Visna in Sheep: Progressive Neurologic Disease and Paralysis

Quick Answer
  • Visna is the neurologic form of maedi-visna, also called ovine progressive pneumonia in the US. It is caused by a lifelong small ruminant lentivirus infection.
  • Affected sheep may develop hind limb weakness, incoordination, circling, head tilt, behavior changes, and slowly progressive paralysis.
  • There is no curative treatment. Care focuses on confirming the diagnosis, protecting flock mates, and making humane quality-of-life decisions with your vet.
  • Testing often involves flock serology plus rule-outs for other neurologic diseases. Necropsy and tissue testing may be needed for a definitive answer.
  • Typical US cost range for evaluation and flock planning is about $150-$900 per animal or case, with necropsy and lab fees potentially adding more.
Estimated cost: $150–$900

What Is Visna in Sheep?

Visna is the neurologic form of maedi-visna, a chronic viral disease of sheep caused by a small ruminant lentivirus. In the United States, this infection is often grouped under ovine progressive pneumonia (OPP). The same virus can affect different body systems, so some sheep develop lung disease, while others develop neurologic signs such as weakness, incoordination, and paralysis.

This is a slow, progressive, lifelong infection. Many infected sheep never show obvious illness for a long time, but they can still carry and spread the virus within the flock. When the nervous system is involved, the disease may appear as encephalitis with circling or altered mentation, or as a spinal form with hind limb deficits that gradually worsen.

For pet parents and flock managers, the hardest part is that visna often looks like other sheep neurologic problems at first. A sheep that starts trailing behind, stumbling, or dragging the rear legs needs a prompt exam so your vet can sort through infectious, metabolic, toxic, and traumatic causes.

Symptoms of Visna in Sheep

  • Progressive hind limb weakness
  • Ataxia or incoordination
  • Paresis progressing to paralysis
  • Circling or head tilt
  • Altered mentation or behavior change
  • Chronic weight loss or wasting
  • Exercise intolerance or trailing behind the flock
  • Recumbency

Call your vet promptly if a sheep develops new weakness, stumbling, circling, head tilt, or trouble standing. These signs are not specific to visna. Other causes, including listeriosis, scrapie, spinal trauma, abscesses, meningitis, metabolic disease, and toxicities, may need different management and some can progress quickly.

A sheep that is down, unable to rise, not eating, or worsening over hours to days needs urgent veterinary attention. Even though visna is usually slow-moving, sudden decline can mean a different emergency is happening at the same time.

What Causes Visna in Sheep?

Visna is caused by maedi-visna virus (MVV), a lentivirus in the retrovirus family. This virus persists in immune cells and is not cleared by the body, which is why infected sheep remain infected for life. The disease is part of the broader small ruminant lentivirus group, which also includes the related virus involved in caprine arthritis and encephalitis in goats.

Transmission can occur through colostrum and milk, and also through respiratory secretions and close contact, especially when sheep are housed closely together. Intrauterine transmission appears to be uncommon, but it can occur. Cross-species transmission between sheep and goats is also possible, which matters on mixed small-ruminant farms.

Not every infected sheep develops neurologic disease. Many remain subclinical, while others develop chronic lung disease, weight loss, udder changes, or less commonly the neurologic form. Age, management, viral strain, and host factors likely all influence which syndrome appears and how severe it becomes.

How Is Visna in Sheep Diagnosed?

Diagnosis usually starts with a history, neurologic exam, and flock-level context. Your vet will look at the sheep's age, progression of signs, exposure risk, and whether there are respiratory, udder, or chronic wasting problems elsewhere in the flock. Because visna can mimic several other conditions, diagnosis often involves ruling out more common or more urgent causes first.

Testing may include serology to identify exposure within the flock, along with other diagnostics chosen to fit the case. Depending on the signs, your vet may recommend bloodwork, cerebrospinal fluid analysis, imaging if available, or testing for important differentials such as listeriosis or scrapie. A positive blood test supports infection status, but it does not always prove that the neurologic signs are caused by visna in that individual sheep.

A definitive diagnosis may require necropsy with histopathology and sometimes immunohistochemistry or other tissue-based testing, especially when a sheep dies or humane euthanasia is elected. This can also help protect the rest of the flock by clarifying whether the problem was visna or another contagious neurologic disease.

