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Border disease
Border disease, or border disease, is a viral disease of sheep caused by a virus similar to that of bovine viral diarrhea. Its name comes from the region where it was first described, but today it exists in most sheep farming countries. The...
What is it?
Border disease, or border disease, is a viral disease of sheep caused by a virus similar to that of bovine viral diarrhea. Its name comes from the region where it was first described, but today it exists in most sheep farming countries. The virus does not attack the adult sheep, which remains healthy: it attacks the fetus. Depending on the stage of gestation at which the ewe is infected, the lamb dies and is resorbed, the ewe aborts, or the lamb is born alive but abnormal. Breeders recognize it above all by these characteristic lambs: they constantly tremble, have an abnormally woolly and coarse fleece on the head and neck, and do not grow. The heart of the problem lies elsewhere: some of these lambs, infected very early in gestation, are born with the virus and excrete it throughout their lives without anything distinguishing them once adults. They are the ones who maintain disease in the herd.
Commonly observed signs
These signs are neither exhaustive nor specific. Only a veterinarian can establish a diagnosis.
Related signs — each one links to its page:
What strikes you first is a poor campaign result without visible disease: many empty ewes, repeated returns to heat, a low number of lambs born.
Abortions at different stages, mummified lambs, stillborn lambs.
Lambs who tremble constantly, especially in the hindquarters and head, trembling which increases when they become excited and disappears when they sleep.
Abnormal fleece: long, coarse, curly hair on the head, neck and back, very different from normal wool.
Lambs that stagger, fall, have difficulty suckling and following their mother.
Abnormally small lambs, sometimes with poorly proportioned limbs and heads.
Persistent growth retardation in lambs which, when larger, remain stunted, lose weight and have repeated diarrhea.
Adult sheep show nothing: they are not sick, and this is what makes the disease difficult to suspect.
Some affected lambs improve with age and lose their tremors: they still retain the virus if the infection occurred very early.
When to consult without delay
This list does not replace veterinary advice. It gathers situations that justify a prompt consultation.
Causes and risk factors
The virus multiplies in infected animals and is transmitted from one sheep to another.
In a non-pregnant ewe, the infection almost always goes unnoticed and leaves lasting immunity.
In a pregnant ewe, the virus crosses the placenta and reaches the fetus. It all depends on the moment.
Very early infection: the fetus dies and disappears, or is born carrying the virus for life, because its defense system, still immature, has learned to consider the virus as part of itself. This is the permanently infected lamb.
Intermediate stage infection: the brain and skin develop poorly, resulting in trembling lambs with abnormal fleece.
Late infection: the lamb defends itself and is often born normal.
Permanently infected lambs shed enormous amounts of virus throughout their lives, through saliva, droppings, urine and discharge. Just one of them is enough to infect an entire herd.
Purchase of animals, especially pregnant females, without quarantine or examination.
Loan or borrowing of rams between farms, a very widespread practice.
Grouping of herds, common pastures, transhumance, fairs.
Contact between sheep and cattle on the same plots or in the same buildings.
Flock never exposed before: all the sheep are then susceptible at the same time, and the introduction of the virus causes a catastrophic campaign.
Conservation, without knowing it, of permanently infected lambs for renewal: these are often small and sickly animals, but not always.
Transmission
Transmission occurs through direct contact between animals, through secretions from the nose and mouth, urine, droppings and birthing products.
The main source is the permanently infected animal, which continuously sheds virus without ever appearing contagious.
An infected pregnant ewe transmits the virus to her lamb, continuing the chain from one generation to the next.
Purchasing a permanently infected animal, or a pregnant ewe carrying an infected fetus, is the most common method of introduction into an uninfected flock.
Cattle can harbor closely related viruses and share them with sheep when they graze together or live together in buildings.
This disease is not transmitted to humans.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
The veterinarian suspects the disease due to the association of poor reproductive results and trembling lambs with abnormal fleece, but confirmation requires analyzes.
Blood tests on a sample of ewes allow us to see if the flock has encountered the virus and when.
The search for the virus itself, on suspected lambs, identifies permanently infected animals. This is the decisive test, because it is on them that everything rests.
An animal found carrying the virus must be rechecked after an interval defined by the veterinarian, in order to distinguish a temporary infection from a permanent infection.
Abortions and placentas can be analyzed: keep them cool, in a closed bag, without handling them with bare hands.
At the same time, the veterinarian rules out other causes of reproductive disorders and serial abortions, in particular brucellosis, Q fever, abortive chlamydia and toxoplasmosis.
