My horse has got pinworm and I'm at a loss as to how he got it. He is kept in a field with his mate - who doesn't seem to have them. Over the fence is 2 others that he talks to but they don't show any signs either. He barely leaves his field. We did have 2 new horses briefly arrive but they were not in contact with mine at all and there was a field separating them. He is utterly miserable and rubbing himself raw. How do I make sure it doesn't happen again?