The Prevalence of Maedi Visna (MV) in the commercial sheep flock in Great Britain
Maedi Visna (MV) is a lentiviral infection of sheep which has a significant impact on the welfare of the individual, infected sheep but also has an impact at the flock level. The disease has a long incubation period and presents mainly as weight loss but can also cause pneumonia, mastitis, paralysis and occasionally arthritis. The loss of condition has many consequences in a flock including reduced fertility, smaller, weaker lambs being born (which have an increased mortality rate), reduced milk production in ewes resulting in reduced lamb growth rates, higher ewe replacement rates due to early culling of thin ewes and higher mortality rates in adult sheep. It is not just the welfare of the infected adult animals which suffers but also the welfare of their offspring. The interstitial mastitis caused by the infection results in reduced milk production leading to hungry lambs. As a consequence unsatisfied lambs chewing on teats leads to an increased incidence of bacterial mastitis, causing significant pain and often death of the ewe or at least loss of milk production in that half of the udder. Diseased sheep are at increased risk of death from other infections such as Pasteurella pneumonia.
By the time the level of infection in a flock reaches 50% the only viable option is to cull the whole flock and restock. If farmers can be made aware of the risk of their flock becoming infected and the steps that they can take to reduce the risk it is hoped that fewer sheep will suffer the effects of MV infection.
A large survey was undertaken in commercial flocks in 1996 and indicated MV infection in 1.5% of flocks. In 2002 another exercise over 150 flocks indicated 3.3% of flocks infected. This project undertook further sampling as there had been a rise in breakdowns in accredited flocks and this was potentially linked to increasing levels of infection in the commercial flock. In some European countries MV incidence is at a high level and one northern European country has an eradication policy. Sheep meat production is a vital part of the UK agricultural economy and efficient production must not be compromised.
The MV Accreditation Scheme
SAC Premium Sheep and Goat Health Scheme (PSGHS) run an MV accreditation scheme which has around 2,500 members representing pedigree breeding flocks. In some of these flocks there are non-accredited sheep as well as accredited sheep and scheme members have to follow strict bio-security rules, with flocks being subject to inspection, to prevent contact between accredited and non-accredited stock.
Analysis of results of breakdowns indicate that there is 2.5 times more likelihood of a breakdown where non-accredited and accredited sheep co-exist on the same holding as compared to a holding solely consisting of MV accredited stock.
To establish the current levels of MV in Great Britain blood samples collected under the Defra brucellosis survey were used for the MV prevalence survey. Over 12000 samples were available for testing.
To reduce the cost of this study blood samples were tested in pools of four. For any pools that tested positive the individual samples were retested.
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