Maedi-Visna in Sheep: Welfare and Control
Disease Overview
Maedi-visna (MV) is caused by a lentivirus (Ovine Progressive Pneumonia Virus/OPPV in North America). It causes progressive pneumonia (maedi) and encephalitis/mastitis (visna). The disease is slow and insidious: infected sheep may carry the virus for years before clinical signs appear. Most affected sheep are over 3 years old when signs emerge. Transmission is primarily via colostrum/milk to lambs, and via aerosol between adults.
Welfare Consequences
Clinical maedi causes progressive weight loss, laboured breathing, exercise intolerance, and eventual death over months to years. Affected sheep struggle to maintain body condition and compete for food. Respiratory distress causes chronic suffering. Visna (neurological form) causes hindlimb weakness, ataxia, and paralysis. Mastitis (hard udder) prevents effective lamb feeding. The chronic nature of suffering makes welfare management challenging.
Diagnosis and Detection
Blood tests (ELISA or AGID) detect antibodies to MV virus. Testing is the basis of accreditation schemes. Routine scanning of the flock, particularly culling serologically positive animals, is the basis of eradication programmes. Sampling should include representative older ewes, which are most likely to test positive.
Control and Eradication
MV-free status is achievable through testing and culling seropositive animals. In the UK, the MV Accreditation Scheme provides official MV-free flock status. Eradication involves testing the whole flock, removing positives, testing annually, and preventing contact with non-accredited sheep. Colostrum management (using heat-treated or substitute colostrum for lambs) can protect lambs born to MV-positive ewes if full eradication is not immediately possible.
Flock Welfare Management
Providing good nutrition, adequate space, and minimising competition reduces the welfare impact on affected animals. Prompt identification and removal of severely affected individuals prevents unnecessary suffering. Farms should have clear protocols for regular welfare assessment and culling criteria. Joining MV-accreditation schemes improves long-term flock health and welfare outcomes.