Farm Ireland

Sunday 18 March 2018

Bulk milk test vital in tracing diseases

It's one of the cheapest and fastest methods on market

Paedar O'Scanaill

With substantial expansion of the national dairy herd already in train, dairy farmers are already hitting the disease challenges that increasing cow numbers inevitably poses.

The evidence shows that as herd sizes expand, the prevalence of diseases such as IBR, Johne's disease, BVD and Mortellaro have increased to the point where they are now widespread in Irish herds.

The challenges to maintaining biosecurity on Irish farms was largely compromised by the level of herd repopulations required in the wake of BSE and Brucellosis. Some of these diseases, such as Johne's, have long incubation periods and may have a serious impact on productivity, even at sub-clinical levels. In addition, all the much-championed emerging markets are becoming much more discerning on milk quality issues, especially zoonotic diseases that can affect humans.

The melamine scares in China ensured that milk quality is non-negotiable.

As a result, there has never been such an appetite for information concerning disease control at farm level.

One of the fastest and cheapest ways for dairy farmers to get a handle on the levels of disease in their herds and, in turn, the changes in this status over time is by using a bulk milk tank test. It isn't a silver bullet since, by its nature it is not animal specific.

Therefore, it can be slow to find some diseases, such as Johne's disease, and there is a risk that a negative result can give a false sense of security.

How does it work?

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When a cow is infected with a viral, bacterial, or parasitic bug, she will mount an immune response in the form of specific antibodies, which will be present in blood and milk. Usually, antibodies will appear after seven days and peak at 10-14 days post-infection and are good indicators of what disease the cow or herd has been exposed to.

However, the onset, size, duration and effectiveness of the antibody response will vary. BVD antibody response is rapid and effective at clearing BVD virus and will remain relatively high for the duration of the animal's life.

On the other hand a cow infected with Johne's disease may result in an antibody response that could be delayed up to two years.

Unfortunately, this type of antibody response will not protect the animal from disease.

Reading the test results

It is essential to appreciate the limitations of the bulk milk test when interpreting results. But it is also vital to understand the disease in order to offer appropriate advice. It is best to consult your own vet once the results are available. It is possible to look at bulk milk antibody changes over time if we take regular samples. Analysis of these results can give an early warning about the possible spread of infectious disease within a herd.

Other factors to be taken into account are as follows:

•Agitation of milk in the tank prior to sampling is crucial.

•Seasonal variations in herds, especially close to drying off time may alter the interpretation. As a result, sampling every three months is necessary.

•Seasonal disease variation can also be observed in some herds, especially those with parasitic disease. Strong positive or low negative antibody levels provide a clear indication of the presence of widespread infection or very low levels of infection with the possibility of freedom from disease. Intermediate antibody levels provide a challenge for interpretation suggesting a current or historic presence of infection in the herd. In many cases, further testing may be necessary.

Peadar O'Scanaill is a Meath-based vet and member of Veterinary Ireland. Email:

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