Recurrent exertional rhabdomyolysis (RER), one form of the painful muscle disorder known as tying up, may be the manifestation of a dominant genetic trait activated by stress, diet, temperament or other factors, according to 2005 research from the University of Minnesota.

RER in Thoroughbreds mainly affects the large muscle groups of the back and hindquarters. Caused by the abnormal release of calcium within muscle cells, the condition can be difficult to diagnose because of its intermittent nature–affected horses appear normal between episodes–and because unrelated factors such as training or lameness may complicate clinical observations.

To isolate the genetic link to RER, the Minnesota researchers first took muscle biopsies from mature, healthy Thoroughbreds who had previously tied up at the racetrack and 23 foals related to those animals. The biopsied tissue then underwent contracture tests, which involve exposure to certain substances to stimulate the calcium regulation mechanism that controls muscle contraction. An abnormal response of the cells indicates RER.

Eight of the mature Thoroughbreds who had tied up tested positive for RER, as did nine of the foals. Next the researchers analyzed the horses’ pedigrees to determine how the RER trait might be passed on.

Their findings suggest that the condition is an autosomal dominant trait, says Stephanie Valberg, DVM, PhD, meaning it is not on the X or Y chromosome and a foal would be affected even if he received a copy of the abnormal gene from only one parent.

But, she adds, specific factors, particularly a high-strung temperament, influence whether RER will be apparent in an individual. “It seems that the underlying trigger for abnormal calcium release within the muscle cell is stress-related, and horses that inherit the trait and are nervous appear to be five times more susceptible to develop signs of RER than calm horses under conditions at the racetrack.”

Valberg says that it is impossible to diagnose RER from pedigree alone, but “breeders should know that if a horse has tied up repeatedly as a racehorse it may well pass this tendency on to his or her offspring. If that horse has many overriding positive traits, this may not be a big factor in breeding the animal, but they should be aware that they may have the same disorder to deal with.”

This article originally appeared in the December 2005 issue of EQUUS magazine. Read about an unusual case of tying up in “A Manageable Muscle Problem” in the September 2007 issue.