Border disease (UK), 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 resulting from in utero infection by a pestivirus.
Border disease has been recognized in most sheep-rearing areas of the world, including the western US. Surviving lambs are persistently infected with border disease virus, and no effective treatment is known.
Etiology, Pathogenesis, and Epidemiology of Border Disease
Border disease is caused by infection of the fetus in early pregnancy with border disease virus (Pestivirus D), a pestivirus (Flaviviridae) closely related to the viruses that cause classical swine fever (Pestivirus C) and bovine viral diarrhea/mucosal disease (Pestivirus A and B).
Lambs that survive border disease virus infection are persistently viremic, and the virus is present in their excretions and secretions, including semen. Cattle, goats, and pigs are also susceptible to infection with border disease virus.
Individuals persistently infected by border disease virus have been demonstrated in sheep, cattle, goats, and pigs from natural in utero infection. The virus can be transmitted to cattle through commingled grazing with persistent or acutely infected sheep. Acute infections in immunocompetent animals are usually transient and subclinical and result in immunity to challenge with homologous but not heterologous viral strains.
Border disease virus acquired in early pregnancy by previously unexposed animals crosses the placenta and invades the fetus. Placentitis occurs 10–30 days after infection and can cause fetal death with expulsion, resorption, or mummification. Abortion can occur at any stage of pregnancy and can pass unnoticed because there is little maternal malaise.
In sustained pregnancies, border disease virus becomes widely distributed in fetal tissues. The most obvious pathological changes are in the skin, skeleton, and CNS. Affected lambs can be born 2–3 days early, and many die before or at weaning.
In survivors of border disease infection, clinical signs gradually regress; however, these animals remain persistently infected and excrete virus for the remainder of their lives, exposing their progeny and flockmates. These clinically “recovered” hairy shaker sheep can die at any time from a syndrome similar to bovine mucosal disease.
In naive flocks exposed to border disease virus, up to 50% or more of lambs born can be affected by disease associated with the virus (1). Thereafter, the prevalence of border disease declines; however, the disease can become endemic when “recovered” lambs are retained for breeding.
Border disease virus is most commonly introduced into susceptible flocks by the addition of persistently infected sheep or pregnant ewes carrying an infected fetus. However, sheep can also acquire infection from transiently or 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 to BVDV-1 and BVDV-2. At least seven Pestivirus species have been identified in domestic ruminants.
Clinical Findings of Border Disease
The infection of a flock by border disease virus is often recognized first at lambing time because of an increase in the number of barren ewes and in the birth of undersized lambs with excessively hairy and sometimes excessively pigmented fleece.
Skeletal abnormalities that are possible in neonate lambs as a result of infection by border disease virus include decreased length from crown to rump, shortened tibia and radius, and shortened longitudinal axis of the cranium. Some lambs exhibit involuntary muscular tremors, particularly of the trunk and hindlimbs. The tremors are less pronounced at rest and exacerbated by purposeful movement. In other cases, skeletal defects such as dropped pasterns and mandibular brachygnathia predominate.
Lambs with border disease have a poor survival rate. In survivors, neurological signs gradually disappear within 3–4 months. Even in the absence of typical hairy shaker lambs, outbreaks of low fertility in ewes and poor viability and ill thrift in lambs are sometimes associated with border disease virus infection. Goat kids can also be affected, and a similar condition occasionally occurs in calves. Other clinical signs include infertility, abortions, and stillbirths.
Lesions of Border Disease
In severe cases of border disease, abnormal development of the cerebrum resulting in hydrocephalus, hydranencephaly, porencephaly, or microcephaly can be evident at necropsy. Cerebellar hypoplasia or cerebellar dysplasia can also occur (see border disease image).
Courtesy of Dr. Kim Newkirk.
Otherwise, the characteristic lesions of border disease are microscopic and involve white matter of the CNS. There is a deficiency of myelin and an increase in interfascicular glial cells, in which myelin-like lipid droplets can accumulate. These changes are most obvious in neonates and gradually resolve.
Diagnosis of Border Disease
Clinical signs
Histological evaluation
Serological testing
PCR assay
Clinical findings usually allow a diagnosis of border disease; however, abnormal hairiness of the coat at birth might not be apparent in rough-coated breeds of sheep.
Diagnosis of border disease can be confirmed by histological demonstration of pathognomonic CNS lesions and by immunocytochemical staining of the virus.
In typical hairy shaker lambs, border disease virus or viral antigen can be demonstrated readily in blood and tissues by virus isolation, fluorescent antibody tests, immunohistochemistry, or PCR assay. When testing blood in neonate lambs, precolostral blood is ideal because colostral antibody can mask the virus for up to 2 months.
Border disease virus can be isolated from serum or buffy coat cells in cell cultures; however, viral antigen detection ELISA using heparinized or EDTA blood is available. RT-PCR assay can also be used to detect viral RNA in clinical specimens and to differentiate between ruminant pestiviruses.
Differential diagnoses in live-born lambs include these disorders:
focal symmetrical encephalomalacia
“daft lamb” disease
Other causes of ovine abortion include the following:
Control of Border Disease
Identification and removal of persistently infected animals
There is no effective treatment for lambs persistently infected with border disease virus.
Bulk tank milk samples can be tested for antibodies against border disease virus to screen for presence of the virus within dairy sheep flocks.
Serological testing should be performed on the dams of affected lambs. Most should have high circulating concentrations of antibodies, providing immunity to the same strain of the virus in subsequent pregnancies. Those that do not have antibody titers should be screened for virus to identify any that are persistently infected.
Lambs that have recovered from border disease should not be retained for breeding; however, they 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 against border disease virus.
Key Points
Border disease virus infection in pregnant ruminants results in embryonic loss/abortion or birth of congenitally infected offspring.
Lambs born after in utero border disease virus infection are characterized by low birth weight and viability, poor conformation, tremor, and an excessively hairy coat.
Surviving offspring remain persistently infected and are the primary reservoir for the virus in a susceptible population.
There is no effective treatment for animals persistently infected with border disease virus or effective vaccine against border disease virus.
Bulk tank milk samples can be monitored for antibodies against border disease virus, and the dams of affected lambs can be serologically tested to ensure that they have high antibody titers against the virus.
For More Information
Menzies PI. Control of important causes of infectious abortion in sheep and goats. Vet Clin North Am Food Anim Pract. 27(1):81-93.
References
Nettleton PF. Border disease. In: Martin WB, Aitken ID, eds. Diseases of Sheep. Blackwell Science; 2000:95-101.
