Testing for Staphylococcus aureus in herds with a low bulk milk somatic cell count

The validity of single quarter milk samples for culturing Staphylococcus aureus was quantified by comparing the results of single samples with intramammary infections (IMI) as defined by the National Mastitis Council (NMC). NMC definitions are based on consecutive samples. During a 20-month study, all lactating cows in five herds with a low bulk milk somatic cell count (BMSCC) were sampled at regular intervals to get a detailed insight in the bacteriological status of these cows. Comparison of the results of single samples with these detailed findings showed that the results of single samples culturing negative on S. aureus were reliable (predictive value negative 0.99). The results of single samples positive on S. aureus, however, were less reliable. Single samples, positive on S. aureus, from cows without an increased SCC, or that had had a SCC > 200,000 only once, very often did not come from quarters in which S. aureus was found in subsequent samples (predictive value positive (PV+) 0.54, respectively, 0.41). PV+ approximately doubled, when cows were selected that had had an increased Somatic Cell Count (SCC) during two or more consecutive milk recordings (PV+ 0.82). The consequences of these findings on selecting cows with subclinicial S. aureus mastitis for treatment or culling are discussed. It was concluded that in low BMSCC herds the validity of bacteriological culturing increases when the information of consecutive measured SCC is used in selecting cows for bacteriological culturing.