Alopecia is the scientific term describing a lack of hair where it should normally be found. Alopecia can affect large areas of skin or be restricted to a single, small patch. Hair loss may be complete, resulting in a bald spot or partial, leaving behind an abnormally thin coat. Alopecia is caused by either increased hair loss or decreased hair growth, but it does not refer to the normal process of shedding.
Hair loss is a symptom of many different conditions in dogs. When trying to narrow down the list, the first distinction to make is whether or not a pet is itchy. Keep in mind, however, that the degree to which itching is present varies greatly between individuals.
Following are the most common causes of itching and hair loss in dogs:
- External parasites: fleas and Sarcoptes mites. Demodex mites are another type of parasite that causes patchy hair loss, but in contrast to fleas and Sarcoptes they tend to cause little if any itching.
- Skin infections: bacteria, yeast, or fungi (e.g. ringworm)
- Allergies: reactions to pollen, mold, dust mites, components of food, or something in the environment that is coming into physical contact with the skin are all possible.
Other underlying causes of itching and hair loss are seen less frequently. These may include some types of cancer affecting the skin (e.g. lymphoma), inflammatory diseases (e.g. sebaceous adenitis), disorders caused by a dog's own immune system attacking its skin (e.g. lupus), and problems with the normal turnover of skin cells (e.g. idiopathic seborrhea).
If a dog is losing its hair but is not particularly itchy, hormonal diseases are most likely. The primary exception to this rule is the patchy hair loss associated with Demodex mites. Veterinarians frequently describe alopecia caused by hormone imbalances as being "bilaterally symmetrical" meaning that both the left and right sides of the body display similar patterns of hair loss. Hypothyroidism, hyperadrenocorticism (i.e. Cushing's disease), and a disorder that has been named alopecia X are common causes of bilaterally symmetrical alopecia. Some rare inherited disorders can also cause similar patterns of hair loss.
Itchy dogs frequently lick, chew, and scratch at affected areas of skin. In fact, these behaviors can be responsible for much of the hair loss associated with itchy skin diseases. This self-trauma may also produce red, oozing sores or other skin lesions. Additional symptoms depend on the underlying cause of the alopecia. Fleas may be visible as the run around on the surface of the skin, particularly around the groin and armpits. Flea dirt (feces) looks like coffee grounds, and tends to be concentrated around the base of a dog's tail. Sarcoptes and Demodex mites, bacteria, yeast, and fungi are only visible under the microscope. Dogs with food allergies may have problems with vomiting and diarrhea as well as itching and hair loss.
In addition to having poor quality coats, dogs with hypothyroidism are often overweight and lethargic. Typical symptoms of Cushing's disease include increased thirst and urination, a ravenous appetite, and a pot-bellied appearance. Most dogs with alopecia X or inherited forms of alopecia seem perfectly normal except for their coats.
In some cases, diagnosing the cause of a dog's alopecia is relatively straightforward. For example, fleas can be seen with the naked eye. However, some dogs are highly allergic to fleabites so only one or two fleas, which may have long ago jumped off the dog, can make these pets miserable. If a veterinarian suspects fleas are involved, he or she will often prescribe aggressive flea control measures even if the parasites can't be found at the time of the office visit. Easily performed diagnostic tests such as skin scrapes, scotch tape preps, fungal cultures, and fecal exams are needed to differentiate between mite infestations, skin infections caused by bacteria and yeast, and cases of ringworm. A veterinarian may tentatively diagnose allergies after other common causes of alopecia and itching are ruled out, but definitive diagnosis requires allergy testing and/or food elimination trials.
Blood tests that measure circulating levels of thyroid hormone are used to diagnose hypothyroidism. The results of routine blood work may be suggestive of Cushing's disease, but ACTH stimulation tests, dexamethasone suppression tests, and abdominal ultrasounds are used alone or in combination reach a final diagnosis. Difficult to diagnose cases of alopecia may require skin biopsies and/or referral to a veterinary dermatologist.
Treatment, Prevention and Prognosis
Treatment, of course, depends on the underlying cause of alopecia. Fleas and Sarcoptes mites respond well to many of the commonly available forms of external parasite control. Dogs can easily become reinfested once treatment stops, so continuing with a preventative medication, most of which are given monthly, is usually a good idea. Many Demodex cases will resolve without any treatment. Once a young dog's immune system matures, it is usually able to keep the mites under control on its own. More severe cases may require treatment with antibiotics and medications that kill the mites, and in rare cases this treatment has to be continued indefinitely. Skin infections usually respond well to topical and oral medications that help the body eliminate bacteria, yeast, and/or ringworm, but treatment may have to continue for several weeks or months.
Treatment for allergies can be very frustrating. If the offending allergen can be removed from a dog's environment, doing so should eliminate the symptoms. Otherwise, medications are available that can decrease a dog's itching, but they may be only partially effective and have to be continued for as long as the dog is in contact with the allergen. Hyposensitization (i.e. allergy shots) should be considered when other treatments fail to keep a dog comfortable.
Hypothyroidism is easily managed with oral medications containing synthetic thyroid hormone, but affected dogs do require treatment for the rest of their lives. Therapy for hyperadrenocorticism is more complicated, but several effective medications are now available for the pituitary form of the disease. These drugs only suppress the overproduction of the hormone responsible for a dog's symptoms, so long term treatment and close monitoring is necessary. When an adrenal tumor causes Cushing's disease, surgery is an option and can be curative. Therapy for alopecia X is not necessary as it is simply a cosmetic issue, but owners may elect to try neutering unaltered pets, melatonin treatments, hormone supplements, and other drugs that can be effective in some cases. If diagnosis and treatment of a dog's alopecia is proving difficult and frustrating, seek the services of a veterinary dermatologist.