A study published Jan. 12 in Equine Veterinary Journal shows strong potential for the development of a blood test that could indicate whether a racehorse is at risk for a catastrophic breakdown.
From an initial pool of 21 genes identified by previous research in horses and humans for their roles in encoding proteins associated with inflammation, bone repair and remodeling, tissue repair, and general response to injury, researchers at the University of Kentucky's Maxwell H. Gluck Equine Research Center were able to zero in on three genes that showed significantly different activity between injured and non-injured horses.
"We had hoped for maybe one gene and that we got three when we only looked at 21, we were impressed, especially when we had to throw out half because they might have been affected by racing," said Dr. Allen Page, co-author of the study and a scientist and veterinarian at the Gluck Center.
Going into the study, researchers knew that strenuous exercise can induce an inflammatory response, so an initial comparison of pre-race and post-race samples allowed researchers to winnow the gene prospects from 21 down to nine.
"Taking into account jurisdiction of collection, sex, age at collection, and category of race (claiming, allowance, or stakes race) as covariates, only three of the nine genes (IGF1, IL1RN, and MMP2) demonstrated significant differences between catastrophically injured and non-injured control horses," the study stated.
The project involved the collection and analysis of 686 blood samples obtained at racetracks in five states. The samples were from 107 catastrophically injured and 579 non-injured horses. The catastrophic injuries included fractured sesamoids, carpal and metacarpal/metatarsal fractures, and mixed fetlock fractures. Mixed fetlock and sesamoid-only fractures accounted for 70% of the injuries in the study, which is consistent with what has been seen industry-wide.
While the study noted no significant effect of race category on the occurrence of catastrophic injuries, the largest number of catastrophic injuries involved horses in claiming races. Other research has shown an increased catastrophic injury risk for claiming horses. Older horses have previously been associated with an increased risk for catastrophic injuries, too, but in this study, the largest number of catastrophic injuries was among horses 2-4 years old (representing nearly 76% of all injuries), while horses 5 and older represented only 24% of the injured population. The representation by age may be due to an over-representation of younger horses (2-4 years old) in the entire study population (70%), the paper noted, because age was not shown to be a potential risk factor for injury.
Among the three genes identified, IGF1 and MMP2 showed increased activity related to inflammation. The IL1RN gene, however, which has potent anti-inflammatory properties, showed decreased activity. Because IL1RN generates a strong anti-inflammatory response, its low activity suggests a "pro-inflammatory" state within the horse, according to Page.
"It is a mirror image of the inflammatory response. When anti-inflammatory markers go down, it can also be a signal of more chronic inflammation," he said.
Also significant with IL1RN is that its decreased expression was found only among horses with sesamoid fractures.
"The finding that only (proximal sesamoid bone) fracture cohorts had significantly lower expression of IL1RN when compared to non-injured controls suggests a possible use for this gene with respect to identifying those horses specifically at risk for a PSB fracture," the study stated.
About the potential for a blood test to better identify at-risk horses, Page said this study is a good first step.
"We really see a blood test as a decision tool. We don't see this as the be-all, end-all as to whether a horse is at-risk or not for an injury," he said. "This is a way to look at a large group of horses and identify the majority of horses that may be at risk for an injury for follow-ups. That may be advanced imaging. We have heard more about CT and MRI scans and now PET scans. None of them, however, are well-situated to quickly image a lot of horses, but we can run through hundreds of samples in a day and be able to identify a good portion of the horses at risk for injury."
Page estimated future blood tests could cost $60-$85 depending on the number of genes being evaluated.
"We have a ways to go but this is an excellent start," Page continued. "We are happy with just those three markers—and not taking anything else into account—correctly identifying 76% of horses that were at risk. That is a huge improvement over what we have currently."
The next step in this research is to identify all the protein-encoding genes in the horse genome, which has just begun with a project funded by the Kentucky Horse Racing Commission.
"That is predicted to be about 22,000 genes, so it is a huge undertaking that requires some high-level computer analysis. Thankfully we have several people in the department who can do that," Page said. "We anticipate adding to the list of three genes we have, by at least double but probably much more."
Another goal of future research will be to collect a much larger number of pre-race and post-race samples, at least 10,000, that can be banked and analyzed when an injury occurs.
"We can then look to see whether what we have reported so far holds up with what we are seeing pre-race. We have every confidence it will," Page said. "We have made huge strides in the last few years toward decreasing injuries but they still occur with some regularity, so anything we can do to further improve the welfare and safety for these athletes, both horses and jockeys, is important."
