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BVD (Bovine Viral Diarrhoea)

Friday, 03 September 2021

BVD (Bovine Viral Diarrhoea)

BVD is a virus that causes reproductive and immunosuppressive problems within both beef and dairy herds.

It is actually a misnomer - BVD stands for Bovine Viral Diarrhoea but this is a clinical sign that we rarely see. Many dairy herd contracts will now require BVD testing of all calves born on farm while many of our beef farmers are part of voluntary schemes. Other farms perhaps test infrequently to confirm they remain clear of disease but not signed up to a specific scheme. Red Tractor are introducing new BVD requirements for beef herds next year so why does the industry care so much about eradicating this virus?



Disease spread

BVD virus can spread via the presence of transient infection (the method in which most viral diseases spread) or persistent infection (the more common route for BVD which can be hard to understand). There are equivalent diseases in sheep (Border disease) and pigs (Classical Swine Fever). While each type predominantly only infects the related species there has been some evidence of spread of the related viruses between sheep and cattle which can complicate eradication schemes.

BVD virus mainly spreads by creating Persistently Infected (PI) calves. These are formed when a cow is infected with BVD during the first 100 days of gestation. The calf’s immune system will recognise the virus as part of its own make up meaning the disease will replicate without being recognised by the immune system. Traditionally it was assumed that PI calves would quickly become sick and die but many will survive for a prolonged period in the herd and may even reach breeding age – when they will produce another PI calf. These PIs are responsible for the majority of infection on farms and their presence is the most important factor in the continuation of BVD. They can spread the virus via nasal discharge, saliva, semen, faeces, milk, urine, blood, vaginal and fetal discharge; the main route is aerosol or nose to nose contact. In Germany there has even been evidence of vets spreading the disease between farms on their equipment and cars(!) The possible outcomes of BVD infection are demonstrated in the schematic below.

BVD Figure 1

Figure 1 - Possible outcomes for a cow infected with BVD when pregnant and non-pregnant



Clinical signs

BVD causes a wide variety of clinical signs – some are directly caused by the virus but most are due to secondary infection due to suppression of immunity. Clinical signs include:

  • Weak calves
  • Foetal ‘monsters’ – small brains, small eyes, cataracts, curly coats, limited lung capacity
  • Abortions
  • Transient decreased milk production
  • Increased incidence of other diseases including Johne’s, TB, clostridial disease, IBR, Salmonella, calf scours etc.
  • Increased incidence of calf pneumonia – a PI can increase the requirement for treatment of pneumonia within a calf rearing system by >40%
  • Increased risk of metritis, mastitis, retained cleansings and ketosis in fresh cows
  • Reduced fertility – due to follicle health issues and early embryonic death
  • Mucosal disease – the end stage for PI calves involving profuse, watery diarrhoea, high temperatures, oral ulceration, severe nasal discharge, and death within a few days.


Diagnosis

Much like for COVID-19, testing for BVD can look to find evidence of previous infection (antibodies) or evidence of current infection (antigen/PCR). There are some basic concepts which help understand what we can achieve from testing:

  • PI calves will always be PCR/antigen positive and antibody negative
  • Antibody positive will either have had the disease previously or have been vaccinated

There are a few types of testing commonly utilised by vets:

  • Check test: tests for antibodies in 9 – 18 month old unvaccinated animals to determine if infection is present on the farm
  • Useful for establishing initial status
  • Useful for monitoring BVD Free herds
  • Tag and testing: tests antigen in animals via a tissue sample taken when the ear tag is inserted
  • Useful for monitoring BVD Free herds
  • Useful for monitoring creation of PI calves if farm is attempting BVD eradication
  • Useful for checking for PIs (bought in or born) without paying for vet time (please cc us into the results though!)
  • Bulk milk Antibody and Antigen testing
  • Useful in dairy herds to monitor BVD Free herds or establish initial status
  • Fetal tissue PCR: tests for the virus in fetal tissue
  • Useful as part of an abortion package to rule out BVD as the cause
  • More useful than antibody testing the aborted cow


Treatment

There is no treatment for BVD. Animals that are transiently infected will develop a natural immunity while animals that are identified as PIs must be culled immediately. It can be tempting to keep a seemingly healthy PI in the hope that you can finish it, but it will be constantly shedding large amounts of the virus. PI animals in contact with pregnant cows will produce more PIs and when in contact with non-pregnant animals will cause immune-suppression and thus cost money.



Control and Eradication

Recent estimates of the cost of BVD in Ireland (before the introduction of their eradication scheme) were €29.1 million for the national beef suckler herd. In the UK it is estimated that a 100 cow suckler herd with no BVD control over 10 years will cost £6000+/year.

The good news is that we can vaccinate for BVD, the bad news is that we cannot simply vaccinate our way out of a problem. Vaccination will protect the majority of cows but if a PI is present then the viral load can still overwhelm the vaccine antibodies and cause disease (like we are currently seeing with COVID-19!). PIs will not respond to vaccination so will continue to spread disease. We are largely unable to distinguish vaccinated animals from those naturally infected with BVD which can make it hard to know if we are successfully eradicating the disease. This means that while vaccination can play a part in BVD eradication it must be done in combination with excellent biosecurity and sensible testing strategies including testing animals to buy in. If vaccinating while attempting to eradicate BVD then testing to determine the herd status needs to be carried out before the vaccine has been given – generally we advise a check test and then to vaccinate the heifers before breeding but strategies vary between farms so please discuss this with your vet!



The national picture

BVD Figure 2

Figure 2 - Much of Europe now has BVD eradication schemes in place. Green = voluntary, Red = Compulsory

BVD eradication in England and Wales is still voluntary but there are compulsory schemes in Scotland and across Ireland as well as across much of Europe. In Northern Ireland the prevalence of BVD has now been reduced from 54% to only 0.3% and in Scotland any farm that has a PI is given a ‘not negative’ status meaning they cannot move cattle off farm until the PI is removed. BVDFree England can be joined by farmers online and it utilises ‘check tests’ (see above) to determine status within a herd and then monitor this going forward. It is highly like that a compulsory scheme will be required in England in the next 5 years so it is worth signing up early to be ahead of the game! In the first year of BVDFree England 10% of cattle were positive for antibodies meaning they had been exposed to a PI calf in their lifetime. Wales has now screened over 80% of their herds for BVD but BVDFree England currently has <50% of breeding cattle registered so there is still some way to go.

From 2022 Red Tractor is introducing new Beef requirements to combine with their Dairy requirements in terms of BVD. These mean that all cattle farmers must have a plan to eliminate BVD from their herd and subsequently prevent its reintroduction. It is recommended that this is done through a national programme. As discussed above, in addition to farm compliance and the risk of future legislation, BVD is estimated to cost between £13 and £31 per cow meaning that there is a significant financial benefit to being BVD Free.

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