HERDA is a genodermatosis of the Quarter Horse and related breeds (Paint) characterised by extreme skin fragility. It is caused by a mutation in the PPIB gene (cyclophilin B) that alters the organisation of dermal collagen. The skin of homozygotes separates and tears easily with minimal trauma, especially on the back and areas in contact with the saddle. The lesions typically appear when training begins, around 2 years of age, and lead to a poor functional prognosis.

Affected breeds: Quarter Horse and breeds with Quarter Horse ancestry (Paint). Tryon et al. (2009) estimated an allele frequency of 0.021 in control Quarter Horses (approx. 4 % carriers) and of 0.142 in the cutting subgroup. The frequency is higher in working lines and lower in the breed as a whole. Limited data outside North America.

- Hyperextensible and fragile skin from training age (typically at 18-24 months)\n- Tears, seromas and haematomas on the back, withers and saddle pressure areas\n- Separation of the dermal layers with minimal rubbing\n- Extensive scars and poor healing\n- Pain on palpation of the affected areas\n- Recurrent secondary skin infections

HERDA was recognised in the early 2000s in working (cutting) lines of the American Quarter Horse, with clinical and histological descriptions of the condition by veterinary dermatology teams. In 2007, Tryon and colleagues identified the causal mutation in the PPIB gene, which encodes cyclophilin B, a protein involved in collagen folding. DNA testing was immediately widely adopted in stock breeds. Subsequent frequency studies revealed a notable carrier prevalence in cutting lines, driving control programmes in breeders' associations.

- Test breeding animals of stock breeds, as a priority if they descend from cutting/reining lines\n- Never mate carrier with carrier: 25% risk of HERDA foals\n- The appearance of a case confirms that both parents are carriers: do not repeat that mating\n- As the signs appear when training begins, early testing avoids investing in the preparation of an affected animal\n- Carriers may be mated to clear animals, prioritising clear offspring to eliminate the allele

Skin biopsy shows disorganisation and separation of the collagen bundles of the deep dermis, compatible with an Ehlers-Danlos-type syndrome. Differential diagnosis with neonatal epitheliogenesis imperfecta, photosensitisation and deep pyodermas. In affected animals, palliative management (protection against trauma, alternative saddles) rarely allows a sporting life; many are euthanased on welfare grounds. DNA testing is conclusive.

1. White SD et al. (2004). Hereditary equine regional dermal asthenia ("hyperelastosis cutis") in 50 horses: clinical, histological, immunohistological and ultrastructural findings. Vet Dermatol 15:207-217. PMID: 15305927
2. Tryon RC et al. (2005). Inheritance of hereditary equine regional dermal asthenia in Quarter Horses. Am J Vet Res 66:437-442. PMID: 15822588
3. Tryon RC et al. (2007). Homozygosity mapping approach identifies a missense mutation in equine cyclophilin B (PPIB) associated with HERDA in the American Quarter Horse. Genomics 90:93-102. PMID: 17498917
4. Tryon RC et al. (2009). Evaluation of allele frequencies of inherited disease genes in subgroups of American Quarter Horses. J Am Vet Med Assoc 234:120-125. PMID: 19119976
5. OMIA:000327-9796. Ehlers-Danlos syndrome, generic in Equus caballus (domestic horse; HERDA, PPIB-related).