Hair loss has a variety of causes. In many conditions, e.g., endocrine disorders, the hair loss is a result of hair follicle cycle changes. The hairs when examined tricho-graphically or histologically are normal in appearance. In the structural dysplasias, a hair shaft defect, hair follicle abnormality, or some combination thereof is present. The defect can be transient or persistent.

Congenital Hypotrichosis or Alopecia: Recognized in various species and breeds within each species. Affected animals are born with hair loss or develop it within the first month or so of life. Hair loss can be accepted as normal (e.g., Mexican Hairless, American Hairless terrier). In most instances, it is considered a disease and can be symmetrically regionalized or generalized. In animals with ectodermal dysplasia, dental and ocular changes can accompany skin changes.

Pinnal Alopecia: Most commonly recognized in the dog but can be seen in the cat. Onset of pinnal hair loss around 1 year of age. Hypotrichosis progresses to complete pinnal alopecia. Exposed skin hyper-pigments quickly and markedly. Some suggestion that it may share the same etiopathogenesis as seasonal flank alopecia. Some claims that melatonin (0.5 mg/kg q12h) improves these animals.

Pattern Baldness: Seen frequently in dachshunds, Boston terriers, and greyhounds but recognized in various other short-coated breeds. Regionalized in most dogs to postauricular region, the ventrum, and posterior thighs. Normal puppy coat replaced with thinner then normal adult in the affected regions and those hairs are slowly lost with increasing age. Hair loss is rarely complete even in advanced case so differentiation from endocrine disease should be easy. Same etiopathogenesis as seasonal flank alopecia? Some claims that melatonin (0.5 mg/kg q12h) improves these dogs.

Black Hair Follicular Dysplasia: Dogs are born normal but start to lose black hairs by weeks of age. Affected hairs lose luster prior to being lost. Focal or generalized.

Color Dilution Alopecia: Recognized in animals (rare outside of the dog) with a diluted (blue or beige(fawn) coat color. Not all dogs with a diluted coat color develop this condition. Onset of signs depends on the breed and degree of dilution. Dogs with a very dilute coat color (e.g., very light gray) lose hair early in adulthood. Non-diluted hairs (e.g., tan points in a Doberman) are not affected. Hair loss can be focal in dogs with bi-or tri-color coats or be generalized.

Trichographically, affected hairs have very large pigment clumps (macromelanosomes) which disrupt the hair shaft anatomy and weaken it. These weak hairs break with trauma (e.g., friction, grooming) and result in hypotrichosis to alopecia. These broken hairs regrow for a certain period of time (can be years) but eventually the hair loss becomes permanent. During the hair loss phase, the animal is prone to a bacterial folliculitis.

Some investigators have stated that treatment with retinoic acids improves the coat in these dogs. This therapy is very expensive and could not be expected to return the dog to normal. Beyond bathing to prevent follicular impaction and bacterial/yeast overgrowth, no treatment is likely to help these animals.

Breed-specific Follicular Dysplasias: Structural defects in the hair shaft and hair follicle have been recognized in various different breeds. Although clinical details vary from syndrome to syndrome, the hair loss occurs in young dogs, usually is regionalized rather then generalized, and tends to be transient early on. With time, the hair loss becomes permanent. No effective treatments reported.

Siberian Husky and Malamute: Onset at 3-4 months of age. Loss of primary but retention of secondary hairs. Coat color change to reddish brown.

Doberman Pincher: Red or black dogs. Onset between 1 and 4 years of age. Flank and saddle region involved initially. With time, the hair along the dorsum is lost but retained elsewhere.

Airedale Terrier, Boxer, English Bulldog, Staffordshire Terrier: Flank and saddle hair loss. May be cyclic or persistent from onset.

Irish Water Spaniels, Portuguese Water Dogs, Curly-coated Retriever: Onset in puppyhood but rarely recognized (attributed to puppy shedding). Hair loss around face and over dorsum. Cyclic initially but eventually permanent.

Trichorrhexis Nodosa: Common? Affected hair has focal weak spot which resembles two brooms pushed together. Hairs break at this point giving the coat an uneven appearance. Usually temporary (topical insult?) but can be permanent. Golden retrievers may be predisposed to the permanent form

Trichoptilosis: Trichoptiotic hairs are those with split ends. All animals will have a small number of these hairs but no coat irregularity is recognized. When large number occur, the coat develops a disheveled look. In most cases, trichoptilosis is due to solar, chemical, and/or physical trauma to the hair shafts and is resolved by correcting the underlying cause. A permanent form has been recognized in the Golden retriever.

