Unlike their human counterparts, horses in general aren’t known for picking up a lot of bad habits. You won’t see a horse smoking, biting its nails, or staring at its phone during a conversation. But one uniquely bad habit that horses can develop is cribbing.
Cribbing is defined as a stereotypical behavior, which is to say the horse performs a functionless and repetitive action, in this case grabbing an object like a fence with its teeth, pulling the object towards itself and sucking air. And while none of this seems like fun to humans (see above re smoking, nail biting, and rude phone manners), horses seem to get a kick out of it.
According to a 2010 study from Applied Animal Behavior Science, a horse with this habit spends between 15%-65% of its time cribbing its heart out. But as anyone who has owned a cribber or been in a barn where one resides knows, it can be difficult to manage, let alone cure. The problem can be so destructive that some equestrian establishments won’t accept your horse if it is a cribber.
Before you blame the horse for the issue, cribbing appears to be a manmade condition. There is no evidence that wild horses crib. The prevailing theories as to what causes the habit include stable management, specifically things such as too much grain, boredom, and stress. An Ohio State University study mentions a finding of a four-fold increase in cribbing in horses that were fed concentrates “immediately after weaning.” Certain breeds seem predisposed to cribbing, particularly warmbloods and Thoroughbreds. There is also an incorrect belief that cribbing is learned by copying other horses, creating an epidemic; studies say not so. In a 2009 report “only 1% of horses were reported to have developed a cribbing habit after exposure to another cribbing horse.”
But cribbing isn’t merely an annoying habit – it can have serious health consequences. For one thing, horses that crib wear out their incisors, which can impact them as they age. Cribbers also exert themselves pursuing the activity and spend less time eating, which can make them poor keepers. Cribbing also increases the risk of colic and stomach ulcers and some studies have shown that it is linked to motor neuron disease (which affects the brain and nerves), and temporohyoid osteoarthropathy (a progressive disease of the middle ear).
So, what’s the solution? Cribbing collars are a common deterrent and if fitted properly can curb the habit, but only when the horse is wearing the collar. Most research suggests that cribbing is a permanent affliction and that owners need to weigh the treatments against the risks. Trying to break the habit might cause more stress and therefore increase the habit.
“While cribbing can be detrimental and there are risks associated with it, we have to consider the horse’s welfare when addressing the cribbing,” says Dr. Kate Robinson, a veterinarian at Ontario-based McKee-Pownall Equine Services. “If we apply a cribbing collar or cover surfaces in hot pepper, we also need to put in the effort to address what might be causing the cribbing in the first place. This may mean increasing turnout time, changing the herd mates around, providing more enrichment such as hay balls. We don’t exactly know why horses crib, but we are doing them a disservice if our only solution is to force them to stop doing it.”
More invasive treatments include surgery to remove certain muscles in the neck, a procedure with inconsistent success. Another procedure surgically implants “crib rings” into the horse’s gums between the upper teeth, which apparently decreases the habit, but for how long depends on if the horse can keep the rings on its teeth during regular wear and tear. These aggressive solutions also raise red flags in terms of animal welfare.
While surgery isn’t an option for many horse owners, pharmaceuticals and supplements are relatively new treatments consider. A recent study out of Midwestern University College of Veterinary Medicine treated horses with the antidepressant fluoxetine, or Prozac, a drug commonly used for humans and canines.
“Fluoxetine is in a class of drugs known as selective serotonin reuptake inhibitors. They increase serotonin levels by slowing their resorption into the neuron,” explains Dr. Laura Waitt Wolker, assistant professor in equine internal medicine and an author of the study. “Serotonin helps improve communication of the neurons and has been found to be deficient in horses that crib. Therefore, this class of drugs shows potential to treat anxiety and stereotypical disorders in horses.”
The goal of Waitt Wolker’s study was to establish the serum levels of fluoxetine and the bioactive metabolite, norfluoxetine, following oral administration to ensure there were no side effects in normal horses. “To do this we collected samples and analyzed them via mass spectrometry. Very simply, this is a procedure that separates a compound into its components so they can be measured,” she explains. “We originally intended to study the effect this drug had on cribbing but we were not able to enroll enough horses to then conclusively determine if it helped them.”
What this means is that the study didn’t have enough ‘power’, a scientific term describing the number of subjects needed to determine if a specific effect really happened versus a coincidence such as an anecdotal one-off report.
“Behavior studies in horses are challenging because often the owners are taking the measurements of cribbing. The information is entirely dependent on their dedication and attention to detail,” Waitt Wolker says. “Things like exercise and feed changes, vacations, horses moving facilities, horses switching trainers, or even owner changes can all affect the amount the horse is cribbing.”
Another drug that has been prescribed for cribbers is trazodone, a serotonin modulator, similar to fluoxetine. In a study published in 2018, a “spectrum of stereotypical behaviors” in 17 of 18 horses was reduced by the use of trazodone.
However, there is no FDA or Health Canada approved formulation and the efficacy and safety at higher doses for equines has not been definitively established. “Absolutely, more studies are needed to determine if this drug is as valuable in horses as it is in people,” Waitt Wolker adds. “Fluoxetine doesn’t have widespread use for cribbing because most horse owners honestly do not try to resolve cribbing with drugs, they use collars, environmental management or just ignore it. The research has not (yet) been performed to conclusively say whether fluoxetine inhibits cribbing behavior.”
Another theory for what causes cribbing is selenium deficiency. A 2018 study suggests “that alterations in selenium, an important component of the antioxidant system, may play a role in the pathophysiology of crib-biting behavior in horses, adding further evidence to the theory that crib-biting may be related to increased oxidative stress and alterations in essential trace elements.”
Selenium occurs naturally in soil, but its levels are low in many parts of Canada where hay is grown, or horses are on pasture. Selenium supplements are available readily through feed stores and tack stores and many commercial grain products have selenium added. “Please talk to your veterinarian and have blood work done to check selenium levels before supplementing, as the safety range for selenium is narrow and we don’t want a horse getting too much either,” advises Robinson, who is familiar with the selenium study. “I have recommended blood work to check selenium and then prescribed selenium supplementation.”
Robinson admits that she’s only used the supplements on a couple of her equine patients, and it seemed to help only one of them. “I do think that checking selenium levels on a cribbing horse and supplementing if indicated is a useful thing to try, but it may not be the solution for every horse,” adds Robinson.
When it comes to the selenium theory, as with the use of fluoxetine, more studies are needed. “I practice in an area of the [United States] that doesn’t have selenium deficiency (Arizona) and I see tons of cribbers,” Waitt Wolker offers. “I practiced in the Pacific Northwest for years, which is well known for its deficient selenium, where I saw both cribbers and diseases caused by low selenium. I saw many animals with a disease induced by low selenium that did not crib and down here we have cribbers with normal selenium.”
Cribbing is a habit with no definitive cause, which makes preventing it next to impossible, although with many stereotypical behaviours, management plays an important part. Providing environmental enrichment tools (toys), free-choice hay, longer grazing times, and direct visual contact with other horses or group turnout are all strategies to improve the welfare of affected horses.