HERDA is a skin disease found primarily in Quarter Horses and their derivatives
Hereditary equine regional dermal asthenia
Breeds known to be affected
Quarter Horse and related breeds
Hereditary equine regional dermal asthenia (HERDA) is characterised by hyper-extensible (fragile) skin which progresses to severe skin lesions, often on the horses back. The disorder affects the collagen that holds the skin in place, making it much easier to tear off than normal. Any rubbing, such as that caused by saddling, will cause the skin to split so affected horses are unable to be ridden. The lesions are painful and prone to infection. HERDA is associated with horses from the Poco Bueno sire line.
HERDA is caused by a mutation in the PPIB gene and is recessive, so the horse must be homozygous (HERDA/HERDA) to be affected. If a horse is a carrier (n/HERDA), it will not show any clinical signs of HERDA. However, there is a 50% chance it will pass the variant to its offspring, so mating to other carriers should be avoided to prevent the birth of an affected foal.
Interpretation of results
n/n: Horse does not carry the mutation associated with HERDA.
n/HERDA: Horse has one copy of the mutation associated with HERDA.
HERDA/HERDA: Horse has two copies of the mutation associated with HERDA.
Reference Tryon et al. Homozygosity mapping approach identifies a missense mutation in equine cyclophilin B (PPIB) associated with HERDA in the American Quarter Horse. Genomics 2007;90:93-102 doi:10.1016/j.ygeno.2007.03.009