Online congress looks at latest thinking on BVD

The past few years have seen steady changes in how vets approach bovine viral diarrhoea (BVD) virus on their farms – eradication programmes in Scotland and the Republic of Ireland, the start of a programme in Northern Ireland and promises the BVD Free project will soon commence in England and Wales is driving this change.

One significant addition in the fight to control and remove this Pestivirus has been the introduction of a modified live vaccine. This vaccine comes with a large volume of rigorously tested trial work data showing a single shot not only offers cattle farmers quick production of immunity for the animal, but also has proven fetal protection.

In November 2015, Boehringer Ingelheim, the company behind the new vaccine (Bovela), presented its first BVD Zero Web Congress. In a novel approach to providing CPD for cattle vets, several leading BVD virus researchers presented a live, online congress from a studio in Barcelona, to a global audience.

It is important to note BVD knowledge has progressed with pace in recent years, with much of the audience probably seeing a lot of the science and thinking for the first time.

History

Volker Moennig, professor emeritus of the University of Veterinary Medicine in Hanover, began with a review of the history of BVD virus, working through the basics of transmission and reviewed control programmes.

First identified in 1948, Dr Moennig postulated BVD virus was probably active for 100 years before that. He also noted we should recognise not the cost of acute BVD virus as the biggest cost, but the persistently infected (PI) animal, as carrier and reservoir for the virus, impacting any naïve animal it comes into contact with and perpetuating the virus on farm.

Globally, two genotypes represent quite different levels of incidence – BVD type two accounts for around 50% of the cases in North America, whereas BVD type one dominates in Europe, with more than 90% of disease.

Type two has been noted as being extremely virulent, impacting calf health in particular, with up to 30% mortality noted in the outbreak seen in Germany in 2012. As type one is often much less pathogenic, this genotype is more likely to affect farm economics, with reduced health and fertility alongside the immune suppression an acute infection creates.

The PI animal was demonstrated in terms of the “basic reproduction ratio” where the risk of spread of BVD virus was high, due mainly to the fact a PI animal continuously sheds virus, creating constant risk to naïve animals.

A PI animal represents tremendous risk to the rest of a seronegative, naïve herd, with Dr Moennig suggesting the industry, and especially vets, should be stressing this more.

Levels of infection

Robert Fux, from the Institute of Infectious Diseases and Zoonoses at Ludwig-Maximilians-University of Munich, then presented his considerations towards different levels of persistent infection recently identified.

One point of particular interest was the white blood cells of a newly infected animal would shed BVD virus particles for up to eight weeks postinfection – much longer than the few days of acute BVD virus viraemia. Could this mean transiently infected animals were more of a risk than previously thought?

A new class of PI was postulated by Dr Fux as “semi-persistent” BVD animals, occurring due to the variability in the immune competence of the developing fetus and then in later life, when challenged in later life by exposure to a very similar, yet a mutation of, BVD virus.

A young bull called “Martin” was identified as a PI animal during early trials of ear notching in Germany. This bull was identified by one lab test as virus negative, but on PCR, was positive. Martin was removed to the university for study when, using immunofluorescence, his cells were shown to be heavily infected with BVD virus.

At six months old, Martin then appeared to produce antibodies to BVD virus, to be clear of virus by 13 months old.

BVD overseas

Jocelyn Amiot, a practising vet from Burgundy, presented the state of BVD virus control in France. Mr Amiot noted outbreaks of classic BVD virus infection were still seen in his practice, giving an example of one such incidence on a beef and dairy farm showing the quantum of this outbreak.

One interesting observation was made by Mr Amiot on immune suppression following acute BVD virus, when he explained he now routinely looked for BVD virus when investigating calf diseases such as cryptosporidiosis.

Two presenters from the US demonstrated the continued level of research into BVD virus. Dan Givens, a professor from the Auburn University College of Veterinary Medicine in Alabama, looked specifically at the risks to healthy cattle of BVD virus introduction. One number struck out – when buying cattle with unknown BVD virus status, the relative risk to purchasing a PI animal was 33% for every 100-head batch. In US feedlots, this would be of particular concern where a PI BVD virus animal in a batch of animals could be spectacular.

BVD type two, Dr Givens noted, was also a concern – for both mortality rate and its ability to mimic mucosal disease in clinical signs and at necropsy. This would be something to consider as a UK practitioner as BVD virus type two has been identified, with a confirmed case in Northumberland.

Finally, James Roth, director of the Center for Food Security and Public Health at Iowa State University, explained the differences in humoral and cellular immunity, relating to how the immune system reacts to BVD virus infection. With a high mutation rate within this RNA virus, Dr Roth showed how the virus functions within the host and why this causes difficulty for killed BVD virus vaccines.

Describing the immune response to modified live and killed vaccines, it was clear the former generates a different immune response, which could help to control BVD virus on farm.

A modified live vaccine makes sense with single shot use, but, more importantly, makes sense at the immune system level. Both humoral and cellular immunity is needed when fetal protection, preventing new PI animals, is the main target to manage this virus.

With relatively high levels of prevalence sitting alongside a very easily transmitted virus, the use of vaccination on farm ranges from 10% in Belgium to 36% in the UK, with an EU average of 25%. Following this event, it is hoped many more farmers are encouraged, where national eradication programmes are not already in place, by their vets to check their herd BVD virus status, search for and remove carrier animals while vaccinating the healthy, naïve stock that remains.