Maedi Visna (MV) is one of the many iceberg diseases in sheep and is named after the Icelandic for clinical signs relating to pneumonia and wasting. In the last 3 to 5 years, there has been an overall increasing trend for chronic viral infections in sheep, and this includes infections with the MV virus. Whilst it remains unclear if this trend is due to increases in sheep movements, stocking rates or general awareness and testing for such diseases, the FUW has been made aware of recent cases of MV amongst its membership.
MV is a very infectious - but slow developing - condition that causes wasting, pneumonia, progressive paralysis, arthritis and chronic mastitis. The virus has a long incubation period and the virus can spread for years before the onset of clinical signs in the flock. Alongside a lengthy incubation period, MV is a highly infectious disease with no vaccine and no cure. There is no compensation for stock culled due to MV.
MV is transmitted by direct contact with respiratory secretions or through the ingestion of milk from an infected sheep or goat. The risk of transmission of the MV virus via artificial insemination is likely to be very low.
An outbreak of MV within a flock can be devastating and the financial costs of an outbreak include high culling rates, high lamb and adult mortality rates, reduced growth rates and reduced conception.
Control relies on blood testing and removing infected animals. However the long incubation period and delayed onset of clinical signs means that infected animals may not test positive on PCR or serology tests for three to six months post infection and may be missed at screening. Thus, repeated testing is necessary at six-monthly intervals to ensure all infected animals are identified and removed.
The Animal and Plant Health Agency have reported an increase in the number of cases of Maedi Visna in lowland sheep in the first quarter of 2022. Local veterinary knowledge also indicates a similar pattern and it is therefore important to be MV aware in order to prevent an incursion of this disease.
Conscientious purchasing can help reduce the risk of introducing MV into the flock and investigative testing to identify infection if there is suspicion of disease will aid in promptly removing infection before the virus is able to spread more widely throughout the flock. Members concerned about MV should contact their veterinary practitioner for further information.