Can a cow’s somatic cell count (SCC) get too low? Does having a low SCC mean that cows are more likely to get mastitis? Will selecting cows for low SCC mean that you are also selecting cows that are more likely to get clinical mastitis or even are more likely to get severe mastitis when they do get mastitis? The fact that you can ask the question so many ways is a giveaway that there is not an easy answer.
Last month, scientists looked back on the research that has been done on this topic in the past two to three decades to try to draw some conclusions on this difficult topic. They found that you get a different answer depending on how you ask the question and on what data you use to determine the answer. I’ll discuss a little about the different approaches to answering the questions and what they think we know.
Before I get too far into that, I will point out that the scientists concluded that there is no real evidence that selecting cows for low SCC leads to an increase in the risk of cows getting clinical mastitis or to an increased risk that cows will get more severe mastitis if they do get mastitis.
So why is this such a difficult topic to research? Partly, it is because the easiest information to get likely does not provide very reliable answers. The easiest information to get is often bulk tank SCC. Using bunk tank SCC doesn’t tell us much about individual cows in those herds. It also turns out that mastitis, for example clinical mastitis in herds with high SCC, is usually different from mastitis in low SCC herds.
Herds with high SCC tend to have more cows with chronic mastitis and clinical mastitis caused by gram positive bacteria like Staph aureus and Streptococcus. Herds with low bulk tank SCC tend to not have as many chronic cows and the clinical mastitis is usually caused by gram negative bacteria like coliforms. Because mastitis tends to be so different between herds with high or low SCC, it is not surprising that it has been difficult to find good answers about whether cows with low SCC have more risk of mastitis.
Research on how resistance to infection in the udder actually works doesn’t provide a clear answer either. It suggests that low SCC may not be an especially good indicator of how well protected the udder is against mastitis. That’s because udder protection goes way beyond just the SCC in the milk produced by that quarter.
The real issue then is not whether SCC is a good reflection of the udder’s vulnerability to infection or to severe infection. The real practical question is whether SCC information is a good way to select for the next generation of cows so they will be more likely to have better udder health. The research suggests that selecting cows with low SCC will not lead to a greater risk of cows getting mastitis or to having more severe mastitis if they do get infected. It is likely to lead to better overall udder health.
Dr. Robert Tremblay is a veterinarian for Boehringer-Ingelheim and lives near Guelph.