Is HERDA Always a Death Sentence? Horse Discovered Homozygous for HERDA With Minimal Signs

Read the full scientific publication here: https://doi.org/10.1016/j.jevs.2021.103756

Video provided by Dr. Tyler Troop, veterinarian and educator at Performance Equine Associates in Oklahoma.

Hereditary Equine Regional Dermal Asthenia, or “HERDA”, is a heritable skin disease that primarily affects the American Quarter Horse breed. Signs or symptoms of HERDA include stretchy, elastic skin that tears easily, slow or non-healing skin wounds or lacerations, hematomas and ulcerations. These signs normally present as early as 1.5-2 years of age, which often corresponds to the beginning of under saddle training for many young horses. Signs also typically worsen over time. Unfortunately there is no known treatment or cure for HERDA, and the majority of horses affected are euthanized within 2-4 years as their condition worsens and the wounds become unmanageable or fatal. While we don’t understand all of the complexities of this disease, what we do know is that HERDA is caused by a mutation on the PPIB gene, responsible for collagen formation, affecting the way collagen folds.

Given the above summary, can you imagine a horse with HERDA living 8 years before exhibiting mild if any signs at all? Not only that, but an active working ranch and roping horse, both before and after his diagnosis.

Etalon researchers, alongside Dr. Tyler Troop, veterinarian and educator at Performance Equine Associates in Oklahoma, have identified a very rare case: A HERDA positive 8-year-old American Quarter Horse gelding presenting with only mild signs. This horse is being ridden daily as a working ranch horse.

Having a normal upbringing, he was started under saddle at 2 years of age (like typical ranch horses) and didn’t present anything out of the ordinary. His current owners have known and loved him since he was just a yearling, and finally purchased him as a show horse (reined cow horse) at the age of 4. He was successful as a show horse and ridden regularly. His owners only observed that his skin seems “more elastic than normal”, and he is mildly more sensitive to bug bites than most horses, but otherwise is healthy and normal. He competed for four years in Roping, Ranch Horse and Reined Cow Horse competitions.

The subject horse being ridden and working as a roping/working cow horse (Photo supplied by the owner).

In November 2020, he was participating in a riding clinic with his current owner and presented with these mild skin lesions (see video. The lesions were smaller and less severe than typical in HERDA cases, yet the skin appearance and elasticity were noticeably similar. This led the attending veterinarian, Dr. Tyler Troop, to recommend genetic testing. Genetic testing for the individual revealed that he is homozygous for the HERDA disease (h/h). To our knowledge, this is the first reported case of an HERDA affected adult horse over the age of five being ridden and working.

This horse’s rare presentation (or lack thereof) of HERDA signals brings to light how little we know of the complexities of some genetic diseases such as HERDA. Not only were the signs so mild that he had been working his whole life, with no suspicion that he could be HERDA positive, but when affected, he recovered rather quickly! A working horse with HERDA, based on our prior understanding of the disease, is extremely odd and quite astounding.

This unusual case opens up for a serious question - how many “mildly affected”, undiagnosed HERDA horses are out there, and what might be affecting their genetics that allows them to overcome the “deadly” aspect of the disease?

This case brings to light the importance of genetic testing for all horses, not just breeding stallions! Many genetic diseases may go undetected if they do not present outward signs, and must be identified through genetic testing. While testing is especially critical for breeding horses, it is also important to test our non-breeding horses in an effort to better understand their genetics. This knowledge enables researchers and horse owners to continue making progress towards better understanding these diseases and their complexities, and create a safer world for our equine partners.