HERDA's Impact on Tendons, Blood Vessels Studied

HERDA's Impact on Tendons, Blood Vessels Studied

Subtle biomechanical differences identified between tissues from HERDA-affected and healthy horses could give HERDA carriers an advantage in cutting and other disciplines requiring movements that strain the limits of athletic flexibility.

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Hereditary equine regional dermal asthenia, more commonly known as HERDA, is often thought of as a skin disease that affects mainly Quarter Horses, primarily in the cutting horse industry. However, a group of researchers recently questioned whether the debilitating disorder could affect tissues other than the skin, specifically those with high Type 1 collagen levels, such as tendons and large blood vessels.

"The HERDA phenotype includes loose, fragile, hyperextensible skin that is easily injured and heals poorly," explained Jacquelyn Bowser, DVM, PhD, Dipl. ACVIM, an equine internist at Mississippi State University, who performed the research during her PhD training. "Seromas (tumorlike collections of serum), hematomas (blood pockets), and ulcerations are reported to occur primarily along the dorsum (back) and progressively intensify in occurrence and severity with age.

"HERDA is also overrepresented in elite cutting horses, where one-third of the top hundred cutting horses (from 1985-2006) were confirmed HERDA carriers," she continued. "Twenty-seven percent of the lifetime earnings from progeny of these top 100 cutting sires are derived from 12 HERDA carriers."

Currently, she noted, HERDA has a prevalence of more than 28% in the cutting horse discipline.

Previously, Cyprianna Swiderski, DVM, PhD, Dipl. ACVIM, the lead investigator on the recent study, worked in collaboration with Marzia Pasquali, PhD, to identify a pattern of abnormal collagen metabolites in HERDA horses' urine and tissues. They used this finding to diagnose HERDA from a urine sample prior to identification of the causal DNA mutation, Bowser said.

"With this evidence of abnormal collagen structure in HERDA, and the predominance of Type 1 collagen in skin, we hypothesized that altered biomechanical properties would also be detected in tendons and vessels, which are also rich in Type 1 collagen," Swiderski explained. The work was funded by the American Quarter Horse Foundation and Mississippi State University.

In their recent study, the team evaluated tissues from horses with two copies of the defective HERDA gene and identified abnormal physical properties in these tissues. They compared the ultimate tensile strength (UTS, the maximum stress the tissue could withstand while being stretched before breaking) and elastic modulus (EM, a measure of tissue stiffness) of superficial digital flexor tendons, deep digital flexor tendons, the aorta, and pulmonary arteries from six HERDA-affected horses to those of six age-matched controls. The team cut pieces of tendon and artery into dumbbell-shaped pieces and methodically tested their UTS and EM.

Upon reviewing their results, the team noted that:

  • The UTS was significantly lower in tendons from HERDA-affected horses than in the control horses. Bowser said this shows that HERDA-affected horses have weaker tendons that could be more easily injured, and that HERDA affects tissues other than skin.
  • The tendons' EM was not significantly different between HERDA-affected and control horses, but the trend was for HERDA-affected horses to have increased stiffness in their tendons. "This could make tendons of HERDA horses function like ‘tighter springs’ relative to tendons of normal horses, increasing the tendon’s ability to store energy during locomotion," Bowser said.
  • The EM of large blood vessels, which are also rich in Type I collagen, was significantly lower in HERDA-affected horses relative to controls. "This would mean that the vessels of HERDA horses have decreased stiffness and are more elastic and highlights a need for further investigations of the effect of HERDA on tendon biomechanics in a larger cohort of horses," Bowser explained.

Bowser noted that these findings could explain why HERDA carriers are overrepresented among elite performing cutting horses: Subtle versions of the biomechanical differences identified in the recent study between tissues from HERDA-affected and healthy horses could give HERDA carriers an advantage in cutting and other disciplines requiring movements that strain the limits of athletic flexibility, she said.

Altering tendon strength and elasticity could do several things: "An increase in elasticity could make it easier for these horses to comfortably perform the positions and rigorous movements that characterize cutting," she said. "In contrast, increase in stiffness of tendons could increase their energy storage capacity during weight-bearing, propelling the horse more efficiently during the stride and decreasing the total effort needed to perform these movements."

Further research is needed into HERDA carriers' tissues before either hypothesis can be confirmed, she said.

"The purpose of our study was to find out if the HERDA allele causes altered mechanical properties in tissues with high Type I collagen, not just in the skin," Bowser concluded. "I can definitively say that the answer is yes, it does. But to what extent this translates to carrier animals, that is still to be determined.

"Sometimes, by selecting for a trait that is desirable, we also get a less desirable condition. … By using careful and responsible breeding (i.e., avoiding breeding HERDA carriers to each other), perhaps we can maximize the benefit and minimize the consequences."

The study, "Tensile properties in collagen-rich tissues of Quarter Horses with hereditary equine regional dermal asthenia (HERDA)," was published in the Equine Veterinary Journal.


About the Author

Erica Larson, News Editor

Erica Larson, News Editor, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in three-day eventing with her OTTB, Dorado, and enjoys photography in her spare time.

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