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Are Genetics to Blame for Fractures? Not So Simple, Epidemiologist Says – Horse Racing News | Paulick’s report

After the Saratoga race meet, which became the headlines for several Grade 1 fatalities, race fans and participants have been asking each other — what more can be done to improve horse safety?

As pre-race animal screening has increased and the use of drugs has become more restricted, many have shifted their focus to breeding. Some have drawn on general breed speed to speed and distance and pedigree, while others have focused on specific sires or sires that they believe are responsible for major health problems.

We asked Dr. Tim Parkin, head of school and professor of animal epidemiology at Bristol Veterinary School, about this question. Parkin is a leading epidemiologist who analyzes data from the Equine Injury Database and together with the research team has identified patterns and risk factors that increase the likelihood that a given horse will suffer a fatality during a race.

Parkin’s work has shown—predictably—that death is a complex phenomenon with many factors influencing its likelihood. Another reason may explain some of the risk between a fatally injured horse and a competitor from the same race, which makes his job more difficult. In 2017, Parkin said his team could explain just 35 percent of the ups and downs in death rates between the start of EID record keeping in 2009 and 2017, when the overall death rate was declining (as it is now).

As it turns out, Parkin believes that the identities of sires and dams in a horse’s pedigree probably don’t make as much difference to the level of risk as we might expect.

“Every time we run a multivariate analysis we put the sire and the dam as something known as random which shows how much influence they have on risk,” he said. “They tend to come out with very little impact when accounting for all the other variables.”

Parkin cited two peer-reviewed studies conducted by a former student on trauma data from Hong Kong and Great Britain.

In a study of Hong Kong data, published in 2013 in the Veterinary Journal, researchers looked back at 15 years of health data and built statistical models to tease out how much heritage influenced horse injury risk. The data was not limited to horses that suffered a fatal accident, but rather horses with a musculoskeletal problem identified in an examination from an official veterinarian, or horses whose musculoskeletal injury was the reason for their withdrawal from the races.

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Researchers looked at the sire, dam and sire in the second generation of each horse’s pedigree. Where additional genealogical information was available, it included information up to eight generations. They also addressed other environmental risks and basic identity data such as gender.

All horses in Hong Kong are imported, as there are no breeding farms there. Interestingly, the researchers found that horses from North America had a reduced chance of suspensory ligament injuries compared to European imports.

“The heritability estimates of fractures, osteoarthritis, and suspensory ligament injuries reported here are small to moderate in magnitude, with large standard errors,” the study’s conclusion read.

Heritability of fracture can range from .03 to .11 on a scale of 0 to 1, depending on the mathematical model used by the researchers. Heritability of tendon injuries was very significant, ranging between .09 and .2.

There was also a genetic correlation between horses’ risk of fractures, osteoarthritis, and suspensory ligament injuries, indicating that those prone to one type of injury appear to be prone to other types of injury, too.

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“The significant genetic correlation between fractures, osteoarthritis and suspensory ligament injuries shows that efforts can be focused on any condition that is very simple, and progress in reducing the genetic risk of the related condition can occur at the same time,” the authors wrote.

A British study looked at horses that had suffered lower leg fractures and injuries to their digital tendons, and examined pedigree data to find out how those conditions can be traced back to their offspring. The researchers concluded that the prevalence of lower limb fractures in the British horse population was .21 to .37 on a scale of 0 to 1.

“SDFT injury and distal distal fracture were significantly genetically correlated,” the researchers wrote in their abstract. “These results show that reducing the risk of the studied conditions can try to use targeted methods of reproduction.”

On the face of it that would seem like a good thing for those who believe the gene pool needs to be fixed. The heritability in this list would seem to indicate that fractures can be reduced by 30 percent if part of the gene can be eliminated.

Unfortunately, the truth is not that simple, says Parkin.

Parkin says he hasn’t done the same heritability analysis on the North American data that the researchers did for Hong Kong and Great Britain, but his models take into account a larger range of additional risk factors outside of genetics than heritability analysis. His first results include first-generation genealogical information in his model, however, it does not give him confidence.

So there is this business of using legacy analysis to reduce risk. Knowing a trait can be inherited between .21 and .37 does not mean that it can be removed from the population, because the species is still a species with limited genetics.

“If (big IF) we were able to eliminate any impact on the reproduction of fractures then the theoretical reduction of 30 percent would happen – but this is not possible,” said Parkin. “New horses have to come from somewhere and I would guess that all sire/mare combinations contribute something to the risk – that some will contribute more and some less. How much and how little is impossible to say from the evidence we have. “

Even if you set out to reduce genetic risk in all people, Parkin said, it would be difficult to use data from people who share a common background to provide specific pointers on how to achieve it. Observers may wonder if Parkina’s models may one day be flagged as having a higher risk of injury than others.

“We can produce the guess that chance is high – but it will be an average over a wide range of interests and there will be significant uncertainty around that estimate,” Parkin said. “So each sire may have a ‘genetic risk’ score but there will be significant overlap with many other sires.”

For now, it seems, breeders will have to make decisions about the best way to improve sound based on the horses and the pedigree in front of them.

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