There doesn’t seem to be much actual data to support it, but my perception in speaking with dairy farmers in a few provinces is that some farmers are buying many more cows than they have in the past. In some cases, they have been buying entire herds. Buying cows often implies that there is an element of risk — the risk could be that the cows in the home herd get sick but there are situations where it is the purchased cattle that get sick. Equally risky is when purchased cattle bring in disease that doesn’t cause cattle in the home herd to look sick because the disease is subclinical. The undetected subclinical disease may then spread to cows in the home herd. One of the subclinical diseases that could be carried into a herd by a purchased cow is contagious mastitis.
It is always difficult to know just how big a risk buying a cow might be. Lessening that risk for contagious mastitis in particular is one reason that many recommend buying heifers rather than lactating cows. Heifers are less likely to be infected already with contagious mastitis microbes.
Just recently, a research group published the results of a trial where they tried to determine how many dairy herds in each province contained cows infected with major contagious microbes: Staph aureus, Mycoplasma bovis, Strep agalactiae and Prototheca. This study looked at milk from the bulk tank rather than from individual cows in the herds. The test they used was the same one available for testing individual cows through Canwest DHI. It is not a culture test; it looks for genetic evidence that the microbe was present. It has an advantage over the culture because it will find the microbes even if they are not still alive. One downside is that the test they used was developed to test milk from individual cows rather than bulk-tank milk. It is not likely that this had a big impact on the results.
Each of the previous studies on how many Canadian dairy herds had evidence of cows with contagious mastitis was limited by how the researchers selected the herds that they tested. In this new study, the researchers worked hard to try to make sure that, for the first time, their results truly reflected the entire Canadian herd.
What did they find? Well, first the good news. Of the 372 herds that were tested, very few had either Strep agalactiae (1 herd) or Mycoplasma bovis (2 herds) in their bulk tank milk. It was a different story for Staph aureus because 172 (46 %) herds were positive. There were 24 (6 %) of herds that were positive for Prototheca. Although that figure for Staph aureus seems high, it was actually lower than what some other studies had found. That could be because some of the other studies looked at more than one sample from each herd, where this study tested only one bulk tank milk sample.
What does this mean? Well, it doesn’t tell us what the risk is when farmers buy individual cows but it does tell us that contagious mastitis is present on many Canadian farms, so buying any cows will likely carry a risk. This risk can be managed by first assuming that all purchased cows are infected until you test them to make sure they are not. There are a number of ways to test, including milk culture and the Mastitis4 test from Canwest DHI. No test is perfect, so make sure you know the limitations of the test you end up using.
Subclinical contagious mastitis is just one of the health risks with purchased cows. They could be infected with Johne’s disease or leucosis virus too. Pregnant cows could be carrying a calf that is infected with BVD virus that will spread the virus after it is born.
Dr. Robert Tremblay is a veterinarian for Boehringer-Ingelheim and lives near Guelph.
