Only going Skin Deep. Do horses really have a thicker skin, and how does this affect their ability to feel pain?

Back in 2015 a Catalyst episode (Horse Whip) was released featuring Dr Lydia Tong, a forensic veterinary pathologist, discussed the differences and similarities between thickness and innervation of skin comparing horses and humans. A report was supplied, but no peer reviewed paper was yet released.

The episode focused on horse racing and the use of whipping in the sport. Often harsh tools and techniques are justified in the horse world by claiming that "horses have a thicker skin" suggesting they don't feel pain at the same level that we do. The report featured on the episode concluded that horses have a thinner epidermis (outermost layer of skin) than humans. It also counted more nerve endings compared to humans. See image below.

Fast forward to 2020 and Dr Tong co-authored and led a study to follow up her previous findings (A Comparative Neuro-Histological Assessment of Gluteal Skin Thickness and Cutaneous Nociceptor Distribution in Horses and Humans).

The study found that the difference between the outer most layer of gluteal skin (epidermis) between horses and humans was negligible. They were comparative in thickness and number of nerve endings. Horses did however have a thicker collagen layer, but this layer has no bearing on ability to feel pain as it is under the sensitive epidermis.

"The results revealed no significant differences between epidermal thickness of humans and horses for left-sided (reference) samples."

"There were no significant differences between the epidermal nerve counts of humans and horses."

"Together, these findings indicate that horse skin is virtually indistinguishable from human skin with respect to the basic anatomical structures relevant to cutaneous pain detection."

Pain is extremely subjective, even between two humans it is impossible to draw a quantitative comparison, as two people experiencing the same stimuli often report different levels of pain. However, we now know that anatomically horses have, at the very least, the same capacity to feel pain on the skin as we do (anything that can detect a fly landing on it must be sensitive).

"These observations must refresh old assumptions made about the capacity of the “thick-skinned” horse to experience pain in comparison to humans."

For years the concept that horses don't feel pain to the same degrees as humans has been used to justify harsh training methods and husbandry, including aggressive whipping, excessive spurring and inappropriate or ill-fitting tack. This 2020 study demonstrates that we cannot feasibly believe this anymore.

Reference:

Tong, L.; Stewart, M.; Johnson, I.; Appleyard, R.; Wilson, B.; James, O.; Johnson, C.; McGreevy, P. A Comparative Neuro-Histological Assessment of Gluteal Skin Thickness and Cutaneous Nociceptor Distribution in Horses and Humans. Animals 2020, 10, 2094. https://doi.org/10.3390/ani10112094