You might check to see if her udder is 'dirty'. Some mares will get a build-up of black, smelly,usually crusty, 'gunk' between the teats, similar in some ways to the 'gunk' that can form inside the sheaths of some geldings and stallions. Since it is next to impossible for the horse to scratch there, they will sometimes try to scratch themselves in a 'nearby' spot. As someone else said, you should be able to see lice, by carefully parting the hair, looking very closely while being patient. I personally would want to be sure my horse HAS lice before treating for them. Another thought...I am a strong proponent of adding stabilized ground flax as a feed supplement. It is high in antioxidant-rich omega-3 fatty acids,which are health-beneficial in several ways, and can really improve the quality of hair coat, skin, hooves.