By Dr. Karen Morellio
, DVM, DACVD, Department of Medical Sciences, School of Veterinary Medicine, University of Wisconsin-Madison
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.
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; severe inflammatory diseases of the skin; 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.
Many dog owners seek veterinary assistance because their pet sheds excessively. You should remember that the natural development and growth of new hair is accompanied by the shedding of old hair. Shedding may be abnormal (excessive) if it results in obvious loss of the hair coat and bald spots. Abnormal shedding may be caused by bacterial infection. However, if the shedding is not accompanied by bald patches or symmetric hair loss, it is likely the shedding is just a stage in the natural replacement of the hair coat.
Itching or pain is a common cause of acquired inflammatory hair loss. Diseases that commonly cause itching or pain include infections, parasites, and allergies. Skin cancer uncommonly causes hair loss. Friction may cause local hair loss, for example, poorly fitted 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). The hair loss may affect an isolated spot or multiple areas, or it may be symmetric or widespread. Inflammation, thickened skin, 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 may be 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 inflammatory or 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.