Breeds Affected: Quarter Horse and related breeds
Samples Accepted: Blood, Hair
Disease Information: Hereditary equine regional dermal asthenia (HERDA) is a serious skin disease wherein a defect in the connective tissue results in skin splitting, tearing and separating from underlying layers. Areas under pressure (saddle, etc.) are particularly affected, and the skin trauma is difficult to heal, leading most horses with HERDA to be euthanized.
Inheritance Information: HERDA is autosomal recessive, meaning that animals with two copies of this allele will be affected. Animals with one copy of the gene will be clinically-normal carriers.
The possible genotypes are:
N/N The horse is normal, and cannot produce HERDA-affected offspring.
N/hrd The horse is a carrier of HERDA, and can pass the allele on to approximately 50% of any offspring. If bred to another N/hrd carrier, approximately 25% of the offspring will be normal, 50% will be carriers, and 25% will be affected.
hrd/hrd The horse is affected with HERDA. This disease is fatal, and affected horses do not usually live long enough to reproduce. If they do reproduce, if bred to a normal animal, 100% of the offspring will be carriers. If bred to a N/hrd carrier, 50% of the offspring will be carriers and 50% will be affected.
– Carriers may be bred to normal animals (N/hrd x N/N) without any risk of producing affected offspring. The offspring should also be tested before breeding to determine if they are carriers or normal.
– Breeding two carriers (N/hrd x N/hrd) is not recommended due to the possibility of 25% of the offspring being affected.
– Affected animals (hrd/hrd) should not be used for breeding.
Test Information: This test is based on a single base pair change in exon 1 of the peptidylprolyl isomerase B (PPIB) (aka cyclophilin B) gene.
Tryon RC, White SD, Bannasch DL. Homozygosity mapping approach identifies a missense mutation in equine cyclophilin B (PPIB) associated with HERDA in the American Quarter Horse. Genomics. 2007 Jul;90(1):93-102. DOI: 10.1016/j.ygeno.2007.03.009.
Further information is available at the Online Mendelian Inheritance in Animals website.