Border disease (Britain) or hairy shaker disease (Australia and New Zealand) is a congenital disorder of lambs characterized by low birth weight and viability, poor conformation, tremor, and an excessively hairy birth coat in normally smooth-coated breeds. Kids may also be affected, and a similar condition occasionally occurs in calves. The disease has been recognized in most sheep-rearing areas of the world, including the western USA.
Etiology, Pathogenesis, and Epidemiology
Border disease is caused by infection of the fetus in early pregnancy with a pestivirus (Flaviviridae) closely related to the viruses of classical swine fever (see Classical Swine Fever) and bovine viral diarrhea/mucosal disease (see Bovine Viral Diarrhea and Mucosal Disease Complex). Surviving lambs are persistently viremic, and the virus is present in their excretions and secretions, including semen. Ruminants and possibly also pigs can be readily infected by contact with these persistent excretors or with acutely infected sheep. Acute infections in immunocompetent animals usually are transient and subclinical and result in immunity to challenge with homologous but not heterologous strains of virus.
Virus acquired in early pregnancy by previously unexposed animals crosses the placenta and invades the fetus. Placentitis occurs 10–30 days after infection and may cause fetal death with expulsion, resorption, or mummification. Abortion may occur at any stage of pregnancy and may pass unnoticed because there is little maternal malaise.
In sustained pregnancies, the virus becomes widely distributed in fetal tissues, but pathologic changes are most obvious in the skin, skeleton, and CNS. Affected lambs may be born 2–3 days early, and many die before or at weaning. In survivors, the clinical signs gradually regress, but such animals remain infected and excrete virus for the remainder of their lives, exposing their progeny and flockmates. Death from a syndrome similar to bovine mucosal disease may occur in these “recovered” hairy-shaker sheep at any time.
In flocks in the first season of a new infection, up to 50% or more of lambs born may be affected with border disease. Thereafter, prevalence declines, although the disease may become endemic when “recovered” lambs are retained for breeding. The virus is most commonly introduced into susceptible flocks by the addition of persistently infected sheep. However, it is not uncommon for sheep to acquire infection from persistently infected cattle. For practical purposes, it should be assumed that sheep and cattle are equally susceptible to all strains of border disease virus and bovine viral diarrhea virus, even though at least 3 antigenic groups of pestiviruses have been identified in ruminants.
Affected flocks probably are recognized first at lambing time by an increase in the number of barren ewes and in the birth of undersized lambs with excessively hairy and sometimes excessively pigmented fleece. Some lambs exhibit involuntary muscular tremors, particularly of the trunk and hindlegs. The tremors are reduced at rest and exacerbated by purposeful movement. In others, skeletal defects such as dropped pasterns and mandibular brachygnathia may predominate. Affected lambs have a poor survival rate. In survivors, nervous signs gradually disappear within 3–4 mo. Even in the absence of typical hairy-shaker lambs, outbreaks of low fertility in ewes and poor viability and ill-thrift in lambs may be associated with border disease virus infection.
In severe cases, cavitation of the cerebrum may be seen at necropsy. Otherwise, the characteristic lesions are microscopic and involve the white matter of the CNS. There is a deficiency of myelin and an increase in interfascicular glial cells, in which myelin-like lipid droplets may accumulate. These changes are most obvious in the newborn and gradually resolve.
Clinical findings usually allow a diagnosis, although abnormal hairiness of the birth coat may not be apparent in rough-coated breeds of sheep. The diagnosis can be confirmed by histologic demonstration of the pathognomonic lesions in the CNS with immunocytochemical staining of the virus. In typical hairy-shaker lambs, the virus may be demonstrated readily in blood and tissues. Precolostral blood is ideal because colostral antibody can mask virus for up to 2 mo. Virus can be isolated from serum or buffy coat cells in cell cultures, but a viral antigen detection ELISA using heparinized or EDTA blood is available. Reverse transcriptase-PCR can also be used for detecting viral RNA in clinical specimens and for typing ruminant pestiviruses.
Other causes of ovine abortion (eg, Chlamydia, Salmonella, Campylobacter, Rickettsia spp, and Toxoplasma gondii) should be considered in the differential diagnosis. In live-born lambs, border disease must be differentiated from swayback (enzootic ataxia), bacterial meningoencephalitis, focal symmetric encephalomalacia, and “daft lamb” disease.
There is no effective treatment. Serology should be done on the dams of affected lambs. Most should have high levels of antibody and be immune to further challenge with the same strain of virus in subsequent pregnancies. Those that do not have antibody titers should be screened for virus to identify any that are persistently infected. Recovered lambs should not be retained for breeding but can be mixed with replacement stock well before breeding season to maximize opportunities for the latter to become infected and develop immunity before subsequent matings. There is no effective vaccine. Bovine viral diarrhea vaccines for cattle cannot be recommended for use in sheep because border disease viruses most commonly isolated from sheep are antigenically distinct from bovine viral diarrhea viruses most common in cattle.
Last full review/revision March 2012 by Peter Nettleton