Bovine Viral Diarrhoea Virus (BVDV), also commonly known as Bovine Pestivirus is recognized as one of the most economically significant diseases in the Australian beef and dairy industries. BVDV is a virus that causes a variety of clinical syndromes, with reproductive disease being the most significant. Based on data from field experiments, the economic impact of introducing a PI animal into a naive herd during mating resulted in a decrease in pregnancy rate by 30%, weaning rate by 35% and turn off at 2 years by 40%.

SOURCES OF INFECTION

Persistently infected (PI) animals are the main source of infection. They secrete large amounts of virus in all bodily fluids and secretions (urine, saliva etc). Close contact between susceptible cattle during yarding, mustering, at agricultural shows and sales can cause infection. One of the most common ways BVDV is introduced into a herd is through the introduction of replacement breeding animals, or over the fence contact with infected animals.  

PERSISTENTLY INFECTED ANIMALS

Which animal is PI?

You won’t know until you test!

Majority of PI animals show signs of immune deficiency, poor weight gain, ill thrift and die from mucosal disease or other BVDV associated illnesses within 2 years. However, some PI animals appear normal and do not show obvious signs of illness. These animals are hard to recognise, making it difficult to diagnose the source of BVDV in your herd. How are PI animals produced? PIs are produced when a pregnant female contracts the virus, the inutero calf becomes a PI. PI animals recognise the virus as normal and therefore don’t develop an immune response to expel the virus from their body.

CLINICAL SYNDROMES ASSOCIATED WITH BVDV

There is a wide spectrum of outcomes associated with BVDV. Infections may be subclinical (no outward signs of disease), clinical or chronic. This depends on the immune status of the animal, the stage of gestation of female animals, and the amount or strain of virus that the animal is exposed to. In Australia, common manifestations of the disease include reproductive syndromes which largely depend on the stage of gestation the animal is infected, a transient viraemia where animals develop an immune response to the virus and finally, PI animals. PI animals are commonly chronic poor doers, immunosupressed and contract severe non-responsive manifestations of common diseases; however, in some cases they can be 100% healthy and die suddenly. This occurs when the virus mutates to a cytopathic strain and the animal dies from acute mucosal disease.  Acute mucosal disease clinical signs: depression, anorexia, drooling, multiple shallow erosions of oral mucosa, pyrexia, profuse watery foul smelling diarrhoea, mucopurulent nasal discharge, dehydration. 

BVDV AND THE REPRODUCTIVE PERFORMANCE OF YOUR HERD

Reproductive syndromes associated with BVDV include: conception failure, normal to prolonged return to oestrus (heat), increase in return to service numbers, spread out calving pattern, abortions, still births, premature calves, poor doing and weak calves, birth defects such as blind calves or calves with neurological problems, ill thrift and death in weaners and young cattle. A good pregnancy rate does not indicate a BVDV free herd. Reproductive losses are associated with the stage of gestation heifers/cows are infected.

TESTING FOR BVDV

There are a range of options for diagnosis of BVDV including on farm analysis and DIY kits, ask your veterinarian for help deciding which method is best for your farm.     

How to test for PI animals – Virus antigen testing.

• PI animals will always be virus antigen positive
• Sample: blood, skin (ear notch), hair

How to determine immunity of a management group (i.e. heifers vs cows) – Antibody testing

• Seropositive means the presence of antibodies (or immunity) against BVDV and indicates past exposure to the virus or current infection.
• Samples: blood
• How: sample 5% of animals (randomly selected) from a breeding herd/management group. This determines what proportion of the group has immunity.
• Very important tool to help develop a management strategy against BVDV associated losses for each group on farm.

MANAGEMENT AND PREVENTION OF BVDV ASSOCIATED LOSSES

Low level immunity herds are at a high risk of infection if they encounter a PI, they are usually PI free (i.e. have not encountered the virus) and require vaccination as severe reproductive losses can occur if they are exposed at critical times during mating and gestation. Management groups with high levels of immunity (seropositive) will not benefit from vaccination and may contain a PI animal; however, as they already have immunity, exposure to a PI during gestation may have no to minimal reproductive consequences.

VACCINATION

Initially two vaccinations are required between 1 and 6 months apart, followed by an annual vaccination. It is common to only vaccinate heifer herds (as they usually have low levels of immunity) and not the cow herd; however, it is imperitive to test herds before making these decisions, to prevent any devastating losses due to BVDV. Speak to your veterinarian to determine what vaccination protocol is suitable for your herd.