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Epitheliogenesis Imperfecta (EI) & Hyperelastosis Cutis
Junctional Epidermolysis Bullosa (JEB),
Epitheliogenesis Imperfecta (EI),
and Hereditary Equine Regional Dermal Asthenia (HERDA or Hyperelastosis Cutis)
in Horses
Introduction
Junctional epidermolysis bullosa (JEB), epitheliogenesis imperfecta (EI), and hereditary equine regional dermal asthenia (HERDA or Hyperelastosis Cutis) are congenital disorders characterized by large skinless areas over the body of newborn foals. EI is also known as aplasia cutis congenita. Though they have similar presentations there are important differences in these diseases. In spite of these differences JEB is often mistakenly labelled EI. While EI is fairly rare and strikes randomly JEB is primarily a disease of Belgians and related breeds and is a inherited disorder. There also is a similar condition reported in American Saddlebreds.
There has been an increasing incidence of hyperelastosis cutis (HC) (also called hereditary equine regional dermal asthenia (HERDA)). The Poco Bueno line is now believed to be associated with this hereditary condition.
This article oulines the clinical signs, diagnosis, and identifying carriers of these various hereditary skin diseases and how to prevent further incidence of this disease.
EI
Not usually as severe a condition as JEB the newborn foal has areas where skin did not develop normally during gestation. This may be a developmental error and not do to a specific genetic defect, though uncertainty exists. The lesions can be small and single to large and multiple and can vary in depth. In cases where the lesions are not too extensive and not progressive, they may scar in or be repaired surgically. Many cases of JEB have been misidentified as EI.
JEB
Clinical Signs
JEB is seen in Belgians and more rarely the American Saddlebred. Affected foals may be born with lesions or begin showing signs 12 hours to 2 weeks after birth. Initially the lesions start as large blisters and vesicles that easily rupture leaving deep ulcers. These start with the coronets, legs, lips, tongue, and the skinless areas spread rapidly. There is remarkable exudation and crusting of serum. Often the foal is first seen following rupture of the blisters and presents with progressive wounds and sloughing of the skin. The foal becomes increasingly depressed until death occurs secondary to the loss of fluid or secondary infection.
Diagnosis
Clinical signs within the commonly effected breeds usually is enough to diagnose the condition. Skin biopsy shows a cleft formed between the epidermis and dermis with minimal inflammation is unrupture blisters. The only similar appearing disease are the autoimmune skin diseases and some drug reactions, which are very unlikely in newborn foals. PAS staining finds the basement membrane attached to the dermal floor and the specimen will be negative for direct immunofluorescence.
Hereditability
The condition in Belgian draft horses has been linked to a mutation of one gene, the LAMC2 gene that codes for a important connective tissue protein. This protein helps attach the layers of the skin together. Carriers are found in the other related draft breeds including Breton, Comtois, Vlaams Paard, and Belgische Koudbloed Flander, and carrier rates varied within France, Belgium, and The Netherlands. The defect in American Saddlebreds is thought to be a little differentmutation but has not been as well characterized yet.
The disease is inherited from the mare and stallion and is a single autosomal recessive gene. That means for the foal to have clinical signs of the diseae he must have received a defective gene from each parent. Both parents had one defective gene paired with a normal gene. Since it is simple recessive the parents do not show signs of the disease. There is now a commercial polymerase chain reaction (PCR) test for JEB in Belgians and related drafts that is performed on the hair roots of mane and tail hair. The Belgian Draft Horse associations now require testing of Belgian stallions registered in the United States and Canada and if positive the status of the horse is recorded on the horse's registration papers.
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