Thanks Shep53, I didn't know that .I will have to watch next year .I keep very detailed notes of each sheep so I will be able to trace those affected and those not.
Another question for those into genetics.......I had 4 cases of flystrike last year, not had any before, they were ewe A her daughter ,ewe B, her daughter ewe C and a lamb out of ewe A. Makes me think there must be a link and I should cull this line but I especially like eweB I have 60 sheep.