Bovine Viral Diarrhoea (BVD) is a disease with a range of clinical presentations and the manifestation of this disease on a property will, to some extent, depend entirely on the type of enterprise being run.
The BVD virus is a pestivirus that is very common in New Zealand cattle herds. The virus is transmitted via a wide range of body tissues, including respiratory secretions, uterine secretions, urine, milk, semen, faeces, saliva and blood. If an animal that has never met the virus before becomes exposed then the result is normally a short period of infection or sub- clinical disease, followed by recovery.
The complicating factor with this disease is that pregnant animals that meet the virus for the first time between 45 and 120 days of pregnancy give rise to calves that are persistently infected with the disease. Such animals are known as PI animals and are an enormous source of virus. These PIs nearly always go on to develop the fatal form of the disease known as mucosal disease (MD). Pregnant animals that are infected earlier than this 45-120 day window will probably lose the foetus and return to service or turn up empty. Cows that are infected after this window will tend to show full-term abortion or have calves born with some of the abnormalities described below.
To the breeder, the disease may present as high empty rates, especially in heifers. Cows and/or heifers that have been confirmed as pregnant may end up empty due to early foetal death. There may be abortions in the later stages of pregnancy or full-term calves may be born showing a number of different abnormalities. These include cataracts, arthrogryposis (contracted tendons) and the inability to walk due to the underdevelopment of an area of the brain (the cerebellum) that is involved in balance.
If purchasing and growing from calves, the disease may present as a period of ill-thrift, scouring and a growth check from which animals recover. To the finisher, nothing more than the occasional death following a period of fading and scouring may be seen. The presence of the disease can cause immunosupression resulting in animals becoming more susceptible and showing increased morbidity to other diseases. If a number of young animals have been purchased from the same property, it is possible for large numbers of cattle or calves to die from mucosal disease.
If you are seeing any of the above on your property then it is possible that you have a BVD problem.
How can I test for BVD on my property?
If you suspect that you are experiencing losses from BVD, the investigation start-point is a blood test to establish the BVD status of the herd. Fifteen blood samples from each age group of animals on the farm should be taken and tested for the presence of antibody to the virus. Any positive result will indicate that an animal has been exposed to the virus in its lifetime.
Careful consideration then needs to be given as to where any positive animals have been in their lifetime and if their exposure to the virus took place on the current property or not. It may be that one particular age group has a large percentage of animals with positive test results, which would suggest the presence of a PI animal in this group. If the presence of PI animals is suspected, these should be detected and culled. This requires blood sampling the whole age group suspected of harbouring the PI and looking for the presence of virus antigen. By applying this method, it is possible to eliminate any PI animals and hence the main source of virus from a property.
Once BVD is removed from a property how can it be kept out?
The simple answer to this is meticulous biosecurity which although achievable, can be very difficult to fully implement. Some suggested measures are listed below.
- All animals introduced to a property should be blood-tested as virus negative. This includes all sire bulls which should ideally not be purchased without their BVD status being known.
- Any animals that are pregnant on arrival must also have their calf tested once it has been born. It is possible for the dam to be negative and the calf positive.
- Avoid close contact with stock from neighbouring properties. Practical ways to do this include double-fenced boundaries, tree-lined boundaries or electrical outriggers around the boundary fence.
- Extreme care must be taken with the use of off-farm grazing.
- Consider vaccination (see comments below).
The systematic vaccination of cattle in herds that have undergone testing and PI removal will build up a high level of immunity within the herd and should be considered in areas with a high density of cattle and especially where biosecurity may be difficult to achieve. The decision whether to vaccinate and if so, which animals should be vaccinated, is something that requires careful planning and should be discussed with your veterinarian.
Non systematic vaccination without PI removal and biosecurity has been tried in other countries but does not work. PI animals exert a huge challenge to the vaccine and in the face of such challenge, vaccines can be overwhelmed.
In beef herds, it is often the case that a PI animal will remain in contact with all age groups. The disease will remain active all the time causing ongoing losses. The disease will spread between younger and older animals because they are kept together. A PI animal may be in constant contact with susceptible pregnant cows, replacements, bulls and the rest of the breeding herd.
If you require further information or advice on BVD or wish to undergo testing to establish the BVD status of your property, then please contact us at Totally Vets.
