I could be wrong but, my money is on Onchocerca worms, a/k/a Neck Threadworms. Caused by Midge flies who, yes, cause sweet itch^^^ Equine-Onchocerciasis in Horses
I've dealt with them and your description fits
It took me two full years to work up the nerve to double-dose with pure Ivermectin; that was after no topicals on the face of this earth, cleared the sores up.
I had to double-dose my Arab four weeks apart, even though the recommendation is two weeks apart for bad cases. He is in his 20's, has gastric ulcers and I was just too much of a Wuss to dose him at a two-week interval.
Not only did his face clear up but so did the sweet itch on his belly line.
I haven't dosed him for two years. If he does start showing signs of NTW's I won't hestitate ----welll yes I will but I will double-dose him anyway because it works