Prevalence study of Staphylococcus aureus in quarter milk samples of dairy cows in the Canton of Bern, Switzerland.

In the present study, the prevalence of S. aureus in mammary gland quarters of dairy cows in Switzerland was estimated and a risk factor analysis was carried out. Dairy cows were selected by one-step-cluster sampling with stratification by herd size. Forty-seven of 50 randomly chosen farms participated in the study, resulting in 603 cows and 2388 quarter samples. Milk samples were collected in all herds on two occasions two weeks apart. In 6% of cows (95% CI: 2.7-9.3%) at least one milk sample was positive for S. aureus and from 2% (0.8-3.2%) of all quarters, S. aureus was cultured at least once. In four quarters a latent S. aureus infection (agent detected and somatic cell count (SCC) <100,000cell/ml) was diagnosed. Multivariable hierarchic logistical regression analysis yielded five significant risk factors for observing S. aureus in a milk sample: high SCC, a S. aureus-positive neighbouring quarter, a palpable induration in the quarter, and a wound, scar tissue or crush injury affecting the teat. The type of housing (P=0.1596) was also a factor that remained in the model. The mentioned risk factors must be considered during the evaluation of herds with S. aureus problems. The occurrence of latent S. aureus infections emphasises that not only quarters with a high SCC but all quarters of all cows must be cultured for control measures to be effective.