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Congenital and Inherited Skin Disorders in Horses

By Karen A. Moriello, DVM, DACVD, Professor of Dermatology, Department of Medical Sciences, School of Veterinary Medicine, University of Wisconsin-Madison
John E. Lloyd, BS, PhD, Professor Emeritus of Entomology, University of Wyoming
Bertrand J. Losson, DVM, PhD, DEVPC, Professor, Department of Parasitology and Parasitic Diseases, Faculty of Veterinary Medicine, University of Liege
Wayne Rosenkrantz, DVM, DACVD,
Patricia A. Talcott, MS, DVM, PhD, DABVT, Associate Professor, Department of Food Science and Toxicology, Holm Research Center, University of Idaho
Alice E. Villalobos, DVM, DPNAP, Director; Director, Animal Oncology Consultation Service; Pawspice
Patricia D. White, DVM, MS, DACVD,
Thomas R. Klei, PhD, Boyd Professor and Associate Dean for Research and Advanced Studies, School of Veterinary Medicine and Louisiana Agriculture Experiment Station, Louisiana State University
David Stiller, MS, PhD, Research Entomologist, Animal Disease Research Unit, U.S. Department of Agriculture, Agricultural Research Service, University of Idaho
Stephen D. White, DVM, DACVD, Professor and Chief of Service, Dermatology, Veterinary Medical Teaching Hospital; Department of Medicine and Epidemiology, School of Veterinary Medicine, University of California, Davis
Carol S. Foil, DVM, MS, DACVD, Professor, Department of Veterinary Clinical Sciences, School of Veterinary Medicine

Horses may be born with any of several different kinds of skin abnormalities. The disorder may be present at birth or may develop weeks to months later.

Congenital Skin Disorders

Epitheliogenesis imperfecta (aplasia cutis) is a congenital skin deformity. Foals are born with small to extensive areas where skin is missing. Secondary infection is common. One or more hooves may be deformed or absent; in some affected animals, there are other associated congenital anomalies. The condition is fatal when extensive, but small defects can be surgically corrected.

A nevus in humans is a congenital pigmented area on the skin that is often called a birthmark. In animals, the term is used for any area where skin is malformed, including abnormally pigmented spots. Some forms of nevi displace the normal structures of the skin, including hair follicles; thus, these patches are hairless. When not extensive, nevi can be surgically removed; otherwise there is no effective treatment.

Dermoid sinuses or cysts occur in Thoroughbred horses. These cysts are lined with skin. Exfoliated skin, hair, and glandular debris accumulate in the cysts, which can lead to infection. The cysts are found on the midline of the back and in rare cases are associated with spinal cord deficits. They can be removed by surgical excision.

Follicular cysts develop by abnormal hair follicle growth and by retention of follicular or glandular products. They may be congenital when the hair follicle does not develop normally. Periauricular (dentigerous) cysts are seen in horses. Although they are present at birth, they may not be recognized until adulthood.

Pigmentary Abnormalities

Skin color abnormalities may be either acquired or hereditary. Abnormalities in skin and coat color are sometimes related.

True albinism is always associated with pink or pale irises and with visual defects and increased risk of skin damage from solar radiation. Albinism is different from extreme white spotting. Lethal white foal syndrome is one that results from breeding 2 Overo Paints. Some animals with extreme piebaldism (spotted or blotched with black and white) or dominant white have associated neurologic anomalies or deafness in one or both ears.

Vitiligo is hereditary but not noticeable at birth. It is mostly seen in Arabian horses (Arabian fading syndrome, pinky syndrome). The onset is usually in young adulthood. Affected animals develop bleached splotches of skin that occasionally also affect the hair coat and hooves. Most splotches are on the face, especially the bridge of the muzzle or around the eyes. Color loss may wax and wane. Complete remission may occur but is rare. Vitiligo causes no other health problems. No treatment is available. Treatments used in people with vitiligo are unlikely to help animals.

Defects of Structural Integrity

Some genetic defects affect the integrity of the skin. Hereditary equine regional dermal asthenia (HERD) is a condition in which the skin produces abnormal collagen and/or elastin. This causes patches or large areas of loose, stretchy, fragile skin and joint problems. Wounds heal slowly or not completely. This condition has been reported in Quarter horses and Arabian cross horses. The condition is often first noticed during training, at the time saddles and tack are first placed on a young horse. Diagnosis is based on visible signs and testing of the collagen structure.

Epidermolysis bullosa syndromes are a group of hereditary congenital defects that affect the attachments between the outer and inner layers of the skin. There are 3 types of epidermolysis bullosa syndromes. In all 3 forms, minor skin trauma results in dermal-epidermal separation and blisters that soon rupture, leaving glistening, flat ulcers. Blisters may be present at birth or develop within the first weeks of life. The most severe blisters are on the lower legs (with sloughing of hooves), mouth, face, and genitals. Except for the simplex form, most occurrences of the disease are fatal. All 3 forms of epidermolysis bullosa have been reported in Belgian foals.