He told a South Island Dairy Event workshop he’s modelled different strategies to combat the virus and found the most cost-effective strategies included vaccinating the herd and screening incoming animals with a virus test.
However, to keep a herd free of BVD farmers had to implement a variety of practices for each stock class – testing and culling, vaccination, and border control.
“The main risks to the herd can be summarised with ‘in, out, and over’,” he said. That is, stock coming into the farm, stock that are off the farm in the first four months of pregnancy, and over the fence contact with neighbours’ stock.”
All animals coming onto the farm should be tested first for the virus to see if they are persistently infected (PI).
“As well, many cows are bought and enter a herd when they are in the later stages of pregnancy and there is the risk the calf, once born, will be a PI. These PI foetuses are commonly referred to as Trojan calves, named after the Trojan horse story.”
He said all replacement calves should be tested early to see whether they are PI. Until they were cleared they should be kept away from the herd so no physical contact could occur.
“If you are only going to vaccinate some of the herd then do the at-risk groups, which are the carry-over cows and heifers, before they are sent away for grazing because they will come back when they are pregnant.”
Bought or leased bulls should be tested and vaccinated before contact with the herd, ideally two months before they are needed.
“The transient infection can cause dramatically reduced fertility in bulls for up to two months after infection.”
Exposure to a neighbour’s infected herd, such as over-the-fence contact, may cause abortions and milk loss.
“However the main issue for the farm is if the contact happens in the first four months of pregnancy and the resulting PI calf is kept as a replacement,” Weir said.
“Major outbreaks often occur when a PI calf is born on a property that has not had BVD before and the farmer is unaware of what the virus can do.”
He said boundary fencing to stop over-the-fence contact and keeping young stock in their own mobs so they had no contact with other animals, especially when grazed off farm, were all management practices that would lower the risk of BVD.
“BVD control is about stopping PIs entering the herd and either stopping them from being made inside pregnant animals or catching them shortly after birth before they can cause too much trouble.
“Remember the linchpins of BVD control are annual calf screening of replacements and herd vaccination.
“It’s well worth working through your situation in more detail with your vet but this is a great start and much better than ignoring BVD.”
The risks of sharing
Bovine viral diarrhoea (BVD) causes reduced conception rates, abortions, foetal deformities, and small and weak calves being born.
It also suppresses the animal’s immune system, increasing the rate and severity of scours and pneumonia in calves and mastitis in cows. Herds which have been recently infected can show a 5% drop in milk production or more.
The virus is found in body fluids (faeces, urine, saliva, and nasal secretions) and is usually spread through direct contact, including over the fence. It has been known to survive up to a week in the environment and so care should be taken when sharing yards and other facilities.
Bulls with BVD will have poor fertility and will pass on the virus to cows during mating.
Animals infected with BVD usually recover and will not infect other animals afterwards but calves from dams infected in the first four months (125 days) of pregnancy become persistently infected (PI). They may have low birth weights, low conception rates, and low milk production but many PIs survive in the herd continually infecting other animals throughout their lives.
