New research offers genetic support for the theory that “roaring” is more likely to be seen in tall horses.
Technically known as recurrent laryngeal neuropathy (RLN), roaring occurs when a portion of a horse’s larynx is paralyzed, blocking the airway. In addition to creating a distinct noise when the horse works, RLN can also greatly restrict athletic efforts.
For the study, veterinarians at Michigan State and Cornell universities selected 550 Thoroughbred horses. An endoscopic examination of the airways was performed on each horse, his height was documented and a blood sample was taken for genetic analysis. Horses with RLN were graded based on how the condition would affect performance. Control animals were older but had normal laryngeal function.
The data produced by gene mapping showed that the region of the equine genome governing height and that associated with RLN overlapped. “Think of it this way: You’ve got the DNA, which is like a long road, hundreds and hundreds of miles long,” explains Ed Robinson, BVetMed, PhD. “And you’ve got distance markers every 20 or 30 miles. Using these markers we were able to determine that the area of the DNA associated with height is very near that involved in roaring. They are at the same distance marker, so to speak. This means that the two characteristics are very likely genetically linked to each other in some way.”
These findings support anecdotal observations that taller horses are more likely to be roarers.
In fact, the data suggests that 16.5 hands is a threshold of sorts: Horses of that height and larger were more likely to be roarers. Robinson says that the apparent link between height and RLN may be a reflection of nerve function: “Nerves have to transport materials that are vital to their survival along their full length in both directions. It’s possible, in a longer necked animal, that some of this transport becomes disrupted, damaging the nerve.” He adds, however, that RLN may also be controlled by its own genes that are near the genes for height. “This study doesn’t explain everything, but it does send us looking in the right direction.”
Regardless of the specific mechanisms that lead to RLN, the study suggests that selective breeding to reduce the incidence of the condition in a population would also produce a decrease in average height. “When the two are so closely linked, you can’t influence one without the other,” says Robinson. “That said, I don’t anticipate anyone basing breeding choices on roaring alone. It does affect performance, but there is surgery to fix it, and it doesn’t seem to be a huge concern among breeders at this point.”
Although this study was limited to Thoroughbreds, Robinson says the findings may be applicable to warmbloods, who are also prone to roaring. “There is quite a bit of Thoroughbred blood in some of the warmblood breeds,” he says. “It would be interesting to see if this same correlation exists.”
Reference: “Genomic analysis establishes correlation between growth and laryngeal neuropathy in Thoroughbreds,” BMC Genomic, April 2014
This article first appeared in EQUUS issue #442.