When a dairy cow gets clinical mastitis again in the same quarter, it is easy to think that the first treatment didn’t work.
When this happens, it is possible that the first treatment failed but it is also possible that there is something about this cow or that particular quarter that makes it more susceptible to getting clinical mastitis.
As diagnostic procedures get better, it has become possible to figure out if the second case of mastitis is caused by exactly the same bacterium, not just the same species of bacterium. It doesn’t make much sense to use these tests routinely but they do make an excellent research tool.
In a recent study from the EU, researchers followed cows in three herds. If cows got clinical mastitis, they cultured the milk to figure out what bacteria were involved. If there was a second case of mastitis in that same quarter more than 2 weeks later, they cultured that too.
If the same species of bacterium was found in both the first and second cases, the researchers checked to see if the two bacteria were genetically identical. If both bacteria were identical, then it was likely that the second case was actually a relapse (called a persistent infection) because the first case wasn’t cured.
They looked at over 2,000 cases of moderate clinical mastitis and, of those, about 22 % were a second case in the same quarter. After they cultured the bacteria from both cases in the same quarter, 145/445 recurrent cases had the same species of bacterium in both the first and second cases. If you were to go just on that information that the same bacterium was found in both cases, you’d think that 32 % of the new cases were actually a recurrence of the old case of mastitis rather than a new infection. In fact, after looking closely at the genes of the bacteria from the first and second infections, they found that only 49 of the 445 cases were caused by the exact same bacterium (11 %). Only 11 % then of the recurring cases in a quarter were likely because the first infection hadn’t been cured. In other words, only 11 % of the time had the bacteria persisted.
Why would you find the same species of bacteria in both a new and older case from the same quarter even if they were not identical? That is because so many cases of mastitis are caused by only a few species of bacteria. In part, that is because only a few species of bacteria can actually invade the udder. It is also because almost all bacteria that cause mastitis come from the barn itself and some bacteria are more common in barns than others. So it is not surprising to see the same species of bacteria causing several clinical cases of mastitis even if they are not the exact same bacterium — there were more new infections with the same species of bacterium than persistent infections.
Looking at it another way though, even those 11 % of cases that look like they hadn’t been cured is likely an over estimate. There is no way to know if that first infection was actually cured but the same quarter got re-infected by the exact same bacterium from another cow or from the barn environment.
The bacteria that were persistent were not really surprising; they were bacteria we already knew were hard to cure. One was Staphylococcus aureus. Others were E. coli and Strep. dysgalactiae. There were two cows in the study that had Trueperella pyogenes, a bacterium that we often associate with heifer mastitis which we also know is hard to cure. We usually think of Strep. uberis as difficult to cure and although it was a common of recurrent cases but most of these cases were actually new infections not persistence from the previous infection.
This research is a good example why it is worthwhile to culture cases of mastitis that appear to come back to help figure out the best treatment protocol.
Dr. Rob Tremblay is veterinarian for Boehringer-Ingelheim and lives near Guelph. Follow his blog at http://bovinehealth.ca.