Hereditary Equine Regional Dermal Asthenia (HERDA), or Hyperelastosis Cutis (HC), is a genetic skin disease that primarily affects Quarter Horses.

It’s characterized by a deficiency in collagen production within the skin cells. Collagen is an important protein responsible for maintaining the strength and elasticity of the skin. [1][2]

Horses affected by HERDA typically exhibit extremely fragile skin, which is prone to tearing even with minor pressure or friction. As a result, they may develop chronic wounds, scars and other severe skin abnormalities.

Skin fragility can severely impact the horse’s quality of life, as recurring and non-healing skin wounds require ongoing management and are prone to bacterial infection.

Understanding the genetic inheritance and signs of HERDA are important for accurate diagnosis, effective management, and informed breeding practices to reduce the prevalence of this debilitating condition within horse populations.

HERDA in Horses

Collagen is a protein found abundantly in the body, serving as a key building block for various tissues, including skin, tendons, ligaments, bones, and cartilage. It provides strength, structure, and elasticity to these tissues, contributing to their resilience and flexibility.

Hereditary Equine Regional Dermal Asthenia (HERDA) stems from a genetic mutation that disrupts collagen formation in the horse’s body. [2][3][4]

HERDA is a form of Ehlers-Danlos syndrome (EDS), a group of genetic connective tissue disorders affecting collagen production. EDS affects various species, including humans, cattle, pigs, cats and dogs. [5][6][7]

While genetic skin disorders may be present in other animals, HERDA is specific to equines, notably Quarter Horses.

EDS arises due to mutations in genes related to collagen formation and processing, resulting in a range of clinical effects between affected horses. Researchers are studying HERDA as an animal model to gain insight into EDS in humans. [1][13]

Horses with HERDA typically seem healthy at birth and do not display signs of disease until they mature. Skin lesions typically emerge between 18 and 24 months of age, sometimes as late as 4 or 5 years old. [1]

Signs of HERDA typically emerge when the horse is started under saddle. During this period, lesions manifest along the horse’s back, particularly in areas where the saddle sits and exerts pressure. [2][4][9]

Causes

The genetic mutation associated with HERDA impacts the protein structure of collagen produced by skin cells. As a result, collagen folding and secretion are impaired.

This prevents the skin layers from sticking together, causing the top layer to separate easily from the deeper tissues. As the skin separates from the layer underneath, it becomes increasingly fragile. [10][11]

HERDA is inherited in an autosomal recessive manner, meaning that affected horses must inherit the mutated gene from both parents to display symptoms.

Heterozygous carriers of the mutated gene do not exhibit signs of HERDA themselves, but they can pass the gene to their offspring. [6]

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Risk Factors

HERDA primarily affects Quarter Horses and is a relatively rare genetic condition. Approximately 3.5% of all registered American Quarter Horse Association (AQHA) horses are heterozygous carriers, meaning they carry one copy of the gene associated with HERDA. [10]

HERDA is notably prevalent in bloodlines associated with cutting horses. It is thought to have originated from specific bloodlines within the Quarter Horse breed, tracing back to the influential stallion Poco Bueno, born in 1944, or his sire, King. [1][13][14]

Around 28% of Quarter horses in cutting and cow horse disciplines are HERDA carriers. Other breeds with Quarter horse ancestry, such as Paint Horses and Appaloosas can be affected. [10][14][15]

Symptoms

Horses with HERDA have exceptionally fragile skin that is prone to tearing even with minor pressure. On palpation, the skin feels loosely attached to the body and stretchy, making it possible to pull it several centimeters away from the body. [1][2]

Lesions on affected horses’ skin are typically focal, occurring in specific areas rather than evenly distributed across the body. They are most commonly found along the back, withers and sides of the body, but the face, legs and belly may also be affected. [1][4][9]

Affected horses may develop scars, ulcers, and tumor-like masses on their skin. These wounds tend to heal poorly, often leaving disfiguring scars as a result. Horses with HERDA may also be at increased risk of developing other conditions, including osteoarthritis. [1][4][12]

Horses with HERDA cannot be ridden due to the fragility of their skin, which leads to severe lesions and wounds when pressure is applied, such as from a saddle, cinch or girth. [9]

 

Eye Complications

Horses affected by HERDA not only experience issues with their skin due to abnormal collagen fibers, but may also experience vision problems.

The genetic mutation responsible for HERDA causes related problems with the horse’s corneas, the transparent dome-shaped outer layer of the eye. [1][8] The cornea focuses light onto the retina for clear vision.

