A hot spot is a painful, swollen patch of skin between 1 and four inches. It exudes puss and gives off a bad odor, and also causes hair loss in the surrounding area over time. The infection gets worse as the dog licks and scratches the spot.
Hot spots can occur anywhere on a dog’s body. In larger breeds with big ears, such as newfoundlands and golden retrievers, they often form under the ear flap. Hot spots can come on quite suddenly and enlarge rapidly. Hot spots tend to appear before shedding when dead hair is trapped next to the skin. This, combined with skin parasites, allergies, ear and anal gland infections, or neglected grooming, can increase the likelihood of hot spots occurring.
Hot spots are really painful. For initial treatment, your vet will often anesthetize your dog, clip away hair to expose the spot, and cleanse the skin. An antibiotic cream or powder will need to be applied to a week and a half to two weeks afterwards, and oral antibiotics are often prescribed as well.
To prevent hot spots, ensure that your dog is dried thoroughly after swimming or bathing when it’s hot and humid out.
Source:
http://pets.webmd.com/dogs/guide/hot-spots-on-dogs-acute-moist-dermatitis
