Use of different somatic cell count cut-points to define intramammary infection at drying off in dairy cows from a herd with a high somatic cell count.

AIMS: To assess the use of different cut-points based on individual cow somatic cell counts (SCC) to define cows with intramammary infection (IMI) at drying-of, in a herd with a high mean bulk tank SCC.

METHODS: Results for SCC from four herd tests during lactation and bacterial culture of milk samples collected before drying-off were obtained for 139 cows from a herd with an average bulk milk SCC of >300,000 cells/mL over the final 4 months of the 2006/07 lactation. Based on culture results, cows were defined as being infected with a major (Staphylococcus aureus, Streptococcus uberis or Nocardia spp.) or any pathogens. Receiver-operator characteristics (ROC) curves were used to determine optimum cut-points for maximum, average and last herd test SCC, for predicting IMI. Multivariable logistic regression models were used to determine which variables were associated with IMI, and the sensitivity (Se), specificity (Sp) and positive predictive value (PPV) were determined for different cut-points.

RESULTS: At the cow level, 75/139 (54.0%) cows had IMI with a major pathogen and 123/139 (88.5%) with any pathogen. A SCC ≥150,000 cells/mL at ≥2 herd tests and a SCC ≥299,000 cells/mL at the last herd test, for cows aged ≥4 years, were associated with IMI with a major pathogen at drying-off (p < 0.05). A SCC ≥150,000 cells/mL at ≥2 herd tests was associated with IMI with any pathogen at drying-off (p < 0.001). A cut-point of ≥150,000 cells/mL at any herd test had the highest Se (0.97 and 0.94), but the lowest Sp (0.19 and 0.44) and PPV (0.58 and 0.93) for infection with major and any pathogens, respectively. A cut-point of ≥150,000 cells/mL at ≥2 herd tests doubled the Sp and increased the PPV without large decreases in test Se for infection with either a major or any pathogen.

CONCLUSIONS AND CLINICAL RELEVANCE: In this herd with a high bulk milk SCC, use of a cut-point of a SCC ≥150,000 cells/mL at any herd test to define IMI would be appropriate, where the goal at drying-off is to ensure that cows infected with any pathogen receive antimicrobial treatment. Where the goal is to reduce the use of DCT in uninfected cows while limiting the number of infected cows not being treated, use of a cut-point of SCC ≥150,000 cells/mL at ≥2 herd tests to define IMI may be more appropriate.