“Hot spots,” also called acute pyoderma, acute moist pyotraumatic dermatitis (AMPD) and acute moist dermatitis, are rapidly developing sores under the hair coat. They are common in thick-coated or long-haired dogs, less so in cats. They most often develop in areas where the hair coat is heavy, such as the back, tail base, and side of the thigh, neck, or face. Hot spots tend to occur more frequently in hot, humid weather. The dog often will scratch or chew at the area, although it can be quite painful (the condition is also called pyotraumatic dermatitis for this reason). When the hair is parted, the skin is seen to be moist and reddened. A pus-like discharge coats the skin and the base of the hairs.
Hot spots begin with a superficial skin injury that causes some moisture to be caught under the hair coat. Bacteria grow in the fluid, causing more skin inflammation, and the affected area rapidly expands as more fluid oozes from the skin, promoting more bacterial growth.
The speed of onset of hot spots is often striking, and a large and painful lesion can develop from previously normal skin in a few hours. Fortunately, other than being uncomfortable, hot spots are not life-threatening and they tend to heal very well.