Some farmers are of the mistaken belief that Maedi visna (MV) is a disease that only affects Texels; however all breeds are susceptible and we have identified infection in a number of Suffolk, Charollais and Lleyn flocks.
We have also found MV positive flocks in the following breeds: Beltex, Bleu de Maine, Oxford Down, Ile de France, Clun Forest, Shropshire, Wensleydale, Friesland, Blackface and mules.
There is evidence that MV is on the increase in Great Britain and there have been recent reports of a number of non-accredited flocks suffering from clinical signs of disease.
In the past many flock owners have not taken MV seriously which was partly because MV was previously at a low level in the national flock and some flock owners therefore believed that the chance of becoming infected was remote. Many vets have never diagnosed a case of MV and the main reasons for this are that the disease is still not very common, the signs can mimic a number of other diseases and veterinary involvement in sheep flocks is at an all time low so cases are not being identified.
What signs does the disease cause?
The disease is due to the effects of a slow virus infection. Increased numbers
of adult sheep in poor condition is the most commonly reported finding in an
infected flock. This has significant economic effects due to increased numbers
of ewes being culled and also because thin ewes are more likely to be barren or
produce fewer offspring.
Ewes in poor condition produce smaller lambs which are less likely to survive. Lower volumes of colostrum and milk are produced resulting in increased lamb mortality and reduced lamb growth rates. Other signs include pneumonia, mastitis, arthritis and hindlimb paralysis.
Some recent cases have involved sheep that were unable to stand due to hindlimb paralysis but they had none of the more common signs of the disease.
Why should I take MV seriously?
Infection is fatal, there is no cure and there is no vaccine available for
control. There are significant economic losses and signs of disease are not
seen until around two to four years after the infection has been introduced, by
which time at least 50% of the flock is likely to have been infected.
A study carried out on a commercial flock infected with MV found that lamb losses were 36% and in adults they were 14%. Four years after the infection had been introduced 68% of sheep were infected. The only practical option was for the flock to be culled.
This case was six years ago but the cost of disease was estimated to be £32 per ewe. Costs of disease would be significantly higher in a more valuable, pedigree flock. It is much easier to try and keep disease out than it is to eradicate it when it is introduced.
What is the MV Accreditation Scheme?
It is a voluntary scheme with over 2800 flocks that have been blood tested and
are accredited free of Maedi visna.
What testing is necessary?
Two clear qualifying tests of all sheep over twelve months old are required six
to twelve months apart in order to gain MV accredited status. A test is carried
out twelve months later and then every two years.
For flocks established with sheep that were purchased as MV accredited a single clear test of all sheep over twelve months old, six months after arrival onto the new holding, is sufficient.
Why should I join the MV Accreditation Scheme?
MV accredited status allows you to sell stock to other members in the scheme.
Commercial farmers are now also starting to take the threat of MV more
seriously and many will only purchase MV accredited rams. It also allows entry
to shows or sales that are designated for MV accredited sheep only. It
therefore increases the potential market value of your stock.
The stringent scheme rules help prevent the flock from becoming infected. The existence of the scheme has helped keep nearly 3000 flocks free of infection.
For more information or an application form for the Maedi visna schemes or the MV Diagnostic Test please contact the Sheep & Goat Health Schemes Office on 01463 226995.