11 Oct 2022
Carolyn Baguley
Job Title
Image © Sina Ettmer / Adobe Stock
BVD remains a very real and constant challenge for farm vets and their clients – especially in England and Wales – with likely policy changes ahead.
Many farmers have engaged with schemes in the voluntary phases, with some having undertaken tissue testing or blood sampling surveillance and vaccination for some time. However, the complex nature of the virus’ transmission and the difficulty of securing full biosecurity in a cattle-dense country are frequently behind unexpected breakdowns.
A high-input, high-output herd of 200 milking cows housed all year and with a year-round calving pattern in the east midlands had registered with BVDFree two years ago. Quarterly bulk milk antigen testing had been performed for more than five years, with consistently negative results, and all calves (including stillborn calves and aborted fetuses) had been Tag and Tested for the past three years, again with negative results.
The herd enjoyed good health and calves, housed in pairs in hutches, usually had little disease apart from the odd case of bovine respiratory disease from time to time. Maiden heifers moved to a nearby farm for summer grazing from April to September and were artificially inseminated while away by the person looking after them at the time. A two-shot primary course BVD vaccine, with the second injection at least three weeks before the start of mating, was also administered, this time by the farmer/owner. The herd was on track to obtain BVDFree accreditation in 2022.
However, in February 2022, a much higher level of pneumonia in the pre-weaned calves at the home farm was noticed. Recent bad weather was initially thought to be the cause, but over a few weeks, it was also noticed that calves were failing to respond to treatment. Sampling and lab work identified Mycoplasma bovis, Pasteurella multocida and Haemophilus somnus, and it was clear that some calves were really quite unwell.
Around the same time, the farmer sent off a batch of BVD tissue tags as he had done before, although he did confess that, with hindsight, these had been kept on the farm for longer than usual. Results identified three antigen positive calves. The author and her team wondered if transient infection was the cause, and re-tested these calves by blood sampling them four weeks later. By then, one of the three had died. The remaining two came back BVD antigen-positive again, so were immediately culled as persistently infected (PI) animals.
Discussions with the farmer sought to establish how the virus had got into the herd and work out what needed to change going forwards to prevent it happening again. The dams of the antigen-positive calves were identified, and it soon became apparent that they were all first lactation heifers. The most likely time for PIs to be created in utero is between approximately 30 and 120 days’ gestation, and this fitted almost exactly with when the heifers would have been away grazing in summer 2021.
The farmer then realised the second dose of the primary BVD vaccine course in April/May 2021 had been accidentally missed. It also came to light the group had been co-grazed with heifers from another dairy farm, with unknown BVD and vaccination status. It is likely that the grazing heifers came into contact with a PI animal belonging to the other farm. The author and her team decided it was lucky only three PIs were born as the situation could have been a lot worse – approximately 25 heifers were away grazing last summer.
The last bulk milk BVD antigen test had been performed before all the 25 heifers in the group had calved, so an extra test was slotted in after the final heifer had calved and they were all contributing to the bulk tank. This test was negative, eliminating the possibility any of the dams were PIs (unlikely, since they should all have been Tag and Tested as neonates). All other youngstock on the farm had also been Tag and Tested, and so the author and her team were happy that no PIs were remaining on farm.
The calf cohort that was ill earlier on this year appears to have caught up and is performing well, although they are not weighed regularly. The immunosuppressive effect of the circulating BVD virus in the neonatal calves had predisposed them to pneumonia infection with pathogens that were already present on the farm, but which were usually kept at bay. Now, without the BVD challenge to pull them down, they do all seem in good health and in line with calves from previous years.
Tissue tags will now be sent off more regularly by the farmer to detect any positive animals as quickly as possible.
The decision was also made to swap to a one-shot live BVD vaccine to minimise the chance of errors and make life easier, and to tighten up on the timing of administration, ensuring that the first dose is always given at least three weeks before the start of mating. The farmer has also agreed to tighten up the biosecurity – in particular, ensuring that his stock is not mixed with stock from other farms when grazed away. The herd will now restart its journey to being BVDFree accredited.
This is a well-run herd, its performance is good, its health gives no cause for concern, and the farmer and team are diligent. It just goes to show how quickly and easily the BVD virus can get into a herd via an unplanned error or biosecurity lapse.
With BVD control likely to move to being mandatory in Wales and, hopefully in time, England too, now is a good time to encourage any clients not yet engaged with testing, surveillance and vaccination to do so.
In the UK’s cattle-dense areas, it’s likely that running BVD-naive herds is unrealistic, so a well-formulated control strategy involving vaccination is likely to be the way ahead for most.