Faced with trembling lambs, he also eliminates other possible causes, notably deficiencies and intoxications, which do not require the same behavior at all.
Treatment
There is no treatment for the virus itself: neither antibiotics nor antiparasitics have any effect on it.
Affected lambs can only receive comfort care: warmth, feeding assistance, quality colostrum, protection against cold and humidity. Mild cases sometimes improve over time.
The most affected lambs, which do not stand up and do not suck, suffer: euthanasia must be considered with the veterinarian.
The real answer is not medical but organizational: identify permanently infected animals and remove them permanently from the herd. As long as they remain, the virus continues to circulate despite all treatment.
An animal recognized as permanently infected must never be sold as breeding stock: this would transmit the problem to another breeding stock.
The complete strategy, from screening to renewal, is built with the veterinarian over several campaigns.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Quarantine any animal purchased, without exception, and have animals intended for breeding checked before introducing them.
Be particularly vigilant with purchased pregnant females: they may be healthy and carry a permanently infected lamb, which will only appear at birth.
Do not borrow a ram without knowing where it comes from and without advice from the veterinarian.
Avoid contact between sheep and cattle when possible, particularly in buildings.
Identifying and eliminating permanently infected lambs as soon as they are identified: this is the measure that breaks the cycle.
Do not keep stunted and retarded lambs from a problem campaign for renewal without having them checked.
In an uninjured herd, the most effective rule is not to introduce anything without control.
Monitor your breeding results from one year to the next: an unexplained drop in the number of lambs is often the first signal.
Vaccination
There is no vaccine specifically dedicated to this disease available anywhere, and the question of vaccination should only be discussed with the veterinarian.
Control is therefore based on screening, elimination of permanently infected animals and strict control of introductions.
In some herds, the veterinarian may discuss controlled exposure of young females prior to wrestling. This approach is delicate, it can never be improvised and can turn against the breeder if it is poorly done.
Possible complications
Very seriously degraded breeding campaign: empty ewes, abortions, lost lambs.
Silent maintenance of the virus in the herd for years due to permanently infected animals.
Fragile permanently infected lambs, which suffer from repeated diarrhea and pneumonia and often die before reaching adulthood.
Increased susceptibility of these lambs to all other diseases, because their defenses function poorly.
Dissemination to other farms during the sale of replacement animals.
Discouragement of the breeder in the face of results which do not improve despite good feeding and good care.
When to see a veterinarian
Lambs are born shaking, with abnormally hairy fleeces on their heads and necks.
A lambing campaign produces far fewer lambs than expected, with unexplained empty ewes.
Ewes come back into heat several times after mating.
Abortions or mummified lambs appear in the flock.
Lambs remain stunted, do not grow and have repeated diarrhea despite good behavior.
You have just introduced animals and the breeding results deteriorate the following campaign.
Prognosis
Adult ewes are not in danger: they remain healthy and, once infected, are protected for subsequent pregnancies.
Mildly affected trembling lambs may improve with age, but often remain stunted in growth.
Permanently infected lambs have a bleak future: they remain fragile, fall sick constantly and most often die young.
At the herd level, the prognosis is good if permanently infected animals are identified and eliminated: the virus then disappears with them. If they stay, the disease sets in for years.
Frequently asked questions
My lambs are trembling: is it necessarily this illness?
No. There are several causes that cause a lamb to tremble, including deficiencies and poisonings, and some are easily corrected. The association of tremors and abnormally hairy fleece on the head and neck is very evocative, but only analyzes allow us to conclude.
Why eliminate a lamb that seems to be doing better?
Because a lamb infected very early in gestation retains the virus all its life, even when its tremors disappear. Nothing then distinguishes it from a normal animal, and it continues to contaminate the herd every day. It is he, and not the sheep, who maintains the disease.
Can I sell these animals rather than destroy them?
An animal recognized as a permanent carrier should never leave as a breeder: you would transmit the problem to another breeding, and it would come back to you sooner or later. The destination of these animals is decided with the veterinarian.
Can my cows contaminate my sheep?
Very similar viruses circulate in cattle and can pass to sheep when they come together. Separating the two species, particularly in buildings, is one of the precautions to take when you want to clean up a herd.
References
WOAH — Organisation mondiale de la santé animale (ex-OIE)
ANSES — Agence nationale de sécurité sanitaire (France)
Content reviewed by the TunisieVet editorial team · Updated 11 August 2026
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