Border Disease in Sheep: Hairy Shaker Lambs, Tremors, and Congenital Brain Defects

Border disease is a viral disease of sheep caused by a pestivirus closely related to bovine viral diarrhea viruses. It is best known for causing "hairy shaker" lambs: small, weak lambs born with a rough, overly hairy fleece and body tremors. The virus affects the fetus during pregnancy, so many of the most important problems show up at lambing rather than in the ewe herself.

This disease can cause more than tremors. Depending on when infection happens during gestation, outcomes may include early embryonic loss, abortion, stillbirth, low birth weight, poor viability, and congenital defects involving the brain, spinal cord, skin, and skeleton. Severe cases may include cerebellar hypoplasia, cerebellar dysplasia, hydrocephalus, hydranencephaly, porencephaly, or microcephaly.

Some lambs survive and gradually look less neurologically abnormal over the first few months of life. Even so, those survivors may remain persistently infected and continue shedding virus in secretions and excretions for life. That makes border disease both an individual lamb problem and a flock health problem.

For pet parents and producers, the big picture is this: border disease is usually managed through early recognition, testing, and preventing spread, not through a single medication that fixes the condition.

Border disease is caused by border disease virus (BDV), a pestivirus in the family Flaviviridae. The most important infections happen when a ewe that has not been exposed before becomes infected during early pregnancy. The virus crosses the placenta, infects the fetus, and can disrupt normal development of the nervous system, skin, and skeleton.

Timing matters. Infection early in gestation can lead to embryonic loss, fetal death, mummification, abortion, or the birth of persistently infected lambs. Cornell notes that infection before about 60 days of gestation can produce classic hairy shaker lambs, while Merck describes early-pregnancy infection as the key risk period for persistent infection and congenital disease. Later in gestation, the fetus is more likely to mount an immune response, which changes the outcome.

The virus is most often introduced into a flock by persistently infected sheep or by bringing in a pregnant ewe carrying an infected fetus. Persistently infected animals shed virus continuously and are the main reservoir. Rams may also shed virus in semen, and sheep can acquire related pestivirus infections from contact with infected cattle.

Many ewes show little or no obvious illness, so the disease may go unnoticed until lambing. That is one reason border disease can spread quietly before anyone realizes there is a flock problem.

Your vet will usually start with the history and flock pattern: abortions, barren ewes, undersized lambs, tremors, rough hair coats, and poor lamb survival. Those clues can strongly suggest border disease, but testing is important because other conditions can also cause weak or neurologic lambs.

Diagnosis may involve testing blood, placenta, aborted fetuses, or tissues from affected lambs. Merck lists virus isolation, fluorescent antibody testing, immunohistochemistry, antigen detection, serology, and RT-PCR/PCR as useful tools. In newborn lambs, precolostral blood is especially helpful because colostral antibodies can interfere with interpretation after nursing.

If a lamb dies or is euthanized, necropsy can be very valuable. Pathologists may find congenital brain defects such as cerebellar hypoplasia or cerebral malformations, and microscopic examination can show the characteristic white matter changes linked to poor myelin development. Testing the dam may also help your vet understand whether she has antibodies or whether a persistently infected animal is present in the flock.

Because several reproductive and neurologic diseases can look similar, your vet may also rule out conditions such as swayback, bacterial meningoencephalitis, toxoplasmosis, campylobacteriosis, chlamydiosis, listeriosis, bluetongue, or Cache Valley virus, depending on your region and flock history.

Prevention focuses on keeping the virus out of the flock and removing animals that keep shedding it. The most important step is to avoid introducing persistently infected sheep or pregnant ewes carrying infected fetuses. Any new additions should be discussed with your vet before purchase, especially if they come from flocks with unknown reproductive history.

Quarantine and testing are central tools. Your vet may recommend testing suspect lambs, dams of affected lambs, and sometimes replacements before breeding. Merck also advises that lambs that recover from border disease should not be retained for breeding, because persistently infected survivors can continue exposing flockmates and future pregnancies.

Good flock records matter. Tracking barren ewes, abortions, stillbirths, weak lambs, and unusual birth coats can help your vet spot a pattern early. If abortions or congenital defects occur, submit placentas and fetuses promptly for diagnostic workup rather than waiting for the next season.

At this time, there is no proven effective vaccine specifically for border disease virus. Some inactivated BVD vaccines have been used in sheep, but Merck notes that their effectiveness for border disease is unproved. That means prevention relies mainly on biosecurity, testing, and breeding management rather than vaccination alone.