Failing to control Maedi-Visna

Maedi-Visna is a lentivirus of sheep that causes lung disease and chronic wasting. It has been designated an “Iceberg disease” by the UK sheep industry levy board with a very large burden of subclinical disease that is often not apparent until losses in an individual flock become catastrophic. Disease prevalence in the UK is thought to have doubled in the last 10 years, however farmer and veterinary awareness of the disease is poor. There is no vaccine and treatment is not cost effective, meaning that the only realistic control option is culling of affected animals.

Current testing protocols use MV gag protein ELISAs. A long lag time between infection and antibody production means that many animals are missed on flock screening and repeated rounds of testing over a period of years are necessary remove all infected animals. Preliminary testing of flocks that have attempted eradication indicates that those that do not keep testing until all animals are negative fail to eliminate the disease and that prevalence rates can even increase substantially in these flocks. The viruses extreme variability confounded attempts to develop a qPCR capable of detecting all variants, indeed deep sequencing was required to establish which strains of virus are currently present in UK sheep as there has been substantial genetic drift since the last sequencing studies (performed more than 20 years ago). More promisingly virus was detectable in nasal swabs of experimental animals at least offering a possibility for sampling methods that can be done by farmers themselves.