Hereditary equine regional dermal asthenia

HERDA makes the skin less resistant to friction

Skin needs to move while resisting everyday friction and minor strain. Collagen fibres form part of the framework that gives tissues strength. In HERDA, the problem is not simply that the horse has too little skin: the production and organisation of this supporting framework are disturbed, leaving the tissue less resistant to normal forces.

The identified variant is in PPIB, which encodes a protein called cyclophilin B. This protein helps collagen fold and be produced correctly. Studies of affected horse cells show that proteins do not work together normally. Collagen takes longer to form its proper shape and to be released by the cells that make it. This gives a biological explanation for the connection between an inherited DNA change and the fragile skin seen in the horse.

References [4] · [1]

HERDA skin wounds are often noticed when a horse is first saddled

Skin may be unusually stretchy, detach or tear easily. Wounds, fluid- or blood-filled swellings and scarring occur particularly along the back and sometimes the neck. Saddle contact can reveal the problem. The saddle does not create the variant: friction exposes a weakness that was already present in the tissue.

Signs are rarely obvious at birth and are often recognised in the first few years as handling or ridden training increases. Severity differs between individuals even with the same variant. Normal-looking skin in a young horse therefore does not rule out HERDA. Testing can establish genetic status without waiting for the horse to develop an injury.

References [3] · [2] · [1]

HERDA has been documented in Quarter Horses and horses with Quarter Horse ancestry

HERDA has been documented in Quarter Horses and horses with Quarter Horse ancestry, including Paint Horses and Appaloosas. Particular lines used in cutting, an equestrian cattle-working sport, have received substantial study. This does not mean every horse in those breeds, pedigrees or disciplines has the disease or even carries the variant.

Relationships help explain how a variant spreads, but do not establish an individual result. Two horses from the same line can have different genotypes. Published frequencies depend on the country, lines and animals sampled. They cannot replace a test correctly linked to the horse’s identity, and should not be applied unchanged to every population carrying a breed name.

References [2] · [3] · [1]

A skin wound alone does not establish HERDA

A veterinarian considers lesion distribution, progression, injury history and relevant ancestry. Other skin conditions and ordinary trauma can resemble parts of the picture. Clinical findings and DNA testing need to be interpreted together; a photograph, a scar or an owner’s impression of skin stretchiness cannot reliably establish the cause.

A skin biopsy involves taking a small piece of skin for examination. It may show how the tissue is organised, although the changes can be difficult to detect. The targeted DNA test answers whether the known PPIB variant is present. A painful wound also needs assessment in its own right. Identifying an inherited cause does not remove the need to assess complications affecting the individual horse.

References [3] · [1]

HERDA DNA testing distinguishes unaffected carriers from horses with HERDA

HERDA follows autosomal recessive inheritance. A horse with one copy of the HERDA variant does not have HERDA, but can pass that copy to a foal. With two copies, the horse develops the disease. Males and females can both be affected, and an affected foal can have two parents whose skin appears entirely normal.

The laboratory tests for a particular variant, not whether the PPIB gene is missing. A negative result means that variant was not detected in the sample. It does not exclude every other cause of fragile or damaged skin. Correct horse identification, sample identification and the name of the test all matter when interpreting or sharing a report.

References [1] · [2]

HERDA care aims to limit skin wounds and pain

There is no curative treatment that permanently restores normal tissue strength. Care aims to limit trauma and address wounds, pain and complications. Tack and friction can cause further lesions, making affected horses unsuitable for riding. The aim is to assess what the horse can tolerate comfortably, rather than judging wellbeing only by whether a wound eventually closes.

HERDA can severely limit a horse’s life, and its outlook depends on how serious the lesions and their consequences are. Some horses can live at pasture with specialised care; others develop repeated lesions and suffering that cannot be reconciled with acceptable welfare. These decisions need veterinary assessment. The outlook for a two-copy affected horse must not be confused with that of a one-copy carrier unaffected by HERDA.

References [3] · [2] · [4]

Choosing parents to avoid producing a foal with HERDA

If the mare and stallion each have one copy of the HERDA variant, every pregnancy has a 25% chance of a foal inheriting two copies, one from each parent, and being affected by HERDA. The remaining possibilities are 50% one-copy carriers and 25% without the variant. These are probabilities for each pregnancy, not a promise that four foals will include exactly one affected animal.

If one parent carries one copy of the HERDA variant and the other has none, their foal cannot inherit two copies of this variant and will not have this form of HERDA. Each pregnancy still has a 50% chance of a foal inheriting one copy and becoming an unaffected carrier. This is why identifying carriers is more informative than relying on normal skin. Decisions should consider both parents’ results, future foal welfare and the genetic diversity of the population.

References [1] · [3]

Questions about the condition: Hereditary equine regional dermal asthenia

Scientific sources about the condition: Hereditary equine regional dermal asthenia

This article brings together the research and the university and laboratory resources listed below. Numbers after each section link to the references used. Sources consulted in November 2026.

1. 1.
   Hereditary Equine Regional Dermal Asthenia (HERDA)

   UC Davis Veterinary Genetics Laboratory · Test documentation

2. 2.
   Hereditary equine regional dermal asthenia

   University of Minnesota Extension · University resource

3. 3.
   HERDA: signs, diagnosis and prognosis

   UC Davis Center for Equine Health · University resource

4. 4.
   Ishikawa et al. (2012) — Cyclophilin B and collagen folding in HERDA

   Journal of Biological Chemistry · Research