On my farm practice placement one of the diseases the vets talked about a lot was bovine viral diarrhoea (BVD). Apparently BVH has a huge economic impact on cattle farming: it can cause poor performance in herds for many years without being treated as farmers may come to accept this as normal. Ongoing infection and re-infection happens via an interesting mechanism including intrauterine transfer to calves and Persistent Infected (PI) individuals constantly shedding virus into the herd’s housing.
BVD infection leads to respiratory problems and poor fertility, as well as compromising the immune system and making cattle more vulnerable to secondary diseases like pneumonia and scouring. In pregnant cows it can cause abortion, embryo death or birth defects, particularly neurological problems. Where cows are infected early in pregnancy their calf becomes exposed to BVD antigens before their immune system is developed enough to recognise it. These calves, if they survive, become the sinister PI cattle responsible for infecting herds more broadly.
What’s the deal with PI cows?
BDV can be transmitted directly or via the environment between infected cows, replicating and causing infection once it touches epithelial cells. Normal individuals will produce antibodies and can become immune, although some will have chronic infections in organs the immune system struggles to reach such as reproductive organs. PI cows, however, “see” the virus as self because it was present before their immune system formed properly. They can never produce antibodies and go around shedding viral material at a thousand times the rate of a normal infected cow.
Many PI calves fail to reach adulthood, often dying from mucosal disease or cerebellar hypoplasia, the aforementioned neurological condition they are likely to be born with. But those that do grow up continue to infect the cows they live with for their entire lives. Identifying and culling these individuals is crucially important to controlling BVD in a herd and restoring herd performance.
What can be done?
As well as removing the PI cows from a herd, biosecurity and vaccination are both important tools. Milk, ear tag and blood samples can be used to test for the presence of antibodies or antigen and look at the infection status of the herd whilst measures such as double fencing and only buying in BVD-free animals can help keep the disease out.
A voluntary BVDFree scheme started in July aiming to eradicate the disease from the UK by 2022. Several Scandinavian countries have proved this to be possible already and practices are encouraging their clients to sign up and gain BVDFree status. This can be expensive but does lead to greatly improved performance of the farm and can be a selling point, for example selling stock certified as BVD-free.
BVD was an interesting disease to learn about even if we never actually dealt with it during my placement week. I’ve listed my references below but a lot of this came from the vets at the practice who were fantastic teachers!
Ususal disclaimer: I’m not a vet, not even a vet student yet, so please don’t use my blog for vet advice! If you notice things I’ve got wrong or not understood properly, please feel free to point them out. Thanks 🙂