My thought before I read your reply about worming the filly was either lice or neck threadworms.
I am leaning toward neck threadworms if she was that full of worms. Onchocerca worms can also get into the eyes and have to surgically removed. Humans can also get the onchocerca worm Horse Health: Neck Threadworms
While I know how to treat for neck threadworms, I am not saying anything because I really think you should consult your vet - especially since the horse is young enough to be called a filly. It would not be wise to try and Powerpak her without consulting your vet first to be sure that really is the issue.
Once he looks at her, he may end up saying she has lice. Horses can pick up lice from chickens and even deer. The Horse: Lice on Horses
My horses will get rainrot, scratches, and sweet itch from July thru mid-September because of our high heat and even higher humidity. I have never seen any of them get it on their necks or poll area.
I hope this helps some