Chronic virus that hits sheep.

Byline: By Iain Carrington

Maedi-visna disease was first detected in Britain in the late 1970s in a group of tups intended for export.

The condition has a worldwide distribution with the exception of New Zealand and Australia.

Maedi and visna are two manifestations of the same lentivirus infection.

This virus is described as slow because of the chronic nature of the disease it causes.

Maedi presents mainly as a chronic respiratory disease, but is rarely seen in sheep less than three years of age despite initial infection often taking place in lambs.

Most infected sheep in commercial flocks will reach the end of their productive lives before obvious disease is noticed.

Initially there will be signs of exercise intolerance, this progresses slowly to respiratory distress which may lead to fainting if the animal is stressed.

The sheep remain bright and eat but there is chronic wasting and coughing is rarely a feature.

These animals often suffer from mastitis with widespread thickening of the mammary tissue and a `flabby' bag, and arthritis has also been noted as a feature in some countries.

Economic losses associated with this disease are mainly due to the mastitis, poor body condition, poor fertility, high mortality among the new-born and poor growth rates in the lamb crop.

Visna is a neurological form of the disease, which is seen mainly in flocks with a high incidence of virus. Initially there is weight loss with tremors, blindness, head tilt circling and inco-ordination. Death usually occurs in three to four weeks.

Transmission of this disease often occurs to the newborn lamb via colostrum and milk from the affected mother. Another important route is through aerosol transmission, especially when animals are housed or kept under more intensive conditions.

Many pedigree flocks are now part of a Maedi-visna scheme which monitors freedom from disease by regular blood testing, purchase of replacements from Maedi-visna free flocks and good biosecurity.

In commercial crossbred flocks, this is less likely to be an economic option, where diagnosis of the problem with identification and culling of affected animals and maintaining a young flock will help control the disease.

Screening of a selection of approximately 12 older and thinner ewes can be carried out to monitor the situation within a flock and identify if disease is present.

This will act as a snapshot of disease status but will not guarantee identification, but if a sample is positive, you have the problem.

Hopefully this will be early enough for a planned approach to control before major economic problems arise.

It was thought that by now the disease would be more widespread but at present this has not happened but an increase in cases has been noted.

Your own vet will be able to advise you about screening and how it can be carried out.