RISK-FACTORS ASSOCIATED WITH HERD-GROUP MILK SOMATIC-CELL COUNT AND PREVALENCE OF COAGULASE-POSITIVE STAPHYLOCOCCAL INTRAMAMMARY INFECTIONS

Fifty nine dairy herds enrolled in the Washington State Dairy Herd Improvement Association Milk Somatic Cell Count Program were split into low and high cell-count groups. Low herds had the lowest percentage of cows with linear-score somatic cell count (SCC) > 4 and high herds had the highest percentage of cows with linear-score SCC > 4. An estimate of prevalence of intramammary infections was made from aseptically-collected milk samples. Logistic regression analysis was used to construct models that would best describe the combination of management factors that best classified herds into high and low SCC groups. Similarly, stepwise linear regression analysis was used to determine management-factor combinations which would best describe herd prevalence of coagulase-positive staphylococcal intramammary infection. A subset of 75 variables was used in models and was from an initial set of more than 250 management factors. Variables from the initial set that were deemed to have a direct effect on mastitis and to be under management control were screened using univariate tests. Those variables found to be either significantly associated with herd SCC-group status and/or prevalence of intramammary infection by coagulase-positive staphylococci were used in the modeling procedures. Additionally, interactions were tested in the models and some variables commonly associated with mastitis control were included as forced variables in these models. Factors which were most likely associated with low-somatic-cell herds were: (1) milking cows with clinical mastitis last; (2) disinfecting the teat ends prior to administration of intramammary antibiotic therapy; and (3) having drier cow bedding. A model containing these three factors classified 87.9% of the herds into the correct group. Factors which described a reduction in the prevalence of intramammary infections with coagulase-positive staphylococci were: (1) disinfection of teat end prior to therapy administration; (2) applying disinfectant solution to the teat after milking (teat dip); and (3) cleaning the milking-system regulator frequently. These three factors were associated with decrease in coagulase positive staphylococcal intramammary infections of 27%, 9%, and 9%, respectively, and in combination described 36.6% of the herd-to-herd variation in infection prevalence by this pathogen.