Our lambs had it last year, a mystery where it came from- somebody suggested thistles to us too. We put out salt licks and kept an eye on the ewes. It was unsightly, but disappeared after a couple of weeks and ewe was fine. I think it just has to run it's course, but I'm expecting it this year as apparently once you have it on your land it remains. Poor little lambies, but it only spoils their good looks for a little while. Do remember to wear gloves though to avoid catching it yourself.