Hi! A few questions: What is the age of the sheep, is it a ewe? Has she been feeding lambs? How long ago is it since they were wormed and if more than six weeks, it might need another dose/change of wormer. If the ewe has lambed a few times they tend to look thin on the back end anyway because everything tends to drop! If she's been feeding lambs this year, particularly twins, it can take it out of them and it can be a while before they are back in condition. You could try taking a droppings sample and sending it to the vets for a worm count but this is a bit of a pain because you have to isolate the sheep for a while to make sure you're getting poo from the right sheep. And of course, you will be charged but not sure how much (someone else on this site might though) Cheers!!