The Disease

Border disease is a form of infectious abortion in sheep (Hussin and Woldehiwet, 1994; Nettleton et al., 1998). On a national scale it is not as important as other causes of abortion (Sands and Harkness, 1978), but on individual farms it can cause serious losses (Sharp and Rawson, 1986). The disease is characterised by barren ewes, abortion, stillbirths and the birth of small, weak lambs, a variable percentage of which show tremor, abnormal body conformation and hairy fleeces ("hairy-shaker" or "fuzzy" lambs) (Jeffrey and Roeder, 1987). The cause of border disease is a virus serologically related to bovine virus diarrhoea (BVD) virus (Nettleton and Entrican, 1995) and swine fever (hog cholera) virus (Edwards et al., 1995), the three viruses being grouped in the genus Pestivirus.

Infection of non-pregnant sheep with the virus causes only a very mild disease, which often passes unnoticed by the shepherd. The disease is, however, serious in susceptible pregnant sheep because the virus can cross the placenta and infect the foetus. The ewe shows no disease but the foetus is affected and may be aborted or stillborn. The outcome of the foetal infection depends on the stage of foetal development at which infection occurs (Jeffrey and Roeder, 1987; Roeder et al., 1987). Foetal lambs infected after 85 days of gestation have an immune system capable of destroying the virus, so that foetal death is rare and virtually all lambs are born normal with detectable antibodies to the virus. Infection in early gestation, before the foetal immune system has developed, will result in damage of the developing nervous system and skin resulting in 'hairy shaker' lambs. Any surviving lambs that have been infected earlier in gestation will be persistently infected with the virus (Nettleton et al., 1992). Some of these carriers mature normally and excrete the virus in all body fluids (e.g. saliva and urine) throughout their lives (Barlow et al., 1980). The persistent infection is due to the fact that the lamb accepts the virus and never mounts an antibody response against it (Terpstra, 1981; Westbury et al., 1979; Woldehiwet and Nettleton, 1991).

Persistently infected lambs that reach maturity (Nettleton et al., 1992) often have a slow growth rate (Roeder et al., 1983; Sweasey et al., 1979) and a reduced fertility. Females that conceive either abort or produce persistently infected lambs, sometimes over a number of years. Rams usually have soft testicles, but can transmit virus in their semen as well as other secretions (Gardiner et al., 1981).

In some persistently infected lambs, a subtle change occurs in the virus and the equilibrium it has established with the host's immune system is upset. This usually occurs when the lambs are 2-6 months old and results in damage to the inner lining (mucosa) of the gut, causing diarrhoea and death (Nettleton, 1991).