Alopecia (absence of hair) and hypotrichosis (less hair than expected) are among the most common hereditary or congenital problems of animals. The initial diagnosis is made via visual examination and a careful workup to rule out other common causes of hair loss (eg, pruritus, infections, parasites). Evaluation of hair follicle structure and density can only be made by histological examination of skin biopsy samples. Congenital or genetic causes of hair loss are managed but not cured. Management focuses on prevention of secondary infections, solar protection, and physical protection of the skin.
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 extremities or correlate with hair color. Ectodermal defects can be congenital or tardive and can be associated with abnormal or absent adnexa, defects in other ectodermal structures (such as teeth, claws, and eyes), or skeletal and other developmental defects (see images of dog with ectodermal teeth abnormality and hypotrichosis in a kitten and a foal).
Courtesy of Dr. Karen A. Moriello and Dr. Gail Kunkle.
Courtesy of Dr. Karen A. Moriello.
Courtesy of Dr. Karen Moriello.
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 Shepherd Dogs. Hairless dog breeds (eg, Mexican Hairless, Chinese Crested, American Hairless Terrier) and cats (Sphynx) 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 (see ectodermal defect in puppy image). All animals with abnormal follicular development are prone to secondary bacterial and yeast overgrowth, comedone formation, hair follicle infections, and hair foreign body granulomas (see image of dog's tongue with ectodermal defect).
Courtesy of Dr. Gail Kunkle.
Courtesy of Dr. Gail Kunkle.
At least 13 types of hypotrichosis have been described in cattle, affecting Angus, Ayrshire, Brangus, Holstein-Friesian, Hereford, Polled Hereford, Guernsey, Gelbvieh, and 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; however, the wool is also of poor quality. In goats, hypotrichosis is associated with congenital goiter. In swine, the Yucatan miniature pig, Mexican Creole hairless pig, and large white Ulster have "normal" hypotrichosis due to an autosomal recessive gene.
In dogs, there are several tardive follicle dysplasias, including color dilution alopecia. The predisposing risk factor is the gene variant mlph, which has an autosomal recessive mode of inheritance. This renders black genotypes blue and liver genotypes beige or fawn.
Color dilution alopecia is most widely recognized in Doberman Pinschers but is also commonly observed in color dilute dog breeds, including Dachshunds, Italian Greyhounds, Greyhounds, Whippets, Yorkshire Terriers, and tricolor hounds. Color dilution alopecia has also been reported in a German Shepherd Dog. “Silver" Labrador Retrievers with color dilution alopecia have been reported. (See color dilution alopecia images.)
Affected dogs are born with normal hair coats; however, before they are 1 year old, they begin to develop progressive folliculitis and hypotrichosis confined to blue- or fawn-colored areas. Recently, defects in the gene mlph have been identified in affected dogs. This disorder is cosmetic and will not affect the dog's overall health. The coat must be managed to control dry skin, excessive scaling, and superficial skin infections.
Courtesy of Dr. Karen A. Moriello.
Black hair follicle dysplasia (a similar but earlier-developing and more complete form of hypotrichosis) occurs in piebald black-and-white dogs. Hypotrichosis develops shortly after birth and affects only black-colored areas. This syndrome is best known in Papillon and Bearded Collie breeds. Genetic analysis in Large Munsterlanders indicates 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 postclipping alopecia in Spitz-type breeds. Familial hypotrichosis of Irish Water Spaniels develops at 2–4 years old, 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 occurs in the Devon Rex. In horses, both color dilution alopecia and black hair follicle dysplasias are occasionally reported, especially in Appaloosas. 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).
For More Information
Welle MM. Canine noninflammatory alopecia: an approach to its classification and a diagnostic aid. Vet Pathol. 2023;60(6):748-769.
Mecklenburg L. An overview on congenital alopecia in domestic animals. Vet Derm. 2006;17(6):393-410.
Also see pet owner content regarding alopecia in dogs and in cats.
