News Q&A: Move to spot test in BVD free herds seems unlikely

Spot testing for BVD, rather than testing all calves, has been “shot down” by researchers who identified birth profiles and movements of calves born in Ireland during 2014 and 2015.

News Q&A: Move to spot test in BVD free herds seems unlikely

Spot testing for BVD, rather than testing all calves, has been “shot down” by researchers who identified birth profiles and movements of calves born in Ireland during 2014 and 2015.

Spot testing, supplemented in dairy herds by testing of milk samples, has formed the basis of the successful eradication programmes run in the Scandinavian countries.

Screening of a limited number (typically 5–10) of home-bred animals for antibodies to BVD virus can provide an effective means of surveillance.

It had been suggested by the technical working group, which advises on BVD eradication in Ireland, that screening 10 animals per herd could suffice.

Screening would be carried out when calves are old enough to no longer test positive due to the presence of any maternally derived antibodies, typically 6-9 months old.

The youngest animals would normally be

selected for screening (but would include animals from each separately managed group).

With most calving in Ireland occurring in the spring, the majority of herds would be screened in the autumn.

However, researchers have concluded that screening of animals aged 6–9 months for BVD is not suitable for all Irish herds.

Why are changes being considered from the current system of testing all calves?

It has been proposed that serological testing of a sample of home-bred young stock would be a more cost-effective surveillance mechanism than continued tissue-tag testing, in herds which have previously been found to be BVD-free.

A negative test result would indicate ongoing freedom from infection in the herd, while the presence of antibodies would suggest that the herd had recently been exposed to BVD virus, which would result in withdrawal of the herd’s Negative Herd Status, and further investigations to identify any persistently infected (PI) animals present.

What research has been done on this?

To examine the potential practicality of a serological testing system, researchers identified birth profiles and movements of calves born in Ireland during 2014 and 2015.

They found that birth profiles for both beef and dairy animals were more evenly distributed throughout the year than often assumed, which should be borne in mind when evaluating the suitability of a single round of serological testing in the autumn for every herd.

A large amount of movement was identified, with

approximately 43% of calves experiencing a move before they reached 10 months of age.

Approximately 19% of calves had moved to other breeding herds in Ireland within this period.

There were distinct patterns, according to movement type, month of birth and herd type.

The majority of herds moved either all or none of their calves in the first 10 months of life.

These results indicate that young stock serological testing is unlikely to be an appropriate surveillance mechanism for all BVDv-free herds, because:

n many herds would not be able to supply a large enough sample of suitably aged home-bred young stock at a single point in time.

n and persistently infected calves which would have been picked up by tissue-tag testing soon after birth would have moved from their home herd, to infect other herds, before serological testing could be conducted.

Did the research indicate what herds are unsuitable for spot testing for BVD?

Many dairy herds sell some or all of their bull calves within weeks of birth, meaning that this group is not available for check testing.

Some 16,500 suckler herds operate a suckling to weaning system, with spring-born calves being sold in the

autumn of their birth year, meaning that there may not be any animals in the preferred age group available for testing, particularly if no females are retained as breeding

replacements.

A further possible issue for both enterprise types is that an extended calving season may mean that not enough calves born in a given year have attained the lower age limit for sampling within a check test, or there may not have yet been sufficient time for the spread of the virus from a recently born PI to older calves.

How have other countries

approached BVD testing?

Ireland’s tissue-tag testing of newborn calves is similar to methods previously implemented in Switzerland and Germany

Spot testing, supplemented in dairy herds by testing of milk samples, has formed the basis of the successful eradication programmes run in the Scandinavian countries.

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