Risk Factors

Much research has already focused on the identification of risk factors for musculoskeletal injury in the Thoroughbred. Risk factors are classified at the level to which they pertain. For example, horse age would be a horse-level or start-level risk factor. Horse age obviously changes from year to year as the racehorse progresses through its career, but essentially remains an attribute of the horse itself. Likewise, the previous racing history of a horse every time it enters a race would be described as a horse-level or start-level risk factor, whereas environmental influences such as racing surface and race distance are attributes of the race and are therefore called race-level risk factors. Use of particular training regimens or training surfaces would be termed trainer-level risk factors.

Horse-level risk factors make an important contribution to the variation in likelihood of injury or fatality. Anecdotally, many racetrack veterinarians look at