CHANGES IN ELECTRICAL-CONDUCTIVITY AND SOMATIC-CELL COUNT BETWEEN MILK FRACTIONS FROM QUARTERS SUBCLINICALLY INFECTED WITH PARTICULAR MASTITIS PATHOGENS

Cows with subclinical intramammary infections were identified by milk bacteriology. The mastitis pathogens included Staphylococcus aureus (n = 9), Streptococcus uberis (n = 10) and coagulase-negative staphyloco cci (n =: 10). Samples of first fore milk, main flow milk and strippin gs milk fractions were collected from each quarter and laboratory meas urements were made of electrical conductivity, milk fat concentration and somatic cell count. Conductivity measurements were corrected for m ilk fat concentration and within-cow inter-quarter conductivity ratios calculated. Repeatability estimates of all measurements between days were calculated. In the case of infected quarters, all conductivity va lues decreased markedly (P < 0.05) from first fore milli to main flow milk fractions. Conductivity differences between quarters of infected cows were substantially lower during the main milk flow phase. For qua rters infected with Staph. aureus an increase in conductivity was obse rved (P < 0.05) from main flow to strippings fractions. For uninfected quarters, conductivity declined as milk fat concentration increased w ith successive milk fractions. Variation, both within and between milk fractions, was greater for somatic cell count than for conductivity. Differences in conductivity between milk fractions from individual inf ected quarters were not accounted for by changes in fat concentration and may result from the mixing of milk from infected and uninfected re gions of the gland. Localized infection may produce a decrease in cond uctivity between fore milk and mid-flow fractions while differential d rainage from an infection site in the secretory tissue may additionall y produce an increase in conductivity from mid-flow to strippings frac tions. Such changes may thus provide information on the location and m agnitude of an infection. The results clearly demonstrate the importan ce of the milk fraction when using conductivity as a diagnostic of int ramammary infection, the highest diagnostic sensitivity being achieved by using first fore milk samples.