Features
Border Disease in Sheep
by Dr Peter Nettleton
Border disease was first reported in 1959 in the region of the English/Welsh border. It is also called ‘Hairy-shaker’ or ‘fuzzy-lamb’ disease, and has been recognised in most sheep-rearing areas of the world.
On a national scale Border disease is less widespread than other causes of sheep abortion, but on individual farms it can cause serious losses. The disease is characterised by barren ewes, abortions, stillbirths and the birth of small weak lambs, some of which can have characteristic features of the disease, namely trembling and/or abnormally hairy fleeces. Such lambs have been called ‘hairy-shakers’ and are a distinctive feature of the disease. In a few outbreaks, however, no ‘hairy-shaker’ lambs are born and the disease is difficult to distinguish from other kinds of abortion.
The disease is caused by a virus, border disease virus (BDV), which is closely related to a similar virus of cattle called bovine virus diarrhoea virus (BVDV). Under natural farming conditions there is no evidence so far of border disease viruses causing disease in cattle but approximately 15% of outbreaks of border disease in the UK are caused by BVDV-1 viruses which probably originate from cattle.
BDV does not survive for long outside its host, being killed easily by heat, sunlight and disinfectants. Nevertheless it is a successful virus because it has evolved a superb way of ensuring a constant reservoir of individual sheep which are always excreting virus
The Disease
When healthy non-pregnant sheep are infected with BDV for the first time it usually results in mild or subclinical disease which the farmer or vet is unlikely to detect. Within 2-3 weeks the sheep will have developed a good immunity to future BDV infection.
The most serious consequences occur when BDV infects susceptible pregnant ewes. The ewes show no clinical signs but virus spreads quickly to the placenta and crosses to the foetus within one week of infection.
The ultimate outcome of the foetal infection depends on several factors including the strain and dose of virus, the breed of the foetus and its ability to repair damage. The most important factor, however, is the stage of foetal development at which the infection occurs. Infection after 85 days of gestation is met by a foetal immune system capable of neutralising and removing the virus so that foetal death is rare and the majority of lambs will be born normal with detectable antibody to the virus.
Infection in earlier pregnancy will result in foetal damage - foetuses may die and be resorbed or can be aborted at any stage of gestation or may even lead to being stillborn.
Up to 50% of infected lambs will survive to full term and a proportion of those born will show clinical disease and will be recognisable as ‘hairy-shakers’. Clinically affected lambs have a low chance of survival. Many die early in life, while survivors have a poor growth rate and an increased susceptibility to other diseases. Apparently normal PI lambs can survive for years, but are at risk of developing a fatal diarrhea and wasting syndrome similar to mucosal disease in PI cattle persistently infected with BVD virus.
All surviving lambs infected in the first half of gestation will be persistently infected (PI) with BDV. Such PI sheep can mature normally but will excrete the virus continuously for the rest of their lives and will be a constant dangerous source of infection.
Spread of Disease
Sheep-to-sheep contact is the principal way in which BDV is spread and the most potent sources of virus are PI sheep. Flocks with no previous experience of the disease are particularly vulnerable and the purchase of PI replacement gimmers or ewes is the commonest way of introducing infection. These animals constantly shed low levels of virus from the skin and excrete it in faeces and urine. The highest concentration of virus, however, is in nasal secretions and saliva. The speed of virus spread in a susceptible flock exposed to one or more persistent excretor will depend on the contact between sheep. Housing during early pregnancy has been shown to result in explosive outbreaks of border disease. To avoid the risk of infection with BVDV pregnant sheep should never e mixed with cattle.
Diagnosis
Diagnosis of BDV may be straightforward if typical ‘hairy-shaker’ lambs are recognisable but confirmation is best sought by sending dead lambs and blood from affected lambs and their dams to the Veterinary Investigation Laboratory for virus detection and antibody testing.
Serological examination of individual sheep for BDV antibody is rarely helpful, but antibody testing of a 10% sample of different age groups of animals can be useful for demonstrating the presence and extent of BDV infection in a flock.
Controlling Border Disease
It is important that pregnant ewes are not exposed to PI carrier sheep. Maintaining a closed flock will help achieve this but blood testing rams (and even replacement females) before purchase to ensure that they are free of BDV will also help keep border disease out of your flock. Recently purchased females should, whenever possible, be kept as a separate flock from mating right through until lambing is over.
If border disease occurs for the first time in your flock every attempt should be made to prevent further outbreaks by removing for slaughter the entire lamb crop in which affected lambs were seen. Also remove for slaughter the carrier PI sheep suspected of introducing the disease.
In endemically infected flocks, susceptible animals retained for breeding should be deliberately exposed, while not pregnant, to known PI lambs. Close herding for at least 3 weeks, preferably indoors, is necessary for BDV to spread effectively and induce protection. Exposure should end 2 months before the start of the breeding season.
The Future
There is an urgent need for a vaccine to protect sheep against BDV. Unfortunately there is insufficient cross protection between BDV and BVDV for the BVDV vaccines used in cattle to be effective in sheep. Therefore these vaccines are not licensed for use in sheep
Footnote: this article first appeared in the Sheep Farmer magazine and is reproduced with their permission