A good way to help horses with the devastating skin condition known as hereditary equine regional dermal asthenia (HERDA) is to limit their exposure to sunlight, according to a new study from the Laboratory for Comparative Orthopaedic Research at Michigan State University.

HERDA, which is passed along through certain Quarter Horse bloodlines, leads to extremely fragile skin that easily tears. It is an autosomal recessive disorder, meaning that horses who inherit only a single copy of the defective gene are carriers who won’t develop the problem; however, when two carriers are mated, their offspring have a one-in-four chance of developing the condition. Severely affected horses can’t be ridden because even normal pressure from tack creates large, open wounds that do not heal. Previous research has shown that HERDA is a form of Ehlers-Danlos syndrome, a group of genetic diseases caused by mutations in the genes that control collagen0 formation.

The risks of sun exposure for HERDA horses have been recognized anecdotally for some time, says Ann Rashmir-Raven, DVM, MS, DACVS, PGCVE. “If you speak to people who are familiar with these bloodlines and this condition, you’ll learn that keeping these horses out of the sunlight improves their skin. And I personally observed that when we’d get horses with the condition at the research clinic, their skin would improve while they were kept indoors during quarantine.”

Building on this knowledge, the Michigan researchers began investigating the role that exposure to UV light may have on the condition. “When human skin is exposed to light, the enzyme known as collagenase is upregulated,” explains Rashmir-Raven. “Collagenase eats collagen. To some degree, this can be helpful; in order for skin to repair itself it needs collagenase. But too much sunlight can push this to the point where it’s destructive. If you have friends who sunbathe regularly, you notice their skin is quite different by the time they are 50. That’s in part because the UV light from the sun increases collagenase activity.”

For the study, researchers took full-thickness skin samples from various anatomical locations on seven horses diagnosed with HERDA and six normal ones. A total of 24 samples were taken from each horse, and half were in-cubated in a collagenase solution to produce the same effect that exposure to sunlight would. Researchers then used a machine to test all of the samples for stretchability.

“We were measuring how much force it took to stretch a sample,” says Rashmir-Raven. They found that it took 24 times less force to stretch HERDA skin after exposure to the collagenase solution than it took to stretch the skin of normal horses with the same exposure.

Rashmir-Raven says the structure of collagen in horses with HERDA likely accounts for their response to the enzyme. “A normal horse has tightly packed collagen, similar to tightly packed, uncooked spaghetti still in the box,” she explains. “A horse with HERDA, however, has very irregularly stacked collagen, a bit like a pile of cooked, loose spaghetti. The problem with this architecture is it provides so much more surface area for collagenase to act on, so collagenase can be much more destructive.” She adds that the pattern of HERDA lesions, with more being found over the back and topline, correlates to sunlight exposure.

This study not only verifies the observations of those who have cared for HERDA horses but provides some guidance on management techniques. “One of the most beneficial things we can do for these animals is to minimize their exposure to sunlight,” says Rashmir-Raven. “Turning them out at night and keeping them stalled for much of the day will help. A UV-blocking fly-sheet can be used in horses where confinement is not possible if it doesn’t damage skin at the withers or elsewhere.”

Ultimately, says Rashmir-Raven, the spread of HERDA should be controlled by the testing of all breeding animals with susceptible bloodlines. And that, she says, can be done in a way that doesn’t remove the desired genes.

“Some carriers are top performers with wonderful genetics,” she says. “You can continue to use them for breeding if you are not breeding to another carrier; otherwise you will produce an affected foal 25 percent of the time. It behooves everyone to test mares and stallions alike and not breed carriers to carriers.”

Reference: “Increased susceptibility of skin from HERDA (Hereditary Equine Regional Dermal Asthenia)-affected horses to bacterial collagenase degradation: a potential contributing factor to the clinical signs of HERDA,” Veterinary Dermatology, December 2015

This article first appeared in EQUUS issue #461, February 2016.