My horses became increasingly itchy over the years (19 and 26 years of age). My gelding got to the point he was knocking over fencing and unhanging sliding barn doors. He had weeping sores on his belly and his neck, and other places. He rubbed his mane, forelock, and tail out. After the usual vet treatments for “allergies”, lice, flies, and rain rot, I started researching on the internet. http://thehorsesback.com/neck-threadworms/ is a starting point. Here is a blog with much much more information about underdeworming and help for itching: http://www.chronofhorse.com/forum/showthread.php?155469-something-to-kill-adult-Onchocerca-19-CASE-STUDIES-POSTED-PAGE-58/page2&s=537bbd0a2c898ac969f1382439386155&highlight=Onchocercca
It took a while to read through The Chronicle of the Horse Forum on this subject, but I finally talked myself into doing the suggested treatment, even though my vet was quite skeptical. I am the one who does not like chemical this and chemical that for cures. In fact I have been deworming my horses with Diatomaceous Earth…underdeworming. I did the double dose of Equimax / Ivermectin recommendation several times as recommended on the Forum. I now have itch-free content horses. I am back to deworming with Diatomaceous Earth (1 cup daily so I don’t underdose again). I grind flax seed for them daily (1 cup) so it is fed immediately. I was reluctant to do the Equimax / Ivermectin treatment, but I am glad I read up on it and did it. Read, research, talk with your vet. It could be something to consider.