Culture of bulk tank milk as a mastitis screening test: A brief review

The culture of a sample of bulk tank milk may be a useful technique by which to screen herds for major mastitis pathogens. Staphylococcus aureus and Streptococcus agalactiae, if identified on a culture of a sample of bulk milk, reliably indicate infection of the udder. Environmental bacteria, such as the other streptococci and coliforms, are unlikely to be indicative of the proportion of cows infected with these organisms.Samples of bulk milk are readily obtainable and can be rapidly and inexpensively cultured to screen large numbers of herds for mastitis-causing bacteria, yet the performance of the test has only recently been formally assessed for its ability to correctly classify herds according to infection status.A single culture of bulk tank milk has been found to be a test with low sensitivity and high specificity for determining the presence of S. agalactiae or S. aureus in the herd. This means that many infected herds will be called negative, but few uninfected herds will be classified as positive.The literature assessing the performance of bulk tank milk culture in comparison with other mastitis screening tests, the use of bulk milk culture for prevalence surveys, and factors affecting these results is discussed.