Research has shown that horses with HERDA may have an increased occurrence of corneal ulcers (open sores on the cornea), thinner corneas, and alterations in corneal structure. [14]

Diagnosis

HERDA is typically diagnosed through a combination of clinical evaluation, genetic testing, and histopathological examination. Veterinarians assess the horse’s skin for characteristic signs such as fragile skin and poor wound healing.

Breeders are critical in the prevention of HERDA. By conducting DNA tests on potential breeding pairs to identify carriers of the mutation, carrier mating can be avoided altogether.

Breeding two heterozygous carriers results in a 50% chance of producing another carrier foal, and a 25% risk of producing a foal affected by HERDA. [1][11]

The AQHA 6-panel Genetic Test

The American Quarter Horse Association (AQHA) offers a 6-panel genetic test that screens for six hereditary diseases that affect Quarter horses, including; [15]

To conduct the 6-panel test, a sample of DNA is collected from the horse, typically through a hair or blood sample, and submitted to a laboratory. The results provide valuable information to owners and breeders, enabling informed breeding decisions.

To be registered with the AQHA, breeding stallions must have DNA and a health panel on record. Breeding mares are only required to have DNA on record to ensure their foals are eligible for registration.

Interpreting Genetic Test Results

Genetic testing is crucial for understanding how HERDA is inherited in horses. Results from the test categorize the horse’s genetic status as: [11]

  • N/N: The horse does not have HERDA or carry the gene, and cannot pass it to their offspring.
  • N/HRD: The horse is not affected by HERDA but carries the gene. They may pass it to half of their offspring.
  • HRD/HRD: The horse is affected by HERDA.

The testing procedure may vary depending on the laboratory, but typically involves collecting 20-30 rooted hairs from the mane or using whole blood samples for analysis.

Diagnostic Biopsy

Biopsy for HERDA involves taking a small sample of skin tissue from the affected horse. This sample is examined under a microscope to assess the structural integrity of the skin and identify abnormalities associated with HERDA, such as disruptions in collagen formation.

While skin biopsy is informative, it does not definitively diagnose HERDA. However, signs like thinning skin and collagen fibers in the sample suggest there may be disease pathogenesis occurring compared to healthy samples. [4][9]

Treatment

There is currently no specific treatment or cure for HERDA in horses. Management primarily focuses on minimizing skin trauma and preventing exacerbation of symptoms.

Some important management practices for horses with HERDA include: [1][2][14]

  • Minimizing sun exposure: Sunlight exposure can exacerbate symptoms of HERDA in horses. UV radiation can degrade collagen and sunlight may aggravate skin lesions, leading to discomfort. Providing adequate shade and using protective gear such as fly sheets and fly masks can help minimize sunlight exposure and reduce the impact of HERDA symptoms.
  • Reducing risk of skin wounds: Avoid activities or equipment that can cause friction or pressure on the skin, such as tight-fitting tack or harsh grooming techniques. Use soft, padded equipment and gentle grooming tools to reduce the risk of skin damage.
  • Feeding a balanced diet: Ensure proper nutrition, including adequate copper and vitamin D, as both play important roles in collagen metabolism. Supplementing with omega-3 fatty acids is also recommended to support healthy skin and coat. Consult with a veterinarian or equine nutritionist to develop a suitable diet plan.
  • Maintaining insect control: Controlling insects is vital for HERDA-affected horses as bites can induce or worsen skin lesions. Using repellents and fly sheets, and maintaining clean living spaces can lessen skin trauma.
  • Avoiding turnout with other horses: The fragile skin of horses with HERDA makes them more susceptible to skin injuries from interactions with other horses.
  • Monitoring affected horses: Regularly inspect the horse’s skin for any signs of lesions, sores, or wounds. Early detection allows for prompt treatment and prevents complications.

It’s important for horse owners to work closely with a veterinarian familiar with HERDA to develop a tailored management plan for affected horses. This may involve a combination of supportive care, medication, and environmental management to help maintain the horse’s quality of life and minimize discomfort.

Wound Care

Although the priority in managing horses with HERDA is avoiding skin trauma, equines with this condition are expected to have recurring skin injuries, which underscores the importance of diligent wound care. Non-healing skin trauma is often painful and is susceptible to secondary bacterial infection.

The main goal of wound management is to keep the wound clean and dry to prevent infection and promote recovery. Depending on the severity and number of wounds, caretakers may be able to manage wounds at home.

