Use and interpretation of somatic cell count data in dairy cows

THE presence of cells in bovine milk, so-called 'somatic cells', has been recognised and studied for many years. More than 95 per cent of somatic cells are leucocytes, including neutrophils, macrophages and lymphocytes. The somatic cell count (SCC) - that is, the number of somatic cells per millilitre of milk - is therefore a useful proxy for the concentration of leucocytes in milk. SCCs in milk are used as indicators of mammary health on the basis that they reflect an immune response and thus the presence of infection. An SCC of < 100,000 cells/ml is often considered to be 'normal', reflecting a healthy mammary gland, whereas an SCC of > 200,000 cells/ml is suggestive of bacterial infection. Although a raised SCC is an accepted indicator of an existing bacterial infection, a very low SCC has been associated with an increased subsequent susceptibility to clinical mastitis. This suggests that somatic cells may provide protection from bacterial colonisation as well as being a marker of infection. This article focuses on how SCCs may be used as a management tool for gauging current infection status - both at the individual cow and herd level.