Association between isolation ofStaphylococcus aureusone week after calving and milk yield, somatic cell count, clinical mastitis, and culling through the remaining lactation

Abstract

Cows with isolation ofStaphylococcus aureusapproximately 1 week after calving and milk yield, somatic cell count (SCC), clinical mastitis (CM), and culling risk through the remaining lactation were assessed in 178 Norwegian dairy herds. Mixed models with repeated measures were used to compare milk yield and SCC, and survival analyses were used to estimate the hazard ratio for CM and culling. On average, cows with an isolate ofStaph. aureushad a significantly higher SCC than culture-negative cows. If no post-milking teat disinfection (PMTD) was used, the mean values of SCC were 42 000, 61 000, 68 000 and 77 000 cells/ml for cows with noStaph. aureusisolate, withStaph. aureusisolated in 1 quarter, in 2 quarters and more than 2 quarters respectively. If iodine PMTD was used, SCC means were 36 000; 63 000; 70 000 and 122 000, respectively. Primiparous cows testing positive forStaph. aureushad the same milk yield curve as culture-negative cows, except for those withStaph. aureusisolated in more than 2 quarters. They produced 229 kg less during a 305-d lactation. Multiparous cows with isolation ofStaph. aureusin at least 1 quarter produced 94–161 kg less milk in 2ndand >3rdparity, respectively, and those with isolation in more than 2 quarters produced 303–390 kg less than multiparous culture-negative animals during a 305-d lactation. Compared with culture-negative cows, the hazard ratio for CM and culling in cows with isolation ofStaph. aureusin at least 1 quarter was 2·0 (1·6–2·4) and 1·7 (1·5–1·9), respectively. There was a decrease in the SCC and in the CM risk in culture-negative cows where iodine PMTD had been used, indicating that iodine PMTD has a preventive effect on already healthy cows. For cows testing positive forStaph. aureusin more than 2 quarters at calving, iodine PMTD had a negative effect on the CM risk and on the SCC through the remaining lactation.