Acanthosis nigricans a condition in which there is abnormal darkening of the skin. It is primarily a disease of dogs and has several different forms and causes. The disease can be divided into a primary form that is a genetic condition found primarily in Dachshunds, and a secondary form that has several causes and can be found in any breed or age of dog. In the secondary form, there is a mobilization of the pigment called 'melanin' to areas of chronic inflammation. It is the melanin that creates the dark appearance of the skin. Acral lick dermatitis (also called lick granuloma) in dogs is a condition which develops as a result of excessive, chronic licking. It usually occurs on the front part of the lower leg, often over the carpus or metacarpus. The constant licking causes inflammation in the skin and, over time, the skin in that area becomes thickened. Because the dog continues to lick, the area can not heal and may become red, raised, and ulcerated. It is likely that the licking and the inflammation also cause the area to be itchy, which causes more licking, and starts an itch-lick cycle. Allergic contact dermatitis occurs in dogs as a hypersensitivity reaction to certain molecules in the pet's environment. Irritant contact dermatitis results when the skin is exposed to noxious substances in the environment. The symptoms and biologic mechanisms involved in these two diseases are similar so they are often discussed together. lowchensaustralia.com take a look at the link above and it will give you all the reasons for hair loss in dogs.