Alopecia is the partial or complete lack of hairs in areas where they are normally present. Hair loss is a sign and its underlying cause must be determined for the condition to be successfully treated. If a dog has hair loss and is also scratching the area excessively, the itching should be investigated first (see Skin Disorders of Dogs: Itching (Pruritus) in Dogs).
There are many causes of hair loss, which can be congenital (the animal is born with the condition) or acquired. Congenital hair loss may or may not be hereditary. It is caused by a lack of normal development of hair follicles. It may be apparent at or shortly after birth. Or, the dog may be born with a normal coat and patchy or widespread hair loss occurs when the dog becomes a young adult.
In acquired hair loss, the dog is born with a normal hair coat. It has or had normal hair follicles at one time, and is or was capable of producing structurally normal hairs. Any disease that can affect hair follicles can cause hair loss. Certain diseases may destroy the hair follicle or shaft or interfere with the growth of hair. Some diseases can cause discomfort, leading to self-trauma and loss of hair. Acquired hair loss can be inflammatory or noninflammatory.
Diseases that can directly cause destruction or damage to the hair shaft or follicle include bacterial, fungal, or parasitic infections; skin trauma such as burns; and (rarely) poisonings caused by mercury, thallium, or iodine. These diseases tend to be inflammatory.
Diseases that can directly inhibit or slow hair follicle growth include nutritional deficiencies (particularly protein deficiencies), or hormonal imbalances such as hypothyroidism. Temporary hair loss can occur during pregnancy, lactation, or several weeks after a severe illness or fever. These types of hair loss tend to be noninflammatory unless a secondary infection of the skin develops.
Itching or pain is a common cause of acquired inflammatory hair loss. Diseases that commonly cause itching or pain include infections, parasites, and allergies. Friction may cause local hair loss, for example, poorly fitted halters or collars. Rarely, excessive grooming may be the cause of hair loss in some dogs.
Signs of hair loss may be obvious or subtle, depending on the disease. Congenital or hereditary hair loss is commonly either symmetric (appearing similar on both sides of the body) or located in one area only. It is not usually accompanied by inflammation.
Signs of acquired hair loss are varied and often influenced by the underlying cause(s). Inflammation, color change, scaling, excessive shedding, and itching are common. Some causes may lead to the development of secondary skin diseases, such as infection or fluid discharge. Itching is variable, depending on the primary cause.
An accurate diagnosis of the cause of hair loss requires a detailed history and physical examination. Key points in the history include the breed's tendency for congenital or hereditary hair loss; the presence, duration, and progression of problems; the presence or absence of itching; evidence of infection; and general health problems.
The physical examination will cover both the dog's skin and its general health. In the physical examination, your veterinarian will note the pattern and distribution of hair loss. The hairs will be examined to determine if they are being shed from the hair follicle or broken off. Your veterinarian will also look for signs of secondary skin infections or parasites and may perform skin scrapings and comb the hair coat for fleas, mites, and lice. The skin scrapings and the materials obtained during combing will be carefully saved and sent to a laboratory for testing.
Your veterinarian may order diagnostic laboratory tests. These usually include smears and culture of the skin to check for evidence of bacterial, fungal, or yeast infections. If these tests do not identify or suggest an underlying cause, a skin biopsy may be performed. Skin biopsies are often needed to confirm bacterial and parasitic causes of hair loss or to identify cancerous causes of hair loss. If your veterinarian suspects a hormonal problem, he or she may order testing of blood and urine samples.
Successful treatment depends on the underlying cause and specific diagnosis. Because identifying the underlying cause of a skin condition may take some time, many veterinarians will provide or prescribe medication to relieve any discomfort or itching your pet has in connection with the hair loss.
Last full review/revision July 2011 by Karen A. Moriello, DVM, DACVD; Patricia D. White, DVM, MS, DACVD; Michael W. Dryden, DVM, PhD; Carol S. Foil, DVM, MS, DACVD; William W. Hawkins, BS, DVM; Thomas R. Klei, PhD; John E. Lloyd, BS, PhD; Bernard Mignon, DVM, PhD, DEVPC; Wayne Rosenkrantz, DVM, DACVD; David Stiller, MS, PhD; Patricia A. Talcott, MS, DVM, PhD, DABVT; Alice Villalobos, DVM, DPNAP; Stephen D. White, DVM, DACVD