Researchers in California are investigating the potential causes of an unusual and devastating gait defect they’ve termed “sidewinder” syndrome.

“Sidewinder isn’t the diagnosis but a description of the how the horse moves, which is sometimes called ‘crabwalking’ or ‘drifting,’” explains Monica Aleman, PhD, at the University of California, Davis. “It’s a very distinct and dramatic gait. It looks as if the horse can’t stop their hindquarters. Their thoracic (front) limbs are usually planted in the ground, but their rear end continues, so the horse swings around in a circle.”

Some horses, says Aleman, lean against a fence or wall for stability and others, if severe might “dog sit” to compensate for the lack of control. “It’s very upsetting to the horse and to the humans watching, she adds: “It can be physically dangerous for everyone as these large animals seem to have no control over their hindquarters.”

To better understand how sidewinding develops, Aleman and fellow researchers collected information on 24 horses treated at UC-Davis and the Animal Health Trust in Newmarket, England, for this type of gait abnormality. They documented each horse’s clinical signs, as well as findings from neurologic, orthopedic and post-mortem examinations and, when possible, reviewed data from muscle biopsies, spinal taps, radiographs, electromyography and other diagnostic techniques.

The researchers found that the sidewinders were of various breeds and both genders. Ten horses had acute, sudden onset of the condition, while six had more subtle onset and progression. Eight horses initially had minor signs that gradually worsened to become severe.

Aleman says the collected data suggests two possible primary causes of sidewinder gait: neurological disease (found in 16 of the horses) and orthopedic disease (found in eight horses).

The most common neurological causes of the sidewinder gait were compression of the spinal cord by vertebrae (five horses), equine protozoal myeloencephalitis (four horses) and damage to the spinal cord from unknown causes (four horses). Among the specific orthopedic conditions found among the sidewinders were osteoarthritis of the hip joint, multiple displaced pelvic fractures and rupture of ligaments stabilizing the hip.

“This study couldn’t tell is why these horses developed these issues,” says Aleman. “It’s possible some were the result of trauma, but it’s impossible to know. Some may have developed slowly over time and gone unnoticed until the horse began spinning.”

One interesting correlation to emerge from the data was the average age of the sidewinding horses: 18.9 years. “This appears to be a condition of older horses,” says Aleman. “Why that is, we aren’t sure. Younger horses can experience all of these conditions, but they don’t usually develop this specific gait as a result. Why older horses? Is there another layer we need to look into? Probably.”

The prognosis for horses with a sidewinder gait is not good, says Aleman. “In this study, 80 percent of the horses were euthanatized shortly after onset because they were so badly affected,” says. “Some horses were treated with rest and anti-inflammatory medication, but most of those were eventually euthanatized as well.” Only two of the study horses were available for follow-up and both were turned out at to pasture with an improved but persistent sidewinder gait.

“I’ve seen several of these horses in person and I don’t know a single one who returned to their previous level of activity,” says Aleman, “although some may have a happy life in the pasture.”

Aleman hopes the identification and description of sidewinder gait, will help advance research into the condition. “If we can find more of these horses to study, we can learn even more,” she says. “If we understand more, maybe we can prevent it. What if we can find them early with subtle lameness? Maybe there is something that can be done.”

Reference: “Sidewinder gait in horses,” Journal of Veterinary Internal Medicine, August 2020. 

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