Pili Torti: Affected animals have curly hairs when they should be straight. Whole body involved when off genetic origin. In acquired cases, not all hairs are affected and defect disappears with correction of the underlying condition.

Shaft Disorder of Abyssinian Cats: Rare condition. Onion-like bulb appears in an otherwise normal hair shaft. Hair breaks at this point.

Traction Alopecia: Uncommon to rare but easy to diagnose. Seen in breeds (e.g., Yorkshire terriers) where rubber bands are used to keep hair out of the dog's face or to prevent damage to a long show coat. Excessive traction is applied to hair follicle which results in temporary or permanent alopecia.

Feline Acquired Symmetrical Alopecia: Symmetrical, apparently spontaneous hair loss is common in cats but most cases are traumatic in origin. The cat licks/chews/pulls normal hairs in response to some pruritic or psychogenic condition. In the rare case, the hair loss, typically on the caudal ventrum and medial thighs, is not traumatic in origin and the cat has no documentable endocrine disease. Hair regrowth seen with T4 or progesterone supplementation.

Feline Paraneoplastic Alopecia: Many similarities with the acquired hyperfragility syndrome of the cat. Associated with liver or pancreatic carcinomas or other serious internal diseases. Gradual development of constitutional signs of illness with a sudden onset on hair loss on the limbs and ventrum. Hair loss in other frictional areas develops shortly thereafter. Exposed skin often develops a glistening appearance and Malassezia yeast are a common finding. Due to the advanced stage of illness of the cat by the time the hair loss is recognized, treatment is difficult.

Alopecia areata: Recognized in most species but rare. Sudden onset of sharply demarcated, focal hair loss. May have one or more areas of involvement. Generalized (alopecia universalis) disease not recognized. If "exclamation point hairs" are seen on trichrography, the diagnosis is straightforward. Since these hairs are rarely seen, the diagnosis typically requires skin biopsy. Spontaneous hair regrowth (may take years) is typical when lesions are few in number and/or size. Regrown hairs usually are white.

Pseudopelade: A recently recognized disorder of the dog and cat. The animal develops one to many well-circumscribed areas of noninflammatory alopecia. Generalized disease sparing the head and neck has also been recognized. In cats, onychomadesis is a consistent finding. Steroids are of no benefits. Some suggestion that cyclosporine or tacrolimus may be beneficial.

Lymphocytic Mural Folliculitis: A recently recognized condition of the cat and horse. One or more areas of spontaneous, asymptomatic hair loss where the exposed skin looks and feels normal. Possible etiologies include drug reaction, very early systemic lupus, or very early epitheliotropic lymphoma.

Granulomatous Folliculitis: A very rare, recently recognized condition when the hair follicle is involved in a granulomatous process with giant cells. No infectious agents visible. Endstage reaction to some eliminated infectious agent?

Anagen Defluxion: The stress is short-lived or not too profound. During the stressful period, hair growth stops and a small defect or band occurs in anagen hairs. Telogen hairs are not affected since they are not growing. After the stress is removed, the hair continues its growth but the band weakens it and it breaks off, typically in 7 to 14 days. Because telogen hairs were not affected, the animal does not develop hair loss but rather the hairs are of very unequal lengths. Diagnosis is via trichrography. The coat will return to normal with no treatment.

Follicular Lipidosis of Rottweilers: A rare condition recognized in dogs of either sex less than one year of age. Hypotrichosis-to alopecia of the tan points. Adjacent black hairs are not affected. Intercurrent thyroid disease may occur. Spontaneous improvement-to-resolution in all dogs.

Medullary Trichomalacia: Recognized trichographically in many different dogs and cats and as a syndrome in German Shepherd Dogs. In the German Shepherd, the coat is involved in one to a few spots. The first recognizable irregularity is that the hairs in the affected area first "stand up" in a large tuft. Alteration in the medullary structure causes the hairs to lose their flexibility and then break off. Spontaneous regrowth occurs in all dogs but the initial event may or may not be followed by another episode.

The trichographic abnormalities seen in the German Shepherd syndrome (medullary depigmentation, medullary swelling, and longitudinal splitting of the overlying cortex with hair shaft fracture) have been seen in many other breeds of dogs and in several cats. In these cases, the areas involved are more numerous or are generalized. In many instances the animal has some other skin disease or a metabolic abnormality and the trichomalacia disappears with the resolution of the other problem(s).