Junctional Epidermolysis Bullosa (JEB) in Belgian Draft Horses: AAEP 2003

Junctional epidermolysis bullosa (JEB) is an inherited disease that causes skin lesions over pressure points of the body in newborn Belgian foals and results in large areas of skin loss. The disease is a lethal condition and affected foals typically are euthanized shortly after birth.

JEB foal
Courtesy Dr. John D. Baird

JEB causes areas of skin loss and is fatal.

Since the gene mutation that causes JEB in draft horses was discovered in 2001, much progress has been made in identifying the carrier status of Belgian breeding animals. John D. Baird, BVSc, PhD, reviewed the cause, characteristics, and prevalence of JEB at the 2003 American Association of Equine Practitioners' convention.

Lesions are found over the hocks, stifles, and fetlocks of JEB-affected foals, and the sores rapidly become more severe. Lesions also can occur around the anus and external genitalia. Ulcers occur around the coronary band and might eventually progress to the loss of the hoof.

On histopathological examination, a split at the junction of the epidermis and dermis of the skin can be observed. Additionally, one will see extensive erosions in the oral cavity of JEB foals, that can result in blood-tinged saliva. "There can be lesions on the soft palate, and you can get very extensive ulcerations involving the tongue," said Baird. Affected foals' teeth are characterized by enamel erosions, and they might appear irregular and serrated. "These foals are born with incisor teeth visible and (normal) foals usually don't have teeth visible until approximately eight days of age," he added.

The first reports of JEB in neonatal Belgian foals were published in North America in 1988 and 1989. The condition also has been reported in American Saddlebreds since 1975.

In 2001, scientists linked the condition in Belgian draft horse to a mutation of a particular gene (LAMC2 gene) that produces part of the laminin-5 protein. This protein is important in the attachment of the epidermis to the dermis as well as mucous membranes throughout the body. "An autosomal recessive mode of inheritance of this mutation has been confirmed," said Baird, meaning that JEB is carried by both sexes, and an affected foal is homozygous (carries two copies of the defective gene, one from each parent). Horses with only one copy of the JEB gene (heterozygous) are carriers of the disease.

In May 2002, scientists developed a commercial polymerase chain reaction (PCR) test that can be performed on the DNA in the hair roots of mane and tail hair. This test was adopted by Belgian draft horse breed associations very quickly, according to Baird. Since  Oct. 22, 2002, mandatory testing of every Belgian stallion registered in the United States and Canada has been conducted to determine if they carry the JEB mutation.

From October 2002 to May 2003, U.S. and Canadian Belgian registries forwarded mane hair samples from both male and female Belgians to a veterinary genetics laboratory, and it was found that 17.1% of those tested were carriers. Of the 252 samples from male Belgians, 34 (13.5%) were identified as carriers, and of the 76 samples from females, 22 (28.9%) were carriers. The carrier status of the horse is recorded on the horse's registration papers. With this information breeders can avoid producing JEB foals by avoiding the mating of carrier animals. "Breeders are now advertising if their horse is a JEB non-carrier in their stud farm ads," Baird said.

Before the development of the commercial PCR test, data was collected from 12 farms in the United States and Canada to see what the incidence of carriers was on breeding farms. Every Belgian horse on these farms was sampled. Eight of the 12 farms had at least one JEB foal born on the farm in the previous five years. The carrier rate on the eight farms that had experienced birth of a JEB foal was 37%, as compared to 22.8% on the four farms where no JEB foal had been born. Overall the carrier rate was 32.3% carrier rate.

Nine of these 12 farms had breeding stallions. On four farms that had a stallion identified as a carrier, there was a 40.8% carrier rate, which compared to a 20% carrier rate on the five farms where the resident stallion was not a carrier.

Carriers were also found in the Breton, Comtois, Vlaams Paard, and Belgische Koudbloed Flander draft horse breeds in Europe, and carrier rates varied within France, Belgium, and The Netherlands.

After examining information from cases of JEB in Saddlebreds in the United States and South Africa, researchers were able to say that the mutation responsible for JEB in American Saddlebreds is different from that in Belgian horses.

Baird gave credit to the Belgian breed associations in the United States and Canada that are working diligently to reduce incidence of JEB in their breed. "It is now possible for horse breeders to avoid the financial and genetic losses associated with the birth of JEB foals in the Belgian, Breton, and Comtois draft horse breeds," Baird's study reported. "The potential exists to eliminate this disease from horse breeds."

About the Author

Stephanie L. Church, Editor-in-Chief

Stephanie L. Church, Editor-in-Chief, received a B.A. in Journalism and Equestrian Studies from Averett College in Danville, Virginia. A Pony Club and 4-H graduate, her background is in eventing, and she is schooling her recently retired Thoroughbred racehorse, Happy, toward a career in that discipline. She also enjoys traveling, photography, cycling, and cooking in her free time.

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