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What is Hereditary Equine Regional Dermal Asthenia (HERDA)?

Hereditary equine regional dermal asthenia (HERDA) is a debilitating genetic condition that affects mainly Quarter Horses. It is characterized by defective collagen, particularly the collagen that holds the outer layer of the skin to the layer directly beneath it. In order for the gene to be activated, it has to be acquired from both parents. Horses with this disorder cannot be ridden as it causes large sores and disfiguring scars along the back and in addition, sunlight should be avoided as it encourages skin damage.

Hereditary equine regional dermal asthenia, known more commonly as HERDA, is a recessive genetic condition in Quarter Horses that can cause severe damage to the skin.

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Symptoms of Hereditary Equine Regional Dermal Asthenia (HERDA) in Horses

Foals that are affected by this disorder appear normal at birth and often for quite some time afterward, but typically begin to show signs somewhere between one and three years of age, often around the time they are being trained to saddle.

  • Corneal ulcers
  • Doughy or mushy feeling skin
  • Hyperelastic skin
  • Scarring on back and withers 
  • Skin lesions
  • Thinned corneas

Types 

Quarter Horses are prone to several genetic disorders. Some other recessive disorders that afflict quarter Horses include: 

  • Glycogen branching enzyme deficiency (GBED) - This is a lethal disease characterized by the inability of the foal to properly store sugar; foals with this disorder are often stillborn, and those who survive the birth process generally succumb to the disease within their first eight weeks
  • Overo lethal white syndrome: (OLWS) - This disorder attacks quarterhorse foals that are born to a parent with a paint horse color pattern; foals with this disorder are white or mostly white at birth and usually die within just a few hours

Causes of Hereditary Equine Regional Dermal Asthenia (HERDA) in Horses

This is a recessive genetic trait that has been traced back to the AQHA sire Poco Bueno for around 95% of the animals that develop this disease. In order for a horse to develop the disorder, they would require the genetic trait to have been passed down from both parents. Although the majority of animals afflicted with HERDA are Quarter Horses, breeds that allow the inclusion of Quarter Horses in their breeding programs, such as the American Paint Horse or the Appaloosa breeds, may also acquire this disorder.

Diagnosis of Hereditary Equine Regional Dermal Asthenia (HERDA) in Horses

There are tests available to determine if a horse has the genetic defect responsible for this disorder. Results of this testing can reveal one of the following classifications:

HRD/HRD (homozygous)

This horse is affected by the disease and carries a copy from both the sire and the dam. All of this animal’s offspring will receive a copy of this disorder and will either have the disease or be a possible carrier of the disease.

n/HRD (heterozygous)

Only one copy of the gene is present giving this horse a 50% chance to pass it on. Horses with this designation may produce foals with this disorder, foals who are carriers, or even foals who are completely unaffected by the disorder

n/n

This is the result if the horse does not have HERDA. Symptoms of this disorder with a result of n/n would require further testing to determine the accurate underlying condition. 

Breedings between normal horses (n/n) and carrier horses (n/HRD) will not result in foals with this disorder but will have offspring with a 50% chance of carrying the disease.

Treatment of Hereditary Equine Regional Dermal Asthenia (HERDA) in Horses

Horses that develop HERDA are unable to be ridden, and open sores can leave them more prone to infections and in some cases, in a great deal of pain. For these reasons, many horse owners and their veterinarians opt to euthanize the patient. Those owners that choose to treat the horse do so knowing that the horse cannot ever be ridden. Direct sunlight should be avoided for these horses as it can worsen the symptoms.

A covering for the horse may be recommended if there are biting flies in the area and contact with other horses should be made cautiously as any skin damage can become acute quickly.  If open sores are present, they will need to be cleaned and evaluated by a veterinarian. In most cases, anti-inflammatories will be recommended for the horse to reduce pain and swelling and if any infections are present, then antibiotics will also be prescribed.

Recovery of Hereditary Equine Regional Dermal Asthenia (HERDA) in Horses

Starting in 2015 the AQHA began requiring a five-panel genetic test to be completed for all breeding stallions in an effort to stop the spread of several genetic disorders through selective breeding. This test covers five genetic tests that are related to the American Quarter Horse breed, looking for genetic markers of three dominant genetic disorders, Hyperkalemic periodic paralysis (HYPP), malignant hyperthermia (MH), and polysaccharide storage myopathy (PSSM), and two recessive genetic diseases hereditary equine regional dermal asthenia (HERDA) and glycogen branching enzyme deficiency (GBED). The testing can be done using hair samples from the mane and tail, and results are updated on the stallions registry papers.