Actually, summer sores are caused by a parasite commonly known as the 'neck threadworm'. Their official name is Onchocerca Cervicalis. They are spread by another parasite called a midge.
I would contact a Vet that is an equine specialist before I just used the Ivomec treatment on my own. While it will kill them, (as will medications used for killing Heartworms in dogs), there can be some awful complications. Some of the immature and mature threadworms can migrate to the horse's eyes and to its tendons and ligaments - primarily in its front legs. When you kill these parasites, the horse can have a terrible reaction to the dead ones. If they have invaded the horse's eyes and have not been located there first, they can cause a terrible case of 'Uvetitis' and can even cause permanent blindness. If they have invaded the ligaments or tendon sheaths in the horse's legs, they can cause lameness and huge swellings that require surgery to remove the dead worms.
They can be killed locally if small amounts of Ivomec Pour-on are applied with a Q-tip. The thought being that this small amount will not kill others in distant parts of the horse like a systemic dewormer will. I have seen this work, but then no one know if there were any parasites present in the horse's eyes when this method was used.
I know Vets that will not treat neck threadworms without a thorough eye exam and may even require a biopsy of the Conjunctiva of the eye of the horse first. This kind of caution comes from having blinded a horse or two and having to pay for the damages.
They can be really simple or they can be very serious, so I would not treat them without taking precautions.
Many of the older Vets and Country Vets still think this dermatitis on a horse's neck and shoulders is an allergy to fly bites, so question a Vet to see what their protocol is for treating Onchocirca and that they even know what they are. Sadly, many do not.