What's in a name ... somatic cell
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 white blood cells (WBCs), being a critical part of the cow's frontline defence against infection in the udder. The remainder are generally damaged and shed epithelial cells from the milk-producing tissue.
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 WBCs in milk. A SCC of 60,000-150,000 cells/ml is often considered to be ‘normal', reflecting a healthy mammary gland, whereas one of >150,000 cells/ml is suggestive of bacterial infection.
In response to infection, SCC can rise very quickly, and to very high numbers, depending on the infecting bacteria, stage of lactation, promptness of detection and treatment.
A herd test SCC for an individual cow (ICSCC) represents the composite result from four quarters; the bulk tank somatic cell count (BTSCC) is the cumulative result from all quarters of all cows contributing to the vat on the day. The BTSCC indirectly estimates the level of subclinical mastitis in the herd. A rough guide is that each 100,000 cells/ml indicates about 10% of cows are infected.
In low BTSCC herds, sudden spikes (> 20%) usually signal new infections, some of which will be undetected clinicals. Such events are more difficult to interpret in high BTSCC herds (> 250,000/ml) as there is greater background ‘noise' of existing and new infections.
Cow-side, the Rapid Mastitis Test (RMT) is a great screening tool to apply before cows enter the vat. Use cautiously in the first 48 hours after calving, as very high numbers of somatic cells in colostrum will cause mild to strong positive reactions. However, infected quarters will still generally show stronger reactions compared to uninfected quarters, especially in heifers.
Events that will increase milk SCC include mastitis - clinical & subclinical; colostrum; cows ‘sick' for any reason; declining milk volume as lactation advances; stray voltage; stress; and involution (drying off process at the end of lactation).
Why are we interested in Somatic Cells?
- Window on udder health at quarter, cow, herd and national herd levels
- Reduced production
- 8kgMS per lactation for every increase of 250,000 cells/ml in the range 100,000-600,000
- Heifers infected in their 1st lactation produce about 8% less milk, an effect carried forward to 2nd and subsequent lactations even if cured
- Processing and manufacturing issues around product mix, yield, shelf-life, and flavour
- Penalty risk
Understand what SCCs tell you (and don't tell you). Have solid systems in place for managing SCC status of the herd. Know what to do when things go astray.
