Visna-Maedi Virus

Also know as: Maedi-Visna — Lymphoid Interstitial Pneumonia

Introduction

This disease is caused by a retrovirus and affects sheep worldwide. It was originally seen in Iceland, hence the name is Icelandic in origin. The disease is slow to progress and is therefore only seen in sheep greater than 2 years old.

There occurs a primary replication in lung macrophages and these then carry infection to brain, lung, udder and/or joints. T cells react causing a Type IV immune complex disease. Target organs become chronically inflamed after 2-6 years.

Visna is the demyelination with subacute meningitis around ventricles and choroid plexus caused by inflammation. Posterior paresis progresses for up to a year until sheep can no longer stand.

Maedi is the lung part of the complex, with alveolar septa becoming infiltrated by lymphocytes and macrophages, there is also smooth muscle hypertrophy. All these result in a loss of condition.

Transmission of the disease is via aerosol, milk or colostrum and therefore there is increased risk of spread of the disease during winter housing.

Clinical Signs

Lack of condition/weight loss, lameness and paresis. Coughing, harsh lung sounds on auscultation, increased respiratory rate and nasal discharge also occur with the respiratory part of the disease complex.

Diagnosis

Clinical signs and history can lead to a presumptive diagnosis of the disease. Due to the clinical signs, sheep affected are usually humanely destroyed.

A post mortem examination can be performed which will show lungs that are heavy, rubbery and do not collapse. Grossly, they will show signs of severe interstitial pneumonia and the impressions of the ribs will remain on the surface fo the lungs after removal from the body. They will appear a mottled, grey colour and speckling may also be present. The diaphragmatic lobes are usually the most severely affected. Bronchial and mediastinal lymph nodes are often enlarged.

Serology can be used on the live animal to detect antibody or antigen by ELISA.

References

Bridger, J and Russell, P (2007) Virology Study Guide, Royal Veterinary College.

Radostits, O.M, Arundel, J.H, and Gay, C.C. (2000) Veterinary Medicine: a textbook of the diseases of cattle, sheep, pigs, goats and horses Elsevier Health Sciences