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Can a Somatic Cell Count Be Too Low??

Dairy Pipeline: June 2000

Ernest Hovingh
Extension Veterinarian
Virginia-Maryland Regional College of Vet Medicine
Virginia Tech
(540) 231-5234

This topic has at times been hotly debated by mastitis researchers, geneticists, dairy producers, and others. A recent study published in the Journal of Dairy Science (June 2000) adds a bit more 'fuel for the fire.' A long-term study (13 years!) was carried out in a single herd in the Netherlands with a low bulk tank somatic cell count (annual average less than 200,000 with only 4 tests over 250,000!). Milk production data were collected approximately every 2 weeks, as well as information about the health and body condition score of the cows. Cases of clinical mastitis were carefully recorded and samples were taken for culture. The researchers were interested in seeing which factors were associated with an increased risk of a cow developing clinical mastitis (abnormal milk and/or a hard and swollen udder) -- 43% of which were found to be caused by E. coli. After taking into account such factors as production, stage of lactation and body condition, they found that a low somatic cell count increased the risk of a cow developing a case of clinical mastitis. The authors of the study correctly note that this study was only carried out in a single, low SCC herd and that care must be taken in applying the results to other herds. However, their results are consistent with some other studies which have found that when cows were experimentally infected with mastitis-causing organisms, those with low SCC developed more severe cases of mastitis. (It must also be mentioned that a few studies have found that a high SCC can put a cow at an increased risk of mastitis!) So what do these results mean? Should we not try to achieve low somatic cell counts? The evidence isn't (yet?) strong or convincing enough to recommend setting a minimum SCC for a cow or a herd. However, this research does reinforce the idea that it is very important to pay close attention to such things as milking practices, environmental management, and cow health -- even if you have attained a very good udder health/milk quality status in your herd (consistently less than 200,000 cells/ml, for example). In general, a cow with a low or very low somatic cell count should be considered to be at least as much at risk -- if not more so -- of developing a case of clinical mastitis as a cow with a higher SCC.

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