There are many reasons why a dog may be losing hair. First, we must decide whether the hair is falling out or whether the hair loss is due to trauma. The most common cause of trauma-induced hair loss is scratching. History should clue us in as to whether or not the dog is itchy. On physical examination, trauma-induced itch is usually associated with broken hairs, linear excoriations, and thickening of the skin. The most frequent causes of trauma-induced hair loss include allergies, ectoparasites, and infections.
If we decide that the hair is falling out, then the next decision is whether the hair is falling out from inflammation or a non-inflammatory skin condition. The most likely inflammatory conditions that we see associated with hair loss are infectious. Hair loss associated with these dermatoses is usually patchy and “moth eaten” rather than diffuse.
Non-inflammatory hair loss is usually more diffuse and often symmetrical. Non-inflammatory causes can be further broken down into two categories: diseases associated with abnormal hair growth and hair cycle abnormalities.
HAIR CYCLE REVIEW
The hair cycle consists of three major phases: telogen, anagen, and catagen. The telogen phase of the hair cycle is the resting phase, anagen is the active growth phase, and catagen is the process by which the hair undergoes involution to the resting phase. Shedding or exogen is separate from the hair cycle.
The hair cycle of people is an anagen dependent cycle with hairs remaining in anagen for 2 to 6 years. Dogs don’t necessarily fit into this model. With the exception of Poodles (and probably other breeds that need regular hair cuts), most dogs have telogen predominant cycles. This means that hairs remain in telogen for months to years without a new anagen hair being initiated. It was shown that the majority of hairs from Beagles and Malamutes were in telogen with only a small percentage of hairs in anagen. These telogen hairs are well anchored and do not pull out easily. Thus, the presence of hairs in telogen alone does not indicate a pathologic state.
There are many factors that regulate the hair cycle. There is no factor that uniquely affects the hair cycle without influencing other areas of the body.
DISEASES ASSOCIATED WITH ABNORMAL HAIR GROWTH
Alopecic (hair loss) disorders in this category are associated with abnormalities in the development of the hair resulting in hairs that are abnormal, fragile, and malformed. The most common dysplasias that we recognize are caused by genetic selection for hairlessness and include the Chinese Crested Dog, Mexican Hairless Dog, and Sphinx Cat. In addition, congenital hypotrichosis is a true dysplasia and may be associated with additional defects in teeth or nails.
Color dilution alopecia and black hair follicular dysplasia are alopecias associated with defective pigmentation. Since hair pigment is acquired during the anagen phase of the hair cycle, the alopecia is associated with hair abnormalities occurring during the growth phase. The alopecia usually occurs gradually during the first 3 years of life and is color restricted. Color dilution alopecia is associated with the coat color dilution gene and has been described in Doberman Pinschers, Great Danes, Whippets, Irish Setters, and Yorkshire Terriers, to name a few. Black hair follicular dysplasia is seen in many breeds with the alopecia restricted to the black haired regions of the coat.
HAIR CYCLE ABNORMALITIES
Diseases associated with hair cycle abnormalities include endocrine dermatoses, hair cycle arrest (Alopecia X), pattern baldness, cyclic flank alopecia, post-clipping alopecia, and anagen/telogen defluxion. What these diseases all have in common is that the hair enters telogen and fails to resume cycling. The telogen hairs eventually fall out and the classic pattern of alopecia (bilaterally symmetrical alopecia sparing the head and extremities) is observed. In addition, many of the other cutaneous signs we associate with endocrine dermatoses may also be seen with these other alopecias associated with hair cycle arrest, including dull, dry hair; failure of hairs to regrow after clipping; pigmentary changes; seborrhea; comedones; and secondary infections.
Histologically, these diseases are also quite similar. In general, histopathology cannot differentiate endocrine diseases from other causes of non-inflammatory alopecia. Therefore, the main reason to biopsy a dog with suspected hair cycle abnormalities is to rule out inflammatory causes of alopecia.
Hypothyroidism is a common endocrine dermatosis associated with hair cycle abnormalities. Hypothyroidism, however, is not commonly associated with the classic bilaterally symmetrical alopecia. Instead, dogs often present with alopecia in areas of wear. Clinical findings include the alopecic “rat tail,” alopecia over the elbows and hips, alopecia around the neck from the dog’s collar, and alopecia along the bridge of the nose. There may also be a preferential loss of guard hairs resulting in the appearance of a “puppy coat.” Because the hairs are not cycling, owners may note that the dog failed to shed. In addition, the hairs may become bleached since they are not shedding out. Hyperpigmentation in alopecic areas may be observed. Recurrent skin infections are also commonly seen.
