...and on the eighth day God created the horse in perfect image, to romp, graze, gallop, play and make manure wherever it darn well pleases, in divine grace.
Summertime can be an occasion for external parasites to cause itchy skin disorders on your horse. A daily "once-over" alerts you to sores, patchy skin or tail rubbing. Remedies to situations can save money and keep your horse comfortable, at any time of the year.
Sometimes horses have allergies to flies or gnats. Other times the external itching comes from other creatures, like mange or lice.
Flies can be brought under control by keeping the barn clean and by using a manure spreader instead of a pile outside the barn door.
Sometimes a manure spreader isn’t possible, so carting the manure away to a pile at least 30 yards away from the barn is best. Wooden planks laid on top of snow or mud make a long boardwalk for winter disposal.
Fly repellants on stable walls and ceilings can help keep insects away as does spraying the animal at least once a day. Do not spray the animal in the face and make sure water and feed have been removed if spraying the interior of stalls.
As stated in the past, I’m not big on natural fly repellants; I like products with pymethrin in them, such as Farnam’s Repel X.
Light scrim sheets can be worn inside or outside to protect the horse’s skin from biting flies, and turning them out at night helps protect not only from flies, but the sun’s harsh rays as well.
As I’ve also stated in the past, it really bothers me to see horses standing in dusty corrals fighting flies under the blazing sun. It especially bothers me to see big draft horses with bobtails standing out in the blazing sun, trying to fight flies. There should be some shade trees, at least, for them to stand under.
Two other external parasites that cause itching are mange mites and lice.
Mange mites burrow under or live on the skin and are considered a contagious disease. The horses must touch one another and the mites can also be picked up from box stalls or tack. They cause intense irritation and itching. (1)
Mange is considered a "reportable disease," and the Vet should be consulted for skin scrapings.
Horse lice are both biting and sucking and are found on horses that are neglected or poorly fed. They can be picked up by direct contact with an infected animal or from dirty stalls and tack.
Both lice and mange have identical symptoms in the early stages: the horse bites and rubs continuously, developing bare patches. On close exam by a Vet or knowledgeable horseman, the lice can be seen along with their minute, white louse eggs. (2)
Chicken lice can be transferred to equine and the two should not be housed in proximity to one another.
Mites have 3 types: Sarcoptic, Psoroptic and Chorioptic. (3)
Sarcoptic mites penetrate and burrow the upper layer of skin. Blisters and nodules form and spread over the entire animal.
Psoroptic mites live in colonies on the skin and do not form burrows. They puncture the skin and secrete poison into the wound, forming vesicles.
The Chorioptic mange mite is found below the knees and hocks. They are similar to
Psoroptic mites, living on the surface, causing similar symptoms.
Other itch-causing creatures are airborn gnats, called "Culicoides." The Indians called them " no-see-ums" or " punkies."
Punxatawney, Pa. was called the land of the punkies, according to Indian lore.
Another name for them is midges, and they cause a summer eczema or dermatitis characterized by severe itching. This can be so severe the horse will wear its mane off.
Black flies, which come out in the spring, can produce an allergic dermatitis, as do horn and stable flies.
These can develop into secondary bacterial infections.
"Pyoderma" is a bacterial skin infection that drains pus. Pyoderma starts because another itchy skin disorder was present there first. Horses bite or rub at a persistent itch, creating self-mutilation of the skin, and a subsequent infection develops.
It is recommended you look for a skin disease before deciding that "Pyoderma" is the only problem with the horse. (4)
"Cellulitis" is a deep skin layer infection, usually caused by lacerations, wounds or scratches. (5)
Your horse could have Cellulitis if the skin appears redder that normal, skin feels hot, too firm or is tender or painful to pressure.
"Folliculitis" or Summer Rash is a hair-pore infection cased by Staph, (bacteria species) occurring in hot weather because of friction to the skin with ill-fitting tack and excessive sweating. 
Folliculitis can develop into "Furunculosis," a deep hair-pore infection with patchy hair loss and draining sinus tracts. (7)
Have a Vet see the horse.
Folks, all that skin junk has made me itchy and I’ve lost my appetite for dinner. Somebody get me a jagger cactus, so’s I can scratch my back.
Maybe I’ll mosey on over to Mom’s kitchen to see what’s cookin’, her good vittles always help to get my appetite back.
Since that will do it for this week, I’ll leave you with the immortal words of Roy Rogers and Dale Evans, "Happy Trails to You."
The information written herein is meant as a guideline and not veterinary analysis.
Always seek the advice of a qualified Vet should you suspect your horse may be suffering from mange, lice or pyoderma.
(1,2) "The Horse Owner’s Vet Book," by E.C. Stratton
(3) "Evans on Horses, 2nd Edition, " by J. Warren Evans
(4,5,6,7) " Horse Owner’s Vet Book," by Drs. Giffin and Gore