In some cases, it may be necessary for a veterinarian or veterinary technician to help with effective, sanitary bandage changes.

General treatments for wound management may include: [16]

  • Clipping and cleaning around the wound
  • Gentle scraping of the wound to remove dead tissue
  • Sutures, if needed to close large wounds
  • Bandaging

Antibiotics may be prescribed as a prophylactic to prevent secondary infection. Bandages usually need to be changed at least once per day to keep the area dry and clean. [16]

The HERDA Guarantee

A HERDA guarantee is a provision often included in breeding agreements by stallion owners. If a stallion carrying the HERDA genotype (N/HRD) sires a foal confirmed to have HERDA (HRD/HRD) before reaching one year of age, the stallion owner may offer a free rebreeding. This offer is usually contingent on not registering the foal.

Note that not all breeders provide a HERDA guarantee. Its availability varies and depends on the specific breeding arrangement between the parties involved.

Prognosis

The prognosis for horses with HERDA is generally poor. There is currently no cure for the condition, and affected horses often experience a diminished quality of life due to their fragile and easily damaged skin. [1]

Some horses with HERDA may be able to live comfortably at pasture, under specialized management, but many owners elect for humane euthanasia.

Fortunately, carriers of the HERDA mutation are able to live normally as carrier status does not lead to any known health issues. [11]

Summary

Hereditary Equine Regional Dermal Asthenia (HERDA) is a genetic skin disorder primarily affecting Quarter Horses.

  • HERDA is caused by a gene mutation that leads to a deficiency in collagen, an important protein for skin strength and elasticity
  • Affected horses have extremely fragile, hyperelastic skin that is prone to tearing along the back and sides of the body
  • Management focuses on minimizing skin trauma and controlling environmental factors such as sunlight and insects
  • There is no cure for HERDA and the prognosis is generally poor

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References

  1. Munroe, G. et al. Hereditary regional equine dermal asthenia (HERDA) in Horses (Equis).
  2. Rashmir-Raven, A. et al. Increased Susceptibility of Skin from HERDA (Hereditary Equine Regional Dermal Asthenia)-Affected Horses to Bacterial Collagenase Degradation: A Potential Contributing Factor to the Clinical Signs of HERDA. Veterinary Dermatology. 2015.View Summary
  3. Bowser, J.E. et al. Tensile Properties in Collagen-Rich Tissues of Quarter Horses with Hereditary Equine Regional Dermal Asthenia (HERDA). Equine Vet J. 2014.View Summary
  4. Tryon, R.C. et al. Homozygosity Mapping Approach Identifies a Missense Mutation in Equine Cyclophilin B (PPIB) Associated with HERDA in the American Quarter Horse. Genomics. 2007.View Summary
  5. Norton, J. Chapter 13 – Gene Testing. In: Equine Emergencies (Fourth Edition). W.B. Saunders. 2014.
  6. Hereditary Equine Regional Dermal Asthenia (HERDA) | Veterinary Genetics Laboratory.
  7. Walker, N.L. et al. Industry Perceptions of HERDA in Performance Horses. Journal of Equine Veterinary Science. 2020.
  8. Monthoux, C. et al. Skin Malformations in a Neonatal Foal Tested Homozygous Positive for Warmblood Fragile Foal Syndrome. BMC Vet Res. 2015.View Summary
  9. Rashmir-Raven, A. et al. Equine Hyperelastosis Cutis Update. Proc. Am. Ass. Equine Practnrs. 2004.
  10. 6 Panel Genetic Testing: What to Know – AQHA.
  11. Quarter Horse – Hereditary Equine Regional Dermal Asthenia. 2016.
  12. Patterson Rosa, L. et al. Hereditary Equine Regional Dermal Asthenia Homozygote Adult Working Horse with Mild Signs – A Case Report. Journal of Equine Veterinary Science. 2021.View Summary
  13. Roberts, JH, Halper, J. Connective Tissue Disorders in Domestic Animals. Adv Exp Med Biol. 2014.View Summary
  14. Mochal, C.A. et al. Ocular Findings in Quarter Horses with Hereditary Equine Regional Dermal Asthenia. J Am Vet Med Assoc. 2010.View Summary
  15. White, S.D. et al. Hereditary Equine Regional Dermal Asthenia (“hyperelastosis Cutis”) in 50 Horses: Clinical, Histological, Immunohistological and Ultrastructural Findings. Vet Dermatol. 2004.View Summary
  16. General Wound Management in Horses. American College of Veterinary Surgeons. 2024.