There are lots of reasons dogs may lose hair (also called alopecia), from infection to irritation caused by parasites. A few of the more common reasons include:
Allergies are a frequent trigger for hair loss in dogs. Like people, dogs can have an allergic reaction to foods, environmental triggers such as pollen, or to parasites like fleas or mites.
Flea bites are behind most dog allergies, however, and along with hair loss from licking and biting the irritated areas, signs of flea allergies include itching and redness. For a particularly allergic dog, it may take as little as one flea bite to start an allergic reaction that lingers for days.
While any dog can have allergies, some are more prone to them than others, including golden retrievers, dalmatians, boxers, Boston terriers, shih tzus, and Labrador retrievers. Treating your dog's allergies may include topical or internal medication, immunotherapy, a change in diet, and avoiding the allergen.
Hair loss in dogs may also be caused by hyperadrenocorticism, or Cushing's disease, a condition caused by the overproduction of the hormone cortisol. Less commonly, it is due to a tumor on one of the adrenal glands.
Medication is available to treat both forms of Cushing’s disease. However, surgery can be done to remove the tumor associated with the adrenal form of Cushing’s disease.
Cushing's disease is more common in dogs 6 years or older. Other signs of Cushing's disease include eating, drinking, and urinating more, as well as having a pot-bellied appearance.
Some dogs are more prone to baldness than others. Bald spots on the outer ear, chest, back, thighs, or lower neck may appear in whippets, Chihuahuas, dachshunds, and Italian greyhounds, usually after their first year.
Most owners learn to live with this, as treatment options are limited. Always make sure your vet has ruled out other causes of hair loss such as infection before you turn a blind eye to this cosmetic problem.
Infection or Infestation (Ringworm, Mites, Bacteria)
Ringworm fungus, pests like mites, and bacteria can all lead to infections that may ultimately cause bald spots in your dog.
Along with hair loss around the eyes, mouth, and elsewhere, signs your dog may have mites include oily skin, thickened skin, itching, and inflammation. Symptoms of ringworm -- a contagious infection of haired skin and claws -- includes circular or irregular hair loss, inflammation, and infected crusts.