Sheep scab is a mange caused by the secretions of mites which live at the edge of the expanding skin lesions. One recent paper calculated the economic cost of sheep scab in Great Britain as £8.3m1. The sheep scab mite is defined as a permanent ectoparasite as it completes its whole lifecycle while living on the host sheep. That said, the mite can survive off the host for up to 17 days, so vector spread of the mite is possible and buildings, gates, fences and buildings should be thoroughly disinfected as part of any irradication programme. Bought in sheep should be quarantined for 21 days and treated for scab on arrival.
1 Nieuwhof G.J. & Bishop S.C., Costs of the major endemic diseases of sheep in Great Britain and the potential benefits of reduction in disease impact Animal Science 81 (2005) 23-29