Alopecia is the absence of hair; hypotrichosis, which is much more common, is the presence of less hair than normal. Although these defects can be generalized, they commonly develop in patterns that spare the extremities or correlate with hair color. These ectodermal defects can be congenital or tardive and can be associated with abnormal or absent adnexa, with defects in other ectodermal structures (such as teeth, claws, and eyes), or with skeletal and other developmental defects.
There are various modes of inheritance in those instances in which familial occurrence has been studied. X-linked ectodermal dysplasia has been reported in German Shepherds. Hairless breeds of dogs (eg, Mexican Hairless, Chinese Crested, American Hairless Terrier) and cats (Sphinx) have been bred for these ectodermal defects. In dogs the defect is due to FOXI3 gene variation. Many sporadic cases of ectodermal defects are described in dogs, most often in males. Affected dogs, including most of the hairless breeds, often have patchy or pattern hypotrichosis as well as associated dental anomalies. All animals with abnormal follicular development are prone to comedone formation, hair follicle infections, and hair foreign-body granulomas.
At least 13 types of hypotrichosis have been described in cattle, affecting Angus, Ayrshire, Brangus, Holstein-Friesian, Hereford, polled Hereford, Guernsey, Gelbvieh, Jersey, as well as Normandy-Maine, Anjou-Charolais, and Simmental crosses. Most have autosomal recessive or sex-linked modes of inheritance. Associated defects include failure of horn development, hypophyseal hypoplasia, macroglossia, dental anomalies, abnormal coat coloration, and death (lethal hypotrichosis). Viable hypotrichosis, hypotrichosis with anodontia, semihairlessness, streaked hairlessness, black hair follicle dysplasia (Holstein), and cross-related hypotrichosis (rat tail) are specific types described in cattle.
In sheep, hypotrichosis is rarely reported, with the best known syndrome affecting the Polled Dorset. This involves the hair of the face most severely, but the wool is also of poor quality. In goats, hypotrichosis is associated with congenital goiter. In swine, two forms of hypotrichosis are known (Mexican Hairless, German), one of which is associated with goiter and death in the homozygote.
In dogs, there are several tardive follicle dysplasias, including color dilution alopecia. This is found in some dogs bearing the coat color genotype dd, which renders black genotypes blue and liver genotypes beige or fawn. This syndrome is best known in Doberman Pinschers but is also commonly seen in color dilute dog breeds that include Dachshunds, Italian Greyhounds, Greyhounds, Whippets, Yorkshire Terriers, tricolor hounds It has also been reported in a German Shepherd. Recently, “silver Labrador Retrievers” with color dilution alopecia have been reported. Affected dogs are born with normal hair coats but before 1 year of age begin to develop folliculitis and hypotrichosis that is progressive and confined to the blue- or fawn-colored areas. Recently, defects in the gene MLPH have been identified in affected dogs.
Black hair follicle dysplasia, a similar but earlier developing and more complete hypotrichosis, is seen in black and white piebald dogs. The hypotrichosis develops shortly after birth and affects only the black-colored areas. This syndrome is best known in the Papillon and Bearded Collie breeds. Genetic analysis in Large Munsterlanders has indicated an autosomal recessive inheritance in this breed. A similar follicular dysplasia is reported in nonpiebald breeds.
Other types of follicular dysplasias of uncertain cause include seasonal flank alopecia of Boxers and Airedale Terriers and various woolly syndromes and post-clipping alopecia in Spitz-type breeds. Familial hypotrichosis of Irish Water Spaniels develops at 2–4 years of age, and a dominant mode of inheritance has been suggested. The condition formerly known as growth hormone-responsive alopecia in Pomeranians and other breeds is now called alopecia X, reflecting the complexity of factors, hereditary and otherwise, influencing these syndromes.
In cats, follicular dysplasia is seen in the Devon Rex. In horses, both color dilution alopecia and black hair follicle dysplasias are occasionally reported, especially in the Appaloosa breed. Congenital progressive hypotrichosis has been reported in a blue roan Percheron horse. Reported hair shaft structural abnormalities of dogs and cats include pili torti, trichorrhexis nodosa, and spiculosis (Kerry Blue Terrier).