Effects of systematic influences and intramammary infection on differential and total somatic cell counts in quarter milk samples from dairy cows.

Effects of bacteriological status, stage of lactation, parity and season of sampling on differential and total somatic cell counts were estimated in quarter milk samples taken from 39 dairy cows. Log somatic cell count was affected by the bacteriological status of the quarter, as well as by the bacteriological status of adjacent quarters. Differential cell counts were affected by presence or absence of pathogens in the quarters themselves, but not by the bacteriological status of the adjacent quarters. Log somatic cell count was clearly affected by stage of lactation, due mainly to physiological variation, but possibly also accentuated by variation in infection rates throughout lactation. With the exception of early lactation, little physiological variation throughout lactation was detected for differential cell counts. Presence of infections seemed to have some indirect effect on trends throughout lactation as regards percentages of granulocytes and monocytes. Variation in somatic cell counts due to parity could be explained by variation in infection rates, rather than being physiologically determined.