There are over 50 conditions that cause hair loss in dogs. When it comes to hair loss on the tail only, that can narrow the list a bit. Much of the time when hair loss is exhibited on the tail, it may also be occurring somewhere else on the dog's body; it may be that since the tail is relatively small in skin coverage, it is the area noted before any other.
Abnormal hair loss on any part of the dog's body is known as alopecia. For some breeds, like the Chinese Crested, this is a natural and normal condition. However, if hair loss other than seasonal shedding is noted in most breeds, it is a time to consult your veterinarian to determine the exact cause and get treatment so the hair can grow back.
When it comes strictly to the tail there are several options for causes of hair loss that are considered relatively common in nature. Hair loss can be a result of parasitic infestations. In these cases the skin simply becomes unhealthy enough to prevent normal hair growth. Most common of these infestations is flea allergy dermatitis. The second is a mite that has moved into the skin; mites cause two types of mange. Most dogs with parasitic-related hair loss will exhibit other problematic areas on their bodies when they are closely examined.
Diseases that can cause tail-hair loss are diabetes; dermatomyositis (which is far less common); follicular dystrophy/alopecia; folliculitis, which is a bacterial skin infection; and growth-hormone-responsive alopecia, which is typically seen in dogs that are under the age of two years. Hypothyroidism can cause general hair loss that includes the tail.
Breed-Specific Hair Loss
There are some breeds that are more predisposed to certain tail-hair-loss conditions than others. Some of the short-coated hounds such as the dachshund, greyhound and whippet as well as sporting breeds like the Irish water spaniel and Portuguese water dog have pattern alopecia. One of the areas affected by these patterns is typically the tail. Follicular dystrophy/dysplasia is another form of typically breed-specific hair loss often noted in doberman pinschers, huskies and boxers.
Tail-hair loss has also been attributed to drug reactions and pressure sores. In the latter, hair loss may appear if your dog lies on its tail frequently on abrasive surfaces like concrete or wire. Areas where hair has been clipped away by a groomer or perhaps for surgeries can experience a delay in regrowth.