Study Finds High Rate of Bit-Related Lesions in Trotting Horses

Finnish researchers found that 83% of the Standardbred horses and 90% of the Finnhorses (a local draft breed) had acute lesions resulting from their bits post-racing. Mares were at a greater risk of mouth injury.

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Study finds high rate of bit-related lesions in trotting horses
Finish researchers classified the horses’ mouth injuries in lettered categories from A to D based on the number, extent, and severity of bruising and open wounds. | Tuomola K, Mäki-Kihniä N, Kujala-Wirth M, Mykkänen A and Valros A (2019) Oral Lesions in the Bit Area in Finnish Trotters After a Race: Lesion Evaluation, Scoring, and Occurrence. Front. Vet. Sci. 6:206. doi: 10.3389/fvets.2019.00206

When used incorrectly, bits can cause severe mouth lesions, even when no blood is visible in the horse’s mouth.

A new study of more than 200 trot-racing horses revealed that 84% of the horses had bit lesions inside the mouth, and more than half of those were moderate or severe. But only 2% of the horses had blood that could be seen without opening the horse’s mouth, said Kati Tuomola, DVM, of the Research Centre for Animal Welfare in the University of Helsinki’s Department of Production Animal Medicine, in Finland.

All bits—including single-jointed snaffle bits—were associated with lesions, regardless of whether blood could be seen on the outside of the horse’ mouth, said Tuomola. Her team’s findings raise serious concerns about an apparently invisible welfare issue in bitted horses.

“The visible blood is very likely only a tip of an iceberg when it comes to oral lesions in horses,” she said.

Study Finds High Rate of Bit-Related Lesions in Trotting Horses
Bit type was the most significant factor affecting lesions, said the researchers. These are the six bits included commonly used in the evaluated horses. | Tuomola, K, Mäki‐Kihniä, N, Valros, A, Mykkänen, A, Kujala‐Wirth, M. Risk factors for bit‐related lesions in Finnish trotting horses. Equine Vet J. 2021; 00: 1– 9.

Severe Bit-Related Lesions Common With Strong Link To Bit Type

Tuomola and her fellow researchers looked into the mouths of 261 harness-racing horses just after they finished racing on any of four tracks in Western Finland. The exam was compulsory: drivers and owners were not allowed to refuse, she said. They classified the horses in lettered categories from A to D based on the number, extent, and severity of bruising and open wounds. They then used data from 229 horses to create a scientific model to better understand risk factors associated with the different categories of lesions.

The scientists found that 83% of the Standardbred horses and 90% of the Finnhorses (a local draft breed) had acute lesions. Of these, 61% and 74% had moderate or severe (Categories C-D) lesions, respectively.

The results are concerning, said Tuomola. “I did not know what to expect (when we started this study),” she said. “But I did not expect this many and such severe lesions, particularly not in bit types that I always thought to be less severe (like the unjointed Regulator Mullen mouth and the straight Happy Mouth).”

Bit type was the most significant factor affecting lesions, Tuomola said. The worst lesions were associated with the single-jointed Crescendo bit, which has thin metal rails that place great pressure on small surface areas. Her group also detected serious lesions especially on the horse’s bars (gum-covered bony part of the jaw where there are no teeth) when unjointed bits—even so-called “gentle” bits with leather or plastic covering—were used. This was likely related to the compression and ulceration of the mucous membranes directly over hard tissues, especially near the first lower cheek teeth. “I find bar lesions nasty, because there is only a very thin mucous membrane covering the bone,” she said.

Although these bits were more likely to be associated with more severe lesions, even the “gentle” snaffle bit created wounds and bruising, said Tuomola. Half the horses racing with single-jointed snaffles or Nurmos bits had C-D category lesions, she said.

Mares at Greater Risk of Bit-Related Lesions

Interestingly, mares in their study were likely to have more severe lesions than geldings. “This was surprising,” Tuomola said. “If we think theoretically, it is possible that their soft tissues are more vulnerable or that mares really are more anxious and flightier than geldings. But scientific studies have also suggested that sex-based stereotypes may compromise mare welfare. For example if a mare has lesion in the mouth and if she expresses conflict behavior (mouth opening, head-tossing …), people might think that the mare is ‘difficult’ because she is a mare, and lesions would then worsen if they’re ignored. But this is theoretical; we do not know if this is the case.”

Even so, people who work with mares should probably monitor their mouths regularly, she added.

No Relation To Performance, but Maybe To Training Difficulties

Although the findings are concerning, they make sense, said Tuomola. “Now that we know the results, it is logical that if trainers have ‘problems’ in driving, it is likely that they try to change the bit type from a basic single-jointed bit to something else, and then we discover the results of that change in the mouth,” she said.

Tuomola’s group didn’t find associations between lesion severity and the horse’s race placings, nor whether the horse broke into a gallop (which can result in disqualification).

“When I worked as a race veterinarian and a horse had lesions, we would sometimes speculate that maybe those lesions resulted from galloping in the race and the driver pulling the reins to get the horse back into trot again,” she said. “But at least in this sample of horses, there was no association between galloping and lesions. Maybe oral lesions result from more long-standing pressure, but we don’t know that. Rein tension measurements would be a good idea.”

The results also contradict a common belief that good performance is a sign of good welfare, she added. “In our study, horses’ performance (placement among top three or prize money) was not different in horses having no lesions or mild lesion status compared to horses having moderate or severe lesion status,” said Tuomola. “It is possible that pain sensation is suppressed under stressful situations. Well-performing horses are not necessarily free from welfare concerns in all life aspects.”

Researcher: ‘Don’t Blame the Bit’

While most horses had bit-related lesions in their mouths, the bit itself might not be to blame, Tuomola said. Bits can be safe if used correctly with good skill and understanding of learning theory, in particular negative reinforcement (well-timed release of pressure in response to the right behavior).

“There were 16% of the horses that did not have any acute oral lesions in the bit area, so it is clearly very well possible to race without lesions,” she said. “The bit itself is not harmful. It’s sort of like good use of a knife. Knives can be very useful. But in the ‘wrong hands,’ they can be very dangerous.”


Written by:

Passionate about horses and science from the time she was riding her first Shetland Pony in Texas, Christa Lesté-Lasserre writes about scientific research that contributes to a better understanding of all equids. After undergrad studies in science, journalism, and literature, she received a master’s degree in creative writing. Now based in France, she aims to present the most fascinating aspect of equine science: the story it creates. Follow Lesté-Lasserre on Twitter @christalestelas.

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