The results of an inbreeding study published June 29 that examines the impact of inbreeding in Thoroughbreds delivers a word of caution to breeders worldwide and at the same time offers hope.
The study showed a strong correlation with inbreeding in Thoroughbreds that never started in a race, and also identified a genetic marker that could be associated with unsoundness. Through genetic testing, breeders can now avoid producing foals that are homozygous for this gene and significantly improve their chances of becoming successful racehorses.
Scientists at the University College Dublin, the University of Edinburgh, and the Irish equine genetic testing company Plusvital conducted the study, which is titled "Inbreeding depression and the probability of racing in the Thoroughbred horse" and published in the peer-reviewed scientific journal "Proceedings of the Royal Society B."
The sobering aspect of the study's conclusions is a strong correlation observed between inbreeding and Thoroughbreds that never start in a race. Researchers measured the inbreeding in more than 6,000 Thoroughbreds throughout Europe and Australia that were all 5 years old and included 13% that had never raced. In order to exactly identify the DNA segments actually inherited from each horse's sire and dam, the study used DNA analysis to determine the level of inbreeding instead of pedigree analysis.
The analysis showed that for every 10% increase in inbreeding there was a 48% predicted decrease in the chance that a horse would ever start in a race, according to Dr. Emmeline Hill, a professor at the University College Dublin's School of Agriculture and Food Science and co-founder of Plusvital.
In broad terms, inbreeding is a natural function of managing a closed breeding population like Thoroughbreds. It is the method breeders use to concentrate desirable characteristics and ideally weed out undesirable traits. New genetic mutations associated with an undesirable trait, however, can arise and start showing up in greater frequency because of a variety of factors, including an association with certain sire lines, for example.
"The effects of inbreeding that we have seen are certainly not new but have only been revealed by the power of genomic data for the many Thoroughbreds that are contained in this data set," said the study's co-author Dr. Josephine Pemberton, a professor at University of Edinburgh. "These results are a warning sign about the rising trend in inbreeding in Thoroughbreds."
Being able to identify and manage for these desirable traits is what makes the discovery of the genetic markers associated with unsoundness so significant.
"The identification of the single genetic marker with a strong negative effect on racing is good news, because it means that it can be managed," Hill said. "If the 'carrier' status of a mare and stallion are known, this information can be used to avoid producing foals with two copies of the genetic marker. The immediate benefit of this will be economic gains for breeders and racehorse owners and improved animal welfare.
"The knowledge of this genetic marker will allow for testing to reduce the negative impact of inbreeding with the goal to increase the number of foals that are born that race," she added.
The study found 1% of the Thoroughbreds studied were homozygous (possessing one copy from the sire and one copy from the dam) for the gene variant negatively affecting bone development but these horses have a 32% increased probability of never starting in a race. A horse's ability to race is not influenced by the gene if it only has one allele associated with the variant negatively affecting soundness in its DNA. But if it is mated to other carriers of the gene variant, the resulting foals would be at-risk. The study found 10% of observed horses were carriers for the gene variant and determined 15% of the stallions represented in the data set were carriers as well.
The presence of the gene is slightly higher in the American Thoroughbred population, according to Hill. While no American Thoroughbreds were used in the study, independent testing indicates the gene is present in 12.9% of the population.
"It still needs to be examined whether this marker is associated with injury risk, but the data is pointing in that direction," Hill said. "It may appear to be a small number of horses, but it has a considerable impact in the population overall. In the long run, the incidence of this marker in the breed could be reduced if a breed-wide program of testing were to be implemented. This would have a positive long-term effect on the genetic health of the population and would not discriminate against the use of any stallion because it is dependent on the mares, too."
Study co-author Dr. David MacHugh, a University College Dublin professor and Plusvital shareholder, said presence of the gene still leaves breeders with plenty of options.
"There is still enough diversity in the gene pool for breeders to proactively manage breeding decisions and use genetically less related stallions, but because there is so much sharing of genetics due to the popularity of particular sire lines, this is not always evident from the pedigree page," he said. "The best way to assess inbreeding and relatedness is through in-depth analysis of DNA variation across horse genomes."
Hill said the identification of the gene provides a powerful tool for breeders worldwide.
"For Thoroughbred breeders, the breeding goal is to produce viable foals that will have productive racing careers and establish their value by winning races. To do this, horses need to be sound and healthy enough to race," she said.
