Jack Sores, or Summer Sores (so-called because they tend to flare up in warm weather and go away in winter) are caused by the larvae of stomach worms. The little critters are carried by flies and will generally make themselves at home in a horse's (donkey's, mule's) stomach, lungs, eyes, or skin. The results can range from annoying and unsightly to fatal. Young and thin-skinned animals are especially susceptible to the pests.
The problem occurs all over the world, and several studies have indicated that roughly half of the equine population is infected with stomach worms. (That's another point in favor of having all horses under human management, rather than having them run "wild and free" -- and sick and miserable.)
Ivermectin is probably the most effective treatment for both internal and external pests (remembering, of course, that this is just your garden-variety cowboy speakin' -- you should always check with your vet about medicines). There are some other treatments that may help in a pinch, including a 0.5% solution of old-fashioned Chlorox. In serious cases of infected sores, surgery may even be needed to remove the dead or diseased skin.
Ultimately, the best treatment is prevention. This includes a regular program of wormers -- about every six to eight weeks in warm weather, less often in winter. Do everything you can to keep flies at a minimum, including fly traps, proper disposal of manure and bedding from stalls, rotating pastures, elimination of fly breeding sites, and removal of fly eggs from tack and the animals' coats. Be especially diligent to keep flies from infecting the horses' food and water. It's probably a good idea to isolate infected animals from the rest of the herd. Faithful application of a good repellant will also help keep infected flies away from our four-legged friends.
:dodgy: