Hereditary Equine Regional Dermal Asthenia
Learn what the condition is, how it may be detected early, how it is treated or managed, and which breeds or species are linked to it.
Condition snapshot
A concise guide to the condition’s pattern, detection, management and urgency.
Breeding advice appliesUse the screening guidance and veterinary or genetics advice before breeding an affected or at-risk animal.
This snapshot is a general guide, not a diagnosis or treatment plan. New, severe or worsening signs require veterinary assessment.
About the condition
Hereditary equine regional dermal asthenia, or HERDA, is a recessive connective-tissue disorder seen mainly in Quarter Horses and related stock-horse lines. A PPIB variant disrupts collagen folding and organisation, leaving skin poorly attached to deeper tissue. Affected foals may look normal, with lesions often emerging between six months and two years when pressure, training or tack increases.
Skin along the back, loin, neck and withers becomes unusually stretchable, loose and easily torn. Seromas, open wounds, scars and permanent hairless areas can develop after minor trauma; healing is slow and the resulting skin remains weak. Some horses show lesions elsewhere, but the dorsal saddle region is characteristic. The disorder is painful when wounds are active and usually prevents safe riding.
HERDA is not contagious and is different from rain scald, dermatophilosis, fungal disease, photosensitisation and acquired trauma. Ehlers-Danlos-like disorders and fragile foal syndrome overlap in collagen fragility but have different age, breed and genetic findings. A biopsy can support abnormal collagen architecture, yet sampling itself creates a wound and DNA testing is more specific for the known variant.
Inheritance is autosomal recessive. Carriers are clinically normal; two carriers have a 25% affected risk per mating. There is no way to strengthen defective collagen permanently. Mild horses may live as unridden companions with strict protection, but recurrent large wounds, infection, chronic pain and inability to use normal tack lead many affected horses to euthanasia. Genetic pairing, not elimination of all carriers, prevents affected foals while conserving bloodlines.
Microscopically defective collagen fails to hold the dermis securely to underlying tissue, so a seroma or flap can form after pressure that would not injure a normal horse. Lesions cluster where saddle and rider forces act. Delayed onset means an affected youngster may be sold or started before the first unmistakable wound appears.
Screening & Early Detection
Examination maps every wound, scar, seroma and loose area and relates lesions to tack, pressure and minor trauma. The veterinarian gently assesses skin extensibility without deliberately causing a tear. History includes age at first lesion, training onset, pedigree and wound-healing pattern. Cytology and culture investigate secondary infection rather than diagnose HERDA.
A validated DNA test for the PPIB variant confirms clear, carrier or affected genotype in appropriate breeds. Hair roots or blood are linked to permanent identification, and parentage is reviewed if findings conflict. Histopathology from a carefully selected biopsy may demonstrate disorganised collagen when genotype or phenotype is uncertain, but a normal biopsy does not always exclude disease.
Differentials such as infection, parasites, photosensitisation, nutritional deficiency, abuse or another inherited fragility syndrome are assessed from distribution and testing. Breeding stock in at-risk lines is screened before mating because carriers cannot be recognised by touch or performance. Relatives are tested individually rather than assigned status from pedigree probabilities.
Ultrasound may document fluid pockets beneath intact loose skin before an open tear develops.
Treatment & Management
There is no cure for the collagen defect. Existing wounds are cleaned gently, protected with non-adherent dressings and treated for documented secondary infection. Sutures may pull through fragile skin, so closure and bandaging require veterinary judgement and minimal tension. Flies, sun and rubbing are controlled, and analgesia is provided when lesions are painful.
Affected horses are not ridden and do not wear saddles, tight rugs or equipment that concentrates pressure over the back. Housing removes protrusions and rough surfaces, companions are selected to reduce biting, and handling avoids skin traction. Even well-managed wounds can recur because apparently healed tissue remains mechanically weak. Corticosteroids, collagen supplements and conditioning exercises do not correct PPIB-related structure.
Quality of life is reviewed honestly according to wound frequency, infection, pain and the restrictions needed to prevent injury. Humane euthanasia is appropriate when large recurrent wounds or chronic discomfort cannot be controlled. Affected animals are not bred. Carriers remain healthy but are paired only with genetically clear horses; all potential offspring intended for breeding are tested. Results are disclosed at sale, and breed-wide screening is preferred to removing carrier families blindly, which would reduce diversity without being necessary to stop affected births.
