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Maedi-Visna
Maedi-Visna is a slow viral disease of sheep. The virus persists for life and signs only appear years after contamination, in animals two years of age and older. Its name comes from Icelandic: “maedi” means shortness of breath, “visna” mean...
What is it?
Maedi-Visna is a slow viral disease of sheep. The virus persists for life and signs only appear years after contamination, in animals two years of age and older.
Its name comes from Icelandic: “maedi” means shortness of breath, “visna” means nervous wasting away. The respiratory form is by far the most common.
The animal gradually loses weight, becomes out of breath, falls behind the herd, but retains its appetite: it is this contrast which must make us think of illness.
Key point
Disease without treatment or vaccine, which sets in silently for years. Pre-purchase screening is the only real protection for a disease-free 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:
Slow and steady weight loss in an adult animal that continues to eat normally.
Shortness of breath, rapid and shallow breathing, at first only during exercise then at rest; the animal stays behind when the herd moves.
Discreet or absent cough; there is no fever, unless there is an added infection.
Dull coat, animal losing its condition despite good nutrition.
Nervous form, rarer: staggering gait, weakness of a hindquarters, fall, then paralysis which progresses over weeks.
Breast form: firm and indurated udder, without heat or pain, with very little milk; the lambs do not gain weight and the ewe seems to have “no milk”.
Arthritis with swelling of the joints in some animals.
Most infected animals never show any signs and yet transmit the virus.
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 disease is caused by a lentivirus of small ruminants, a virus which remains in the animal's defense cells throughout its life.
It causes chronic inflammation that gradually thickens the tissue in the lungs, reducing the surface area available for breathing.
Depending on the organs affected, the same infection results in lung, nerve, breast or joint damage.
The body produces antibodies but never manages to eliminate the virus: a positive animal remains infected and contaminating for life.
Purchase of animals without known health status and without screening.
Raised in buildings with high density and long wintering period.
Feeding lambs with raw milk mixture.
Mixing of flocks and sheep-goat cohabitation.
Reuse of needles and blood sampling equipment.
Older herds: the proportion of positive animals increases with the age of the herd.
Transmission
Through colostrum and milk from the infected ewe to its lamb: this is the main route, and it explains the persistence of the disease from generation to generation.
By respiratory route, between adult animals in close contact, especially in buildings and during periods of high density.
By equipment soiled with blood, and by needles reused from one animal to another.
By purchasing an infected animal without apparent signs: this is the usual way in which the virus enters an unaffected herd.
The mixture of sheep and goats allows the virus to pass between the two species.
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 in adults who lose weight and become short of breath without fever, with a preserved appetite.
Diagnosis in living animals is based on a blood test looking for antibodies. A positive result means the animal is infected for life.
Screening is done at the herd level: a sample of adults is analyzed to estimate the situation, rather than a single animal.
An autopsy on a dead or culled animal provides very telling elements, particularly on the appearance of the lungs.
The veterinarian rules out other causes of chronic wasting in adult sheep, which can give a similar picture.
Treatment
There is no treatment capable of eliminating the virus, either in the affected animal or in the herd.
Care is limited to comfort: well-ventilated premises, quality food, treatment of bacterial respiratory infections which are added, on veterinary prescription.
These treatments only slow down the deterioration; the development remains unfavorable.
The decision to cull an affected animal is made with the veterinarian, taking into account its condition, its respiratory distress and the risk it poses to the rest of the herd.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Only introduce animals from screened herds, and screen before any introduction.
Regularly screen the herd by blood test, and eliminate positive animals. This is the basis of any sanitation program.
Separating lambs from their mothers at birth and feeding them without colostrum or raw milk from infected ewes: this demanding behavior is the only one that really breaks mother-lamb transmission.
Never feed raw milk mix to lambs.
Use one needle per animal.
Avoid letting sheep and goats coexist when the health status is not known.
Ensure good ventilation and limit building density.
Possible complications
Progressive respiratory failure leading to death or reformation.
Bacterial pneumonia which attaches to already damaged lungs.
Weak lambs and increased mortality when the mothers' udders no longer produce.
Progressive paralysis in the nervous form, with no possible return.
At the herd level: anticipated culls, drop in productivity, difficulty selling breeding stock.
When to see a veterinarian
An adult sheep loses weight regularly even though it eats normally.
An animal runs out of breath at the slightest effort or stays behind the herd.
A ewe has no milk even though her udder appears firm and well developed, and her lambs do not gain weight.
An animal staggers, weakens in the hindquarters or becomes progressively paralyzed.
Several adults die off in the herd over the same period: ask for screening.
You are considering buying breeding stock: talk about it before purchasing, not after.
Prognosis
The outcome is unfavorable for the animal which shows signs: the pulmonary or nervous damage does not regress and leads to death or culling within a few months.
A positive animal without signs can live normally for a long time, while remaining contaminating.
At the herd level, sanitation is possible but requires years of screening, sorting and rigor on introductions.
Identifying the disease early allows above all to prevent it from spreading and to preserve the value of the breeding stock.
Frequently asked questions
My sheep is losing weight but eating well: is it necessarily Maedi-Visna?
No, several chronic diseases of adult sheep give this picture. This is precisely why the animal must be examined: only the veterinarian, with a blood test and sometimes other examinations, can make the difference.
Can a positive animal heal?
No. The virus stays in the body for life. A positive animal is infected and contaminating, even if it shows no signs for years.
How can I protect my flock if I want to buy sheep?
By only buying from screened herds, asking for the results and having the animals screened before mixing them with yours. This is the only truly effective measure, since there is no vaccine.
Is this dangerous for humans or for my other animals?
This virus is not transmitted to humans. On the other hand, it can pass between sheep and goats when they are raised together: it is better to know the status of the two species before mixing them.
References
WOAH — Organisation mondiale de la santé animale (ex-OIE)
Content reviewed by the TunisieVet editorial team · Updated 09 August 2026
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