By: Morgan Hulbert
Epidermolysis Bullosa, EB for short, affects both humans and animals. It is categorized into 3 main groups- EB simplex, dystrophic EB and junctional EB (JEB) based on the level of skin separation that it causes (Mömke et al., 1). Within the category of Junctional Epidermolysis Bullosa (JEB) there are two sub-categories: non-Herlitz type and Herlitz type, the latter being the type of EB we are concerned about within the Belgian draft horse breed. Non-Herlitz type rarely causes death, whereas Herlitz type is always deadly (Mömke et al., 1).
Once known as Epitheliogenesis Imperfecto, Herlitz type JEB is characterized by a decrease in the ability of certain skin layers to stay together due to a lack of skin protein (JEB/ DNA Info, Belgian Draft Horse Corporation of America). The skin becomes very fragile, ulcers form, and even the slightest pressure to the skin can cause injury. Shortly after birth, patches of hair loss are observed, especially around the hip joints, shoulder blades and places where pressure has been applied to the foal. Because the hoof wall is attached to the horse by the same structures that keep the skin held in place, the horse’s hooves also begin to separate from the underlying structures, and the hoof itself eventually falls off. The condition becomes so severe that the animal dies or must be euthanized (JEB/ DNA Info, Belgian Draft Horse Corporation of America).
The disease is purely genetic; it is not caused by a pathogen such as a bacteria or a virus, nor is it contagious. JEB is specifically an autosomal recessive genetic condition, meaning the disease is passed from parent to offspring, potentially, if the parent has at least one recessive allele. If there is just one recessive allele copy, then the horse will not have symptoms of JEB, it will only be a carrier. The horse will have the potential, during reproduction, to pass that one recessive allele onto their offspring. Depending on the resulting offspring’s alleles, they will either have JEB or carry the genes for it. If the resulting offspring has two copies of the recessive allele, identified as “bb” for example, then the foal will have the deadly conditions of JEB. If the foal has a dominant allele and a recessive allele, identified as “Bb” for example, then the dominant allele (B) will ‘override’ the recessive. In this condition the foal will have no symptoms, but will be a “carrier”. In this scenario, there is the potential, if this foal became breeding stock, for him or her to pass that recessive “b” allele onto its offspring. In the final scenario, if both parents have dominant alleles, identified as “BB” in this example, they will not have JEB nor will they be able to pass it to their offspring. This is explained a bit more below.
The above boxes are Punnett squares- they predict what offspring could be born. The letters on the outside of the box show the alleles the parents have; but when breeding occurs, the parents will only give one allele to the offspring. The pairs of letters inside the box indicate the possible pair of alleles the foal will have when the parents are bred. Referring to the left hand box above, there is a 25% chance that if two “JEB carrier” parents (Bb x Bb) were bred that they would produce a JEB affected foal, 50% chance they would create offspring without the JEB condition, but they would be JEB carriers, and 25% chance they would create offspring that would not be affected nor would it be a carrier. The right hand above box is an example if we bred two “non-carrier JEB” horses. There is 100% non-carrier offspring; there is absolutely no possibility of having a JEB foal born, or even having a “JEB carrier” born.
In order to register a prospective breeding stud colt with the Belgian Draft Horse Corporation of America, it is mandatory to have the horse DNA tested to see what alleles they possess. The goal is to prevent the occurrence of the condition in the Belgian breed. If the breeder knows what alleles the stallion carries, then we can be selective in our breeding. Because the site in the horse’s DNA where the JEB causing mutation occurs is known, testing can be done to determine what alleles the horse carries. The Belgian Draft Horse Corporation of America has partnered with UC Davis Veterinary Genetics Lab to complete the testing. Upon registration, the owner fills out the appropriate forms, and sends in the adequate fees- $95.00 for JEB testing for stallions- and the corporation sends back paperwork and an envelope in which pulled mane or tail hairs are placed. The hair is sent to the lab to be tested, the results sent to the corporation and either “carrier of JEB” or “non-carrier of JEB” is indicated on the registration papers (JEB/DNA Info, Belgian Draft Horse Corporation). In this way, effective breeding can e practiced.
Herlitz-type JEB is a very deadly condition that affects the Belgian breed. However, with the identification of the JEB causing mutation, and the development of DNA testing, breeders and the Belgian Draft Horse Corporation are able to partner to identify those whom are carriers of the mutation. With monitoring and mindful breeding, JEB can be minimized and potentially eliminated from the Belgian pedigree.
About the author: Morgan graduated from Vet Tech school in 2017, then transferred to Binghamton University where she is currently a senior Biology student. She hopes to pursue research in human and animal infectious disease upon graduation. She grew up on a small hobby farm where her family raised Belgians, beef cows, sheep and an array of foul. firstname.lastname@example.org
“JEB/ DNA Info.” Belgian Draft Horse Corporation of America.
Citation: Mömke S, Kerkmann A, Wo¨ hlke A, Ostmeier M, Hewicker-Trautwein M, et al. (2011) A Frameshift Mutation within LAMC2 Is Responsible for Herlitz Type Junctional Epidermolysis Bullosa (HJEB) in Black Headed Mutton Sheep. PLoS ONE 6(5): e18943. doi:10.1371/journal.pone.0018943