Hoof wall separation disease (HWSD) is a genetic defect characterized by a hoof wall that easily breaks and cracks, and a normal appearing coronary band. The breaks and cracks begin to occur in young ponies. In severe cases the pony bears weight entirely on the sole of the foot which can lead to severe lameness.
HWSD is caused by a frameshift mutation in the gene SERPINB11, (c.504_505insC), and is inherited as an autosomal recessive trait. This means that carriers are completely normal and only animals with two copies of the mutation will show clinical signs of the disease. HWSD appears to occur only in the Connemara Pony and the carrier frequency has been estimated to be about 14.8%. A DNA test for this specific mutation can determine if ponies are normal or if they carry one or two copies of the mutation. Ponies that carry two copies of the mutation are highly likely to be affected with the disease. Some cases are milder while others are more severe. We have identified at least one case where the pony does not appear to have clinical signs associated with having two copies of the mutation. This indicates that the mutation is not fully penetrant; however it has very high penetrance of 96.8%.
Ponies that have only one mutant copy of HWSD (N/HWSD) are normal, but they are carriers of the disease. When two carriers are bred to each other, the resulting foals can be affected. At the time that this test was released, approximately 15% of Connemara were carriers of HWSD (N/HWSD); however, the number of carriers can change with each generation. Ponies that are carriers of HWSD are completely normal, and they can be safely bred to non-carrier, normal ponies (N/N) in order to maintain diversity within the breed and to select for other positive attributes in carriers.
Testing for hoof wall separation disease is important in assisting clinicians, owners, and breeders in identifying affected and carrier horses. Breeders can use results from the test as a tool for selection of mating pairs to avoid producing affected horses.