Breaking down BVD
October 2023

The BVD (Bovine Viral Diarrhoea) bulk milk results will have no doubt been flooding many of your mailboxes now or within the next month. This article is not about the disease itself but focussing on what the graph means.

To get the most out of it we will break down the graph for you so

a) you understand what you are looking at and

b) that together we can evaluate the risks and costs of having BVD infection on your farm (or keeping it free from BVD).

These stars quantify the antibody test result which indicates your herd’s exposure to BVD. The level of antibody needs to be interpreted with care and in context with your herd’s individual management and biosecurity risks. Most herds that have had BVD infection in the past few seasons will have a result that sits in the high to very high range.

The arrow at the top of the graph is very important as it represents the test for detecting the BVD virus. The white arrow is a negative result, meaning that there were no cows infected with BVD milking into the vat on that day. A red arrow means there is at least one cow in the herd that has BVD virus infection. A red arrow warrants further investigation as we know there is an infection in the herd but not whether this is a persistent infection (PI) or a transient infection (TI). However, a white arrow may also need further investigation depending on the individual scenario; for example, if there is a large jump in antibody level.

It is important to remember that the bulk milk test is good for screening for BVD infection within the herd, however, it is not without its limitations. The biggest limitation is that there may be BVD positive animals on the farm, but a negative bulk tank would show up if these cattle were not being milked into the vat. This can include pregnant cows, lame cows, sick cows, dry cows or even bulls (if they have not already been tested and vaccinated).

To help you understand; the graph above shows that last year there were high circulating antibodies within the herd, but previous two tests last season have returned negative BVD infection in the herd.

The first milk result for this season so far has returned a positive virus result, which means there is at least one milking cow in the herd with BVD infection.

To break this down further there are a few details to be ascertained:

  • Was there a BVD positive animal on the farm last season that was not contributing to the vat at the time of testing?
  • Have the replacement heifers been tested prior to entering the herd?
  • Has there been any other cattle brought onto the farm since the last season?

Once we answer these questions, we can then start a focused investigation to target testing in the hunt for the infected cow.

The economic costs to a naïve herd have been conservatively reported as being $22.22 per cow/year. The biggest economic impact can occur when a persistently infected first calver enters the herd causing transient infections to the existing milking herd and a subsequent drop in milk production.  

There are a few aspects to the disease which can complicate an investigation, so it is a good idea to get in contact with us (if we haven’t already with you) to help you get the best out of this screening tool!