We had all our hoggs itchy one winter, no-one could find anything in wool or skin scrapings, but we decided to try Dectomax as they were so miserable. Not much of an improvement, so we tried Crovect (thin line right down onto the skin, not a broad mist over the fleece like you would for fly) and that did the trick. Vet said it must've been biting lice as they are not affected by Dectomax but are by Crovect.