Like an ill-fitting pair of shoes that makes your feet sore—or creates a blister on your little toe—a saddle can create a sore on your horse's back if it doesn't fit right. A girth sore might show up the first time you ride your horse in the spring. This could seem strange because you used that same cinch all last summer and it didn't cause a problem. However, your horse is soft and fat after his layoff and his skin is more tender. The girth rubbing on tender skin (with a layer of fat underneath it) was like you suddenly doing a lot of hand work without gloves; unless the skin on your hands has a chance to toughen up gradually, you will get blisters and raw spots.
The first sign of trouble might be a dry spot when you take the saddle off (the extra pressure in a small area has inhibited circulation and the horse was unable to sweat) or ruffled hair. The pressure or rubbing might break off some hair, leaving a rough-looking area at that spot. The hair might be standing up rather than lying smoothly. If you keep riding that horse with problem tack, he will develop a sore.
A sore starts as inflammation of the skin, and you should be able to feel it before you can actually see it. If you run your hand over the horse's back, you might find a raised, hot, or swollen area. If you continue to use the horse, this lump is subjected to more rubbing (since it protrudes upward), making the condition worse.
Too much pressure in one area can kill skin cells due to lack of proper circulation; blood is pressed out of that spot. A sore results if enough cells die. The affected skin sloughs off. The tissue death is call