(RACE HORSES) Dr. Matthew Binns, previously Professor of Genetics at The Royal Veterinary College in London and a founder of the Horse Genome Project has attempted to answer the question of whether the Thoroughbred horse is becoming more inbred. Scientists from the journal Animal Genetics describe the current trend of greater inbreeding as “worrisome.” Dr. Binns suggests rechecking Thoroughbreds inbreeding levels every 5-10 years to ensure “that dangerous levels of genetic variation are not lost from this fantastic breed.” We here at Global Animal agree. — Global Animal
A group of racing Thoroughbreds make the turn for home. Photo credit: AP Photo/Benoit Photo

BBC Nature, Matt Walker

“Just hours before the Kentucky Derby, trainer Larry Jones got up early with his filly Eight Belles and took her to the track for a ride before the big race.

This was supposed to be a day of tempting history for Jones and Eight Belles.

They were taking on 19 colts and trying to make Eight Belles the fourth filly, and the first since Winning Colors in 1988, to win the “Run for the Roses.”

This was to be a day of celebration for owner Rick Porter and his entourage no matter where she finished. She was the first filly to enter the Derby since 1999.

Now there will be a necropsy and then cremation.”

The excerpt above is taken verbatim from a USA Today news report filed hours after the racehorse Eight Belles was euthanized on the track.

She was put to sleep after fracturing both her forelegs while pulling up after the race, in which she finished a glorious second, running, in the words of USA Today, “the race of her 3-year-old life”. 

The tragedy occurred just two years after the Derby winning horse Barbaro fractured his leg in the second round of the US Triple Crown, a race called the Preakness.

These and other inexplicable injuries to racehorses (in July the racer Rewilding snapped his leg challenging in the King George VI and Queen Elizabeth Stakes at Ascot in England, a tragedy I witnessed from the stands) inevitably leads to questions about the quality of the ground on which the horses run, the age at which they run, whether fillies should race colts, and most pertinent of all from a science or natural history viewpoint, has breeding caused a weakening of the racehorse talent pool?

New scientific research just published helps inform this last point; for it suggests that Thoroughbred racehorses around the world are becoming more inbred.

Not only have Thoroughbreds become more inbred over the past 40 years, the research shows, but the rate of inbreeding has accelerated over the past 15 years.

Thoroughbred horses, by definition, suffer relatively high levels of inbreeding. Just 21 horses mated at the turn of the 18th Century founded most of the racehorses running today, accounting for 80% of the genetic makeup of the current population. Other genetic analyses have shown that Thoroughbreds are the most inbred of horse breeds examined.

“However, to put these results into a broader context, the Thoroughbred is not as inbred as most pedigree dog breeds,” Matthew Binns, an expert in horse genetics, tells me.

But questions about the quality of the breed led Dr Binns, previously Professor of Genetics at The Royal Veterinary College in London, UK, and a founder of the Horse Genome Project, to investigate further.

Dr Binns has spent a significant portion of his career investigating the genetic basis of racehorse performance, and has a massive set of genetic data taken from horses sampled from the 1960s onwards.

“I realised I could answer the question about whether the Thoroughbred was becoming more inbred,” he told me.

Dr Binns and his colleagues, including Dr Jackie Cardwell from the Royal Veterinary College in London, UK who did the statistics, Drs Bailey and Lear from the Gluck Equine Research Centre in Lexington Kentucky, US and Drs Lambert and Boehler, colleagues at Equine Analysis in Lexington, Kentucky, US where Dr Binns now works, analysed the genetic profiles of 467 racehorses born between 1961 and 2006.

They analysed 50,000 separate markers, known as SNPs, on the genome of each horse, and then compared them to each other.

“DNA markers measure what was actually inherited rather than assuming an average as would be obtained by pedigree. For example, two full brothers on average share 50% of their DNA, but the real figure could theoretically range from 0-100%, depending on whether they inherited the same or the different chromosome from each parent.”

The study showed that there had been a small but significant (i.e. real) increase in inbreeding over the past 40 years, and that most of the increase was from the mid 1990s to present.

“Which is the time period during which many things have changed in the breeding of Thoroughbred horses,” says Dr Binns. “In the 1960s it was usual for each stallion to cover 40-50 mares per season, in the mid-1990s this number jumped to 150+.”

Nowadays, high quality stallions are also “shuttled” around the world to cover mares, for example, being sent to the southern hemisphere to breed with mares during the quiet season for breeding in the northern hemisphere.

This in part is to meet the modern demand for producing yearlings that sell for high prices at auction rather than the previous breeding goal of producing superior racehorses.

Overall that means fewer stallions are siring a greater proportion of offspring.

The current trend toward greater inbreeding is “worrisome”, say the scientists in the journal Animal Genetics, which has published their research.

Dr Binns says he doesn’t believe the inbreeding is, at the moment, greatly contributing to the number of fractures sustained by racehorses, and there is no evidence it directly led to the fractures of Eight Belles, Barbaro or Rewilding.

But he suspects it is contributing to the failure rate of pregnancy among breeding Thoroughbreds. So called “reproductive depression” is one of the first signs of inbreeding problems seen in populations of animals.

Scientists working with rare and endangered species face similar issues. In zoos and captive breeding programmes, researchers try to maximise “outbreeding” of their rare animals. That is to reduce the inevitable loss of genetic variation that occurs within a population due to a phenomenon called “genetic drift”.

They even set themselves a benchmark of maintaining 90% variation over 100 years within a population of animals.

As yet it isn’t possible to say whether Thoroughbreds are being bred to destruction.

It isn’t possible to link injuries to horses to inbreeding, or to conclusively say that inbreeding is damaging the fertility or fecundity of these horses.

But the trend isn’t good.

And no-one wants to be watching the Derby, Kentucky Derby or Prix de l’Arc de Triomphe in 2018 and to see another horse fall, broken under its own weight and heritage. 

To avoid such problems in Thoroughbreds, and to maintain the genetic health of these most athletic of animals, Dr Binns suggests that the Thoroughbred industry should periodically, every 5-10 years, re-check to see what the levels of inbreeding are.

That way, he says, it can “make sure that dangerous levels of genetic variation are not lost from this fantastic breed.”

More BBC Nature: http://www.bbc.co.uk/blogs/wondermonkey/2011/11/bred-to-destruction.shtml