Impact of maedi-visna on sheep breeding flocks

Maedi-visna (MV), is a chronic wasting disease of sheep and goats caused by the small ruminant lentivirus (SRLV), maedi-visna virus (MVV). With no known cure or treatment, an asymptomatic period of sometimes several years and an infection that ultimately results in death, the finding that prevalence is on the rise within the UK is of great concern. In this study, a diagnostic was developed for detection and quantification of as a yet unidentified SRLV strain circulating within the UK in 2014. Identification of the viral strain was attempted to characterise this current circulating strain. Tissues and blood samples were collected from 28 seropositive rams over a period 28 months as part of a longitudinal case study after which semen harvested from 13 of these rams was used in an artificial insemination (AI) trial to estimate the risk of MVV transmission within a natural mating model. The viral strain was partially characterised at the molecular level and found to show similarities with previously reported UK strain (EV1). A qPCR assay was developed and showed successful detection of virus within both blood and tissue samples of seropositive animals but failed to detect any viral sequences with inseminated naïve ewes 7 weeks post insemination. In addition, proviral loads within blood were shown to be higher than previous reported findings. Finally, regression modelling of milk production data collected from a UK dairy flock suggested an outbreak of MVV of an unknown strain to cause a reduction in milk yield within seropositive ewes. Overall, this study demonstrates the impact of disease of a newly identified circulating strain of MVV within the UK