Alopecia, also called hypotrichosis, is absence of hair in an area where it is normally present. It may present no risk to the patient and may be nothing more than a cosmetic problem, albeit one which may cause the owner considerable distress. Alternatively, it may be a manifestation of an internal disease with potentially serious consequences. A methodical approach and accurate diagnosis are prerequisites for successful management of these cases. Breed predilections have been reported for many of the acquired endocrine diseases and are of particular importance in follicular dysplasias, which have been recognized in Curly Coated Retrievers, Irish Water Spaniel, Portuguese Water Dog, Siberian Husky, Doberman Pinscher and other breeds.
Dogs generally possess a dense hair coat that covers the entire skin surface, with the exception of the nasal planum, footpads, lips, nipples, and anus. The length, thickness, density and color vary between individuals and especially between breeds. Dogs exhibit compound hair follicle grouping whereby a primary hair and 2-15 secondary hairs emerge through the same pore. Hair has important thermoregulatory, protective and sensory functions. A cyclical pattern of hair shedding, followed by new growth and replacement has evolved. The growing or anagen phase is characterized by the production of a hair shaft. Anagen usually accounts for 80-90% of the duration of the hair growth cycle. The cessation of active hair growth signals the onset of catagen, a short transitional phase during which the inferior section of the hair follicle undergoes regression. Shedding of the hair only occurs after the next anagen phase has begun and a new hair shaft has entered the follicular canal. In the dog, the hair growth cycle in each body region is asynchronous and the hair coat is replaced in a mosaic pattern. The hair growth cycle reflects the intrinsic rhythmic activity of the follicle, which results largely from local regulatory processes that are poorly understood. Several hormonal systems influence follicular activity which probably facilitate seasonal changes in follicular activity in response to environmental factors, such as photoperiod and ambient temperature.
Abnormalities of the hair coat are usually recognized by dog owners early in the course of a disease.
Alopecia may be a feature of myriad skin diseases, including parasitic, allergic, and endocrine diseases, as well as tumors. Diseases characterized by alopecia can be divided into scarring (or potentially scarring) and non-scarring disorders. In scarring alopecia, there is destruction or distortion of the follicle, usually as a consequence of an inflammatory process, which means that hair regrowth is not possible. A common example would be deep pyoderma secondary to demodicosis. By contrast, non-scarring alopecias generally result from structural or growth cycle abnormalities of the follicle without inflammation, and can be classified into four subtypes:
- In atrophic alopecias there is abnormal hair growth cycle leading to a shortened anagen; telogen phase (follicles are "asleep"), prolonged hypothyroidism, hyperadrenocorticism.
- Follicular dysplasias are marked by disorder of structural proteins causing malformation of shaft or follicle such that hair growth is impossible (congenital alopecia in Chinese Crested Dogs).
- Matrix cell/melanocyte abnormalities is characterized by abnormal dispersal of melanosomes and/or disruption of matrix cells (color dilution alopecia, black hair follicular dysplasia)
- In traumatic alopecias the hair is removed as a result of excessive itch or a psychogenic disease (alopecia in allergic dermatitis)
Acquired, symmetrical, affecting both sides of the body, non-itchy alopecia with easily epilated hairs and variable disturbances of skin and hair pigmentation is the classical picture of atrophic hair follicle diseases associated with hormonal disorders. Follicular dysplasias (diseases characterized by incomplete or abnormally formed hair follicles and hair shafts) are less common but may clinically closely resemble an endocrine system disease.
Symmetrical, non-inflammatory alopecia acquired during the first year of life is suggestive of some follicular dysplasias; matrix cell/melanocyte abnormalities; congenital endocrine diseases, e.g. congenital hypothyroidism, or pituitary dwarfism. The most common acquired endocrine diseases, hypothyroidism and hyperadrenocorticism, are usually seen in middle-aged or elderly dogs. Moderate or severe itching (pruritus) in a dog with symmetrical alopecia may indicate a trauma or a scarring (inflammatory) disease. However, dogs with basic non-inflammatory diseases, such as endocrine disease, may also be pruritic if the disease is complicated by a secondary microbial infection, the most common being staphylococcal pyoderma.
In endocrine diseases such as hypothyroidism and hyperadrenocorticism, there may be features indicating abnormalities of other organ systems. For example, the owners of dogs with hyperadrenocorticism often report increased urination, thirst and hunger. In hypothyroidism, there may be lethargy and weight gain.
Telogen effluvium (telogen defluxion) is a transient disorder of excessive hair shedding resulting from a synchronous cessation of anagen, and usually follows pregnancy, lactation, severe illness or a similar stressful event that has occurred in the 1-3 months previously.
Seasonal or cyclical episodes of non-inflammatory symmetrical truncal alopecia followed by spontaneous hair regrowth are typical of seasonal flank alopecia and also occasionally occurs in follicular dysplasias, such as that seen in the Portuguese Water Dog.
A lightening of the coat color is common in dogs with hyperadrenocorticism. Changes in coat color and coat quality are also seen in many of the follicular dysplasias. Pattern baldness is a rare disease usually seen in the Dachshund marked by symmetrical non-inflammatory alopecia of the ears and/or abdomen. Intense darkening of the alopecic areas is most commonly seen in dogs with seasonal flank alopecia and "alopecia X" which is dependent on hyperadrenocorticism. Both male and female, neutered and entire, dogs are affected. Symmetrical alopecia affects the trunk, thighs, neck, and changes in coat color and coat quality may also be seen. Hair regrowth is often observed at sites of trauma, such as skin biopsy sites.
The microscopic examination of skin scrapings for demodectic mites is performed in cases of canine alopecia. In dogs with non-inflammatory symmetrical alopecia microscope examinations of plucked hairs (trichograms) and skin biopsy specimens may also be of value. Skin biopsy is required for the diagnosis of follicular dysplasias and matrix cell/melanocyte abnormalities. Biopsies are also indicated in cases where the diagnosis cannot be readily confirmed by other means. Urine and blood analyses are also appropriate. Specific treatment is available for most of the endocrinopathies but it is not possible in dogs with follicular dysplasias or matrix cell/melanocyte abnormalities. In general, the treatment of dogs with symmetrical alopecia by hormonal supplementation, without a specific indication based on laboratory testing, is discouraged. Dogs with alopecia X may respond to castration. Drug therapy often results in hair regrowth within 4-12 weeks. In neutered male dogs, methyltestosterone supplementation, followed by weekly maintenance therapy, may be attempted. However, behavioral changes, seborrhea oleosa and hepatic disease are potential adverse effects. Although some dogs respond favorably to growth hormone supplementation, this hormone is harmful to the immune system, may cause diabetes and is difficult to obtain and therefore is not recommended. Supplementation with estrogen or testosterone is potentially hazardous and they should only be used in cases where other more common endocrine diseases have been excluded definitively. Some owners prefer to accept the alopecia once the veterinarian has explained the potential adverse effects of supplementation with these hormones.