Association between milk anti-Staphylococcus aureus antibody, somatic cell count, and milk yield in lactating dairy...

To assess the association between the sample/positive (S/P) ratios from a milk IgG ELISA, specific for Staphylococcus aureus and the somatic cell count (SCC) in cow composite milk samples taken at routine herd testing, and the association between the S/P ratio and milk solids (MS; kg fat + protein/cow/day) in cows in a pasture-based, seasonal-calving, New Zealand dairy herd.

Data from cow-composite milk samples were retrospectively analysed to determine the associations between the S/P ratios from a milk IgG ELISA specific for S. aureus, SCC (transformed to linear scores (LS) for analysis), and MS from cow-composite milk samples collected on four occasions across two lactations. A generalised linear mixed model was used to assess the effect of age, breed, month in lactation, and their interactions on the S/P ratio. To assess agreement between cows defined as infected based on S/P and on SCC, cows were classified as uninfected or infected using test positive cutpoints ≥ 0.38 for the S/P ratio and ≥ 150,000 cells/mL for SCC, and the agreement between these categories was compared by Gwet's agreement coefficient (AC). A generalised linear mixed model was used to assess associations between S/P and MS production with age, breed, and herd test date, with their interactions as explanatory variables, and cow as a random effect.

The S/P varied by month of lactation, and there was an interaction between age and breed. There was a moderate agreement between the dichotomised S/P ratios and the dichotomised SCC (Gwet's AC = 0.59 (SE = 0.02); p < 0.001). When the MS production of cows defined as S/P test negative, suspicious or positive was analysed, there was an age by S/P interaction whereby there was no association between S/P category and MS amongst 2-year-olds, while in older cows, test-positive cows had lower MS than test-negative cows.

This observational study demonstrated moderate agreement between S/P ratio and SCC. Additionally, there were negative associations between the S/P ratio and MS amongst older cows and in late lactation. Thus, the S/P ratio of an S. aureus-specific IgG ELISA is correlated with a well-defined marker of mastitis (SCC) and with milk yield depression. Further assessment of these associations using microbiology, and investigation of associations between the antibody results and long-term survival in the herd would further strengthen the validity and utility of the test.