Treatment Options for Visna in Sheep

Spectrum of Care means you have options. Here are treatment tiers at different price points.

Budget-Conscious Care

$150–$350
Best for: Sheep with progressive signs where the goal is comfort, practical decision-making, and limiting immediate flock risk without extensive testing.
  • Farm call or clinic exam
  • Basic neurologic and physical assessment
  • Supportive nursing care plan
  • Isolation from the flock while contagious causes are considered
  • Quality-of-life discussion and monitoring
  • Humane euthanasia discussion if the sheep cannot stand or is declining
Expected outcome: Guarded to poor. There is no curative treatment for visna, and neurologic deficits usually progress.
Consider: Lower upfront cost range, but diagnosis may remain presumptive. Important rule-outs may still need testing if signs are atypical or rapidly worsening.
Most Common

Recommended Standard Treatment

$350–$900
Best for: Most flock situations where pet parents need a practical diagnosis, a humane care plan, and guidance to reduce spread within the flock.
  • Veterinary exam and neurologic localization
  • Serologic testing for maedi-visna/OPP exposure status
  • Targeted rule-out testing for other neurologic diseases
  • Short-term supportive care, anti-inflammatory or nursing recommendations as directed by your vet
  • Flock biosecurity review and segregation or culling plan
  • Necropsy planning if the sheep dies or euthanasia is chosen
Expected outcome: Poor for meaningful neurologic recovery. Some sheep remain stable briefly, but progressive weakness and paralysis are expected over time.
Consider: More useful information for flock management, but still no antiviral cure. Costs rise if multiple flock mates are tested.

Advanced / Critical Care

$900–$2,500
Best for: High-value breeding animals, diagnostically unclear cases, mixed-species farms, or situations where a precise answer will strongly affect flock decisions.
  • Referral-level evaluation or hospital assessment
  • Expanded diagnostics such as CSF analysis, imaging where available, and advanced infectious disease workup
  • Intensive nursing support for recumbent sheep
  • Definitive postmortem testing with histopathology and specialized tissue assays
  • Broader flock investigation and consultation for eradication or accreditation planning
Expected outcome: Still poor for the affected sheep if visna is confirmed, but advanced workup can be valuable for protecting the rest of the flock.
Consider: Highest cost range and labor commitment. Advanced testing may clarify the diagnosis, but it does not change the lack of curative treatment.

Cost estimates as of 2026-03. Actual costs vary by location, clinic, and individual case.

Questions to Ask Your Vet About Visna in Sheep

Bring these questions to your vet appointment to get the most out of your visit.

  1. Based on the exam, what are the top causes of my sheep's weakness or paralysis?
  2. Does this look more like visna, listeriosis, scrapie, trauma, or another neurologic problem?
  3. Which tests would give us the most useful answers first, and what is the expected cost range?
  4. Should this sheep be isolated from the rest of the flock right now?
  5. If maedi-visna is confirmed or strongly suspected, what should we do about flock testing?
  6. Are there realistic supportive-care options for this sheep, or is humane euthanasia the kindest choice?
  7. If this sheep dies or we elect euthanasia, should we submit tissues for necropsy?
  8. What management changes would most reduce spread to lambs and flock mates?

How to Prevent Visna in Sheep

Prevention focuses on biosecurity, testing, and flock management, because there is no vaccine for maedi-visna. The most effective programs aim to identify infected animals, reduce contact between infected and uninfected groups, and prevent transmission to lambs. Your vet can help build a plan that fits a small hobby flock, seedstock operation, or mixed sheep-goat farm.

Common prevention steps include purchasing animals from low-risk or tested flocks, quarantining new arrivals, avoiding unnecessary mixing of sheep and goats, and reducing close-contact housing when possible. Because transmission can occur through colostrum, milk, and respiratory secretions, flock-level control may also involve lambing management, segregation, and strategic culling of positive animals.

If visna or OPP is a concern in your flock, ask your vet about a long-term surveillance plan. Repeated testing over time is often more useful than a single snapshot. Good records, careful sourcing of replacements, and early investigation of chronic weight loss, respiratory signs, or neurologic disease can make a major difference.

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