If that isn't neck threadworms, a/k/a onchocerca worms, I'd lose a lot of money on the bet. Parasite Information - Neck Threadworms
If your horse is prone to colic or ulcers, consult with your vet first. Have him or her recommend a strong microbial ready to start feeding with the next meal.
EquiOtic and Succeed are good ones for example
If you do the double-dose with pure ivermectin, expect to see the sores get worse - a lot worse for up to 72 hours after worming. That is the NTW's dieing off under the skin and give the horse one last moment of misery.
Topicals are great to keep the flies off them. If the sores are open, I use Triple Antibiotic ointment and diaper rash cream. The diaper rash cream stays on and keeps the flies off