Atrial Fibrillation a Heritable Trait in Standardbreds
A healthy heart is an important part of a racehorse’s success. But veterinarians have noted that a form of cardiac arrhythmia, known as atrial fibrillation (AF), appears to be showing up more commonly among Standardbred trotting and pacing horses. And researchers in Canada recently confirmed that this growing trend has a lot to do with genetics—it appears AF has a fairly strong heritability.

“The heritability of atrial fibrillation was found to be quite significant for a disease trait,” said Flavio Schenkel, PhD, of the University of Guelph’s Ontario Veterinary College Department of Animal Biosciences.

“Based on our finding, AF-positive parents will increase the incidence of this disease in the herd and in the population of Standardbred horses as a whole,” he said. “The increase might be initially slow, but will be cumulative and will gain momentum over generations.”

In their study, Schenkel and colleagues examined the racing and health records of more than 1,200 Standardbred racehorses, including more than 200 that had been treated for AF at the Ontario Veterinary College Teaching Hospital. They also analyzed those horses’ pedigrees, which included more than 12,000 animals, to look for genetic links among AF-positive horses.

One of the study’s co-authors, Peter W. Physick-Sheard, PhD, also of the University of Guelph, led a similar study in 2014. He was the first to estimate AF’s heritability in Standardbreds—and it was moderate for a disease trait (0.096). But his study did not take gender or racing gait (trot or pace) into account, the researchers said.

This new study found much higher heritability estimates (up to 0.17), especially in male horses and in pacers, Schenkel said. In fact, pedigree analysis showed that most AF-positive horses born over a 29-year period came from only five ancestors. And one of those stallions sired nearly half the affected horses in a single year, only two years after arriving at stud.

“Our findings highlight the genetic background of this disease, which will enable horse owners to make a more informed decision about whether or not to use a horse for breeding when the horse’s disease status is known, with an aim to decrease the incidence of AF in the Standardbred population,” the authors stated.

However, that doesn’t necessarily mean breeders can wipe the disease out of racing altogether, Schenkel said. “Eliminating AF from the Standardbred horse population is a difficult task to be achieved in the short term, given its level of heritability,” he said. “However, breeding could lower the incidence of this disease by avoiding mating individuals with known or estimated high likelihood of being genetically more prone to AF.”

A better understanding of how AF transfers from parent to offspring is an important next step, he said. Through genomic research, scientists could eventually identify the actual, multiple genes involved. With that knowledge, genetic testing for AF could open many doors for better health, welfare, and management of AF-prone horses.

“The Standardbred horse industry should support future genomic research aiming to map and understand the genes and biological pathways involved in the liability for this disease,” Schenkel said. “This could potentially lead to a genetic test for early prediction of higher liability for AF, allowing for better management of herds/population to decrease incidence of this disease over time.”

The study, “Estimates of heritability of atrial fibrillation in the Standardbred racehorse,” was published in the Equine Veterinary Journal.