Junctional epidermolysis bullosa (JEB) is an inherited disease that causes moderate to severe blistering of the skin and mouth epithelia, and sloughing of hooves in newborn foals. This condition is also known as red foot disease. Affected foals are typically born alive, but soon develop skin lesions at pressure points. The condition worsens with time and the foal eventually succumbs from severe infection or has to be euthanized.
JEB in Belgian Draft horses has been shown to be the result of a specific mutation in a gene that affects the production of normal and healthy skin (F. Spirito et. al., J Invest Dermatol 119:684-691, 2002). To date, this mutation has been found only in Belgian Draft horses and derivatives of that breed.
JEB is inherited as a recessive trait. Animals that carry two copies of the mutated gene (homozygous recessive) will develop the disease. Animals that carry one copy of the mutated gene and one copy of the normal gene (heterozygous) are carriers of JEB. Carriers do not develop the disease and have normal epithelia, but they have a 50% chance of passing on the mutation to their offspring. If we use N to represent the normal gene and J the mutated gene, an affected animal is designated J/J, a carrier animal is N/J and a normal animal is N/N.
Since affected animals do not survive to breeding age, JEB foals are produced by matings between carrier animals (N/J x N/J). This means that 25% of the offspring produced by these matings will be affected (J/J), 50% will be carriers (N/J) and 25% will be normal (N/N). Matings between carrier (N/J) and homozygous normal (N/N) animals produce 50% normal (N/N) and 50% carrier (N/J) offspring. The expected outcomes for each of these mating combinations are shown below.
|N/J x N/J matings have a 25% chance of producing NN, 50% for NJ and 25% risk of producing affected JJ foals.||N/J x N/N matings have a 50% chance of producing either NN or NJ. All offspring will be normal but 50% will be carriers.|
|N||25% NN||25% NJ||N||50% NN||50% NJ|
|J||25% NJ||25% JJ|
VGL offers a DNA test that detects the mutation that has been associated with JEB in Belgian Draft horses and in other breeds derived from Belgian Draft stock. It is possible that matings of NN horses can produce affected foals if the sire and dam are carriers of a different mutation that also leads to this disease. At the present time, no other mutation has been reported in the scientific literature.
Breeders can reliably use test results to enhance breeding strategies to avoid producing affected foals. Carriers do not need to be removed from the breeding pool. A successful breeding program can use matings of carriers (N/J) to non-carriers (N/N) without the worry of producing an affected foal.