A Comparative Neuro-Histological Assessment of Gluteal Skin Thickness and Cutaneous Nociceptor Distribution in Horses and Humans

SIMPLE SUMMARY: This study was performed to increase the understanding of the capacity of horse skin to detect pain when directly compared to human skin. The study focused on gluteal skin where horses are most often struck with whips during racing. The study was designed to inform the debate surrounding the use of whip strikes in horse racing where there is increasing pressure on the global racing industry to justify whip use. At the core of the debate is the question—do horses experience pain when being whipped? The study used microscopic studies of skin from 10 deceased humans and 20 euthanased horses to explore any differences between the species in their skin structure and nerve supply. The results revealed no significant difference between humans and horses in either the concentration of nerve endings in the outer pain-detecting layer of skin (epidermis) or in the thickness of this layer. In horses, this layer was deeper on the right than on the left. The collagen layer (dermis) of skin which is not involved in pain detection was significantly thinner in humans than in horses. These findings show that, although horse skin is thicker overall than human skin, the part of the skin that is thicker does not insulate them from pain that is generated during a whip strike, and that humans and horses have the equivalent basic anatomic structures to detect pain in the skin. ABSTRACT: The current project aims to build on knowledge of the nociceptive capability of equine skin to detect superficial acute pain, particularly in comparison to human skin. Post-mortem samples of gluteal skin were taken from men (n = 5) and women (n = 5), thoroughbreds and thoroughbred types (mares, n = 11; geldings, n = 9). Only sections that contained epidermis and dermis through to the hypodermis were analysed. Epidermal depth, dermal depth and epidermal nerve counts were conducted by a veterinary pathologist. The results revealed no significant difference between the epidermal nerve counts of humans and horses (t = 0.051, p = 0.960). There were no significant differences between epidermal thickness of humans (26.8 µm) and horses (31.6 µm) for reference (left side) samples (t = 0.117, p = 0.908). The human dermis was significantly thinner than the horse dermis (t = −2.946, p = 0.007). Epidermal samples were thicker on the right than on the left, but only significantly so for horses (t = 2.291, p = 0.023), not for humans (t = 0.694, p = 0.489). The thicker collagenous dermis of horse skin may afford some resilience versus external mechanical trauma, though as this is below the pain-detecting nerve endings, it is not considered protective from external cutaneous pain. The superficial pain-sensitive epidermal layer of horse skin is as richly innervated and is of equivalent thickness as human skin, demonstrating that humans and horses have the equivalent basic anatomic structures to detect cutaneous pain. This finding challenges assumptions about the physical capacity of horses to feel pain particularly in comparison to humans, and presents physical evidence to inform the discussion and debate regarding the ethics of whipping horses.