Bovine Viral Diarrhoea/Pestivirus
What’s that ?
Bovine viral diarrhoea or BVD is a disease of cattle and other ruminants caused by a Pestivirus. This virus occurs worldwide although only one type is found in Australia. Infection with BVD can cause major economic losses due to reduced growth rates, reproductive losses, reduced milk production and increase in susceptibility to other diseases. In New-Zealand, the annual losses to dairy farmers from BDV was estimated to be around $ 127 million. In Australia, infection with BVD has been found in 60 % of cattle and 90 % of herds.
What are the signs that my cows may be infected with BVD ?
BVD infection in a herd is often insidious, ie you don’t know it’s there. The clinical signs are highly variable and depend on the age and pregnancy status of the animals infected.
In non-pregnant cattle, the infection often goes unnoticed but some may develop a 2-3 weeks illness with fever, lethargy, loss of appetite and reduced milk production. Discharge from the nose and eyes as well as ulcers in the mouth can also be seen.
Infection of pregnant naïve cows (ie cows that have never been exposed to the virus) is probably the biggest issue with BDV. Here again, the signs vary with the stages of gestation.
|1-4 months gestation
|5-4 months gestation
|5-9 months gestation
|Reduced conception rate, embryonic death, abortion, stillbirth, mummified fetuses. Any calf that survives the infection at this age and is born alive will be persistently infected.
|Abortions, birth of viable to non-viable calves. Calves infected at 120-150 days of gestation will be born with congenital defects (cerebellar hypoplasia).
|Abortions. Calves born after 175 days of gestation will be born alive and viable although they may be more prone to develop more serious diseases later in life. Live calves will be immune to BVD.
What are persistently infected or PI calves ?
PI calves acquire the infection during pregnancy at a time where their immune system is not yet operational, usually around 30-125 days of gestation. Calves infected at this age cannot fight the virus and become infected for life. Most PI calves are born to cows that have had a short term infection during pregnancy but a small number PI calves will be born to PI cows (a PI cow will ALWAYS produce a PI calf). While the literature describes PI calves as being unthrifty, stunted and with poor growth rates many of them are actually indistinguishable from uninfected calves.
Why are PI calves a problem ?
PI calves shed huge quantities of the virus in the environment for their entire life and represent a major source of infection for other animals in the herd. PI calves can also be poorer-doers and have a shortened life-span. Most importantly, PI calves represent a major problem to producers because they may look completely normal yet infect the entire herd silently. PI calves exposed again to the BVD virus later in life develop a severe and invariably fatal illness called mucosal disease characterized by lethargy, fever, severe diarrhoea, ulcers in the mouth, feet and vulva. Death usually occurs within 3-7 days.
How does BVD spread ?
The virus is shed in a range of bodily fluids such as urine, milk, saliva, faeces, respiratory secretions and semen. Aborted foetuses are also a source of infection. There are 2 main ways the virus can be transmitted. Transiently infected and PI bulls can transmit infection during natural mating or AI, thus infecting distant herds.
1) Horizontally between an infected animal or a PI animal and a susceptible one. Contact between infected objects (halters, buckets, feed/water troughs,..) and naïve cows is also a means of transmission.
2) Vertically, from cow to foetus during pregnancy. This is the only way PI calves can occur.
What strategies can I put in place to minimise transmission of BVD to my herd ?
There is no “one size fit all” approach with BVD. Strategies that work for your neighbours may not work for you.
However, a general management plan includes:
- Establish the likelihood of introducing BVD onto your property
- Manage cattle movements and new introductions
- Good on-farm biosecurity practices (fencing, gates, visitors,…)
- Identify persistently PI animals
- Vaccinate at risk animals