and as published in three major dairy research journals. It examines in particular non-pathological influences on somatic cell counts (SCCs) levels as they are unique for goat milk and different from cow milk in order to aid towards progress for establishing equitable quality standards for goat and sheep milk. Different testing equipment and procedures are of variable reliability and applicability to goat milk, unless appropriate correction factors and calibration with goat milk are used. The only reliable method for arbitration seems to be the pyronin Y-methyl green stain. Culturing goat milk samples indicated that in commercial goat herds the variation in SCC can be as much as 90% due to non-infectious factors, and a correlation with standard plate counts of bacteria may be only at r=0.44. Among pathogens found in goat udders, Staphylococcus aureus seems to cause higher levels of SCC than coagulase negative staphylococci. Prevalence of pathogenic species in different herds varies between S. aureus, S. epidermidis, and coagulase negative staphylococci. Polymorphonuclear neutrophils predominate in goat milk and at higher levels than in cow milk under non-infectious conditions. Histopathological sections of goat udders with high SCC showed absence of mastitic conditions and that healthy goat udders can have high SCC levels normally. Stage of lactation always influences SCC. In contrast to cow milk tests, a single SCC test in goat milk has little value unless stage of lactation and parity is incorporated. Curvilinear and linear regressions have been calculated for the prediction of SCC versus stage of lactation. Estrus may also elevate SCC, but definitive studies in goats are lacking. Goat milk samples before, during and after milking differ significantly in SCC. Hand milking may cause higher SCC than machine milking, but studies have been inconclusive. The few studies of effects of SCC on productivity of goat milk, cheese and yoghurt have been inconclusive so far, partly because the incidence of subclinical and clinical infections in goat udders is less than in cow udders, and any risk of antibiotic contamination is also less. Different threshold levels have been proposed for SCC, but their predictive value of udder infections has been very variable.