Dogs with Cushings syndrome often present with bilaterally symmetrical alopecia sparing the head and extremities. In addition, dermatologic abnormalities may include thin skin, poor wound healing, bruising, calcinosis cutis, and recurrent infections.
Sex hormone dermatoses are uncommon causes of hair cycle abnormalities. Sex hormones are produced by the adrenal glands and the gonads. Hyperestrogenism has been diagnosed in intact dogs with cystic ovaries or testicular tumors. It can also be caused by estrogen supplementation. In addition to skin and coat changes, females will often have enlarged nipples and vulva and irregular heat cycles. Males may display feminization. Hyperandrogenism is usually associated with testicular tumors. Symmetrical alopecia is not a common presentation of this condition.
Pattern baldness is a hair loss that usually starts at less than one year of age and progresses over time. Breeds commonly affected include Dachshunds, Boston Terriers, Greyhounds, and Water Spaniels.
Cyclic flank alopecia is a seasonal truncal alopecia that primarily occurs in adult Airedales, Boxers, and English Bulldogs. The alopecia is usually symmetrical and is often associated with hyperpigmentation of the skin. The alopecia usually develops during periods of short daylength, being most noticeable in winter and early spring. The alopecia does not always recur each year. The condition appears to be worse up north. The hair usually regrows within several months; however, some dogs have a prolonged period of alopecia.
Hair that fails to regrow following clipping has been coined “post-clipping alopecia.” This is often seen in plush-coated breeds but may occur in any dog. This is probably not one disease entity but many. The plush-coated breeds may have Alopecia X or simply have been shaved during the normal telogen phase of the hair cycle. Post-clipping alopecia is also often associated with surgeries in which the dogs received an epidural. In these cases the hair grows back from other surgery sites but fails to regrow from the site of the epidural. The reason for this is unknown.
Telogen defluxion is an uncommon cause of alopecia in dogs. Illness or severe stress causes abrupt cessation of anagen and synchronizes hair follicles into telogen. One to three months following the stressful incident, the telogen hairs are shed synchronously as new hair cycles begin. The dog experiences a temporary baldness as new hairs grow.
Because all of these alopecic conditions can mimic an endocrine dermatosis, it is important to always rule out hypothyroidism and hyperadrenocorticism prior to deciding that this is a hair cycle abnormality.
For years, breeders of plush-coated breeds and Miniature/Toy Poodles have recognized a bilaterally symmetrical alopecia with hyperpigmentation of the skin that is not caused by hypothyroidism or hypercortisolemia. The breeders refer to this condition as “black skin disease” or “coat funk,” depending on the breed affected. In the veterinary literature it has gone by many different names including adult-onset growth hormone deficiency (hyposomatotropism), growth hormone-responsive alopecia, castration-responsive alopecia, biopsy-responsive alopecia, congenital adrenal hyperplasia-like syndrome, and most recently Alopecia X.
The fact that many varied treatments (growth hormone, castration, methyltestosterone, mitotane, and melatonin) have resulted in hair regrowth suggests a failure of the hair follicle to cycle properly and that the treatments merely serve to initiate anagen. Hormones are known to influence the hair follicle cycle. Castration, methyltestosterone, and mitotane may result in hair regrowth by merely manipulating hormone concentrations. Melatonin may induce hair growth by decreasing sex hormone concentrations in intact dogs; however, in neutered dogs, this is unlikely. Melatonin may indirectly induce hair growth by increasing growth factors. In addition, melatonin has been shown to block estrogen receptors in human breast cancer cells. Estrogen receptors are present in telogen follicles and appear to have a regulatory role in the hair cycle. Preliminary studies with estrogen receptor blockers, however, have failed to cause hair regrowth. This is possibly dose-related.
Understanding the normal hair cycle control in dog breeds affected by this condition will hopefully allow us to identify the pathomechanism of the alopecia. Recent work has shown that breeds such as the Nordic breeds have hairs in a prolonged telogen phase which may last for years. This is in contrast to Poodles in which the hair cycle has a prolonged anagen phase, similar to that found in people. Interestingly, both of these breed types develop Alopecia X. More research is needed to unravel the mystery behind this alopecic disorder.