This month's article describes a form of BVD virus that can affect sheep.
Studies have shown that at least one third of sheep flocks have been exposed to the virus which causes Border Disease, also known as Hairy Shaker Disease. Where the virus is present in a flock, it can lead to abortions, barren ewes, and also lambs born with low birthweights, congenital deformities and poor survival rates.
One very distinct sign that the Border Disease virus (BDV) is present, is when lambs are born with ‘hairy shaker’syndrome. This happens when they become infected whilst in the womb. The virus affects the nervous system resulting in a tremor; it also affects the cells which produce the fleece so that a hairy coarse coat develops instead of the normal wool coat.
Border disease is a world-wide disease of sheep which was first recognised in the UK in 1959. Goats are also susceptible. It is closely related to Bovine Viral Diarrhoea Virus (BVDV) of cattle, and there can be cross-infection of both viruses between cattle and sheep.
Spread of Infection
The BDV is spread by contact with bodily secretions. When a non-pregnant animal comes into contact with the virus, it causes a transient viral infection. This is unlikely to have any noticeable effects, and the animal should then develop a degree of immunity to the virus.
However, when a pregnant ewe or doe encounters the virus, it crosses the placenta and infects the developing foetus, commonly causing death. Early foetal death will result in absorption of the foetus and this will only be recognised at scanning or lambing when high numbers of barren ewes will be found. When foetuses become infected later on in the pregnancy, high numbers of abortions, stillbirths and non-viable lambs will be seen. Those that survive the birthing process may show hairy shaker signs.
Effect of Infection
In early pregnancy the foetus' immune system is not fully developed. If a foetus survives infection at this point, its immune system will recognise the virus as part of its own genetic content and so will never produce antibodies to it. These lambs will be born Persistently Infected (PI), in the same way that the BVD virus leads to PI calves.
Although physically normal at birth, PI lambs will shed the Border Disease virus throughout their whole life, infecting the following year’s lamb crop, and keeping the disease within the flock. If PI animals are bred from, they will always produce PI offspring.
There is no vaccine for Border Disease in the UK. The cattle vaccines for BVDV are not recommended for use, as although the two viruses are similar, the strains most commonly seen in sheep are different to those seen in cattle.
To investigate whether BDV is present in your flock, blood samples should be taken from the dams of affected lambs. Those ewes that have no antibodies to BDV should then be tested for presence of the virus itself, to see whether they are PIs and should be culled.
Screening for the Virus
Buying in a PI ewe-lamb can have devastating effects on a flock which has not encountered the virus before. So when buying in replacement breeding stock, it is worth carrying out screening tests to ascertain the status of the source flock, as well as your own flock.
In flocks where Border Disease is endemic, check that there are no PIs in the breeding ewes (and tups) and then, prior to breeding, mix the whole breeding flock with affected, recovered lambs. This will ensure maximum immunity within the flock. This approach should be fully discussed with your vet, as it is only appropriate in certain situations.
In Maedi Visna (MV) accredited flocks, it is relatively inexpensive to screen for BDV as samples can be taken at the same time as MV bloods.
Once it is known whether or not BDV is endemic in your flock, a customised BDV plan can be drawn up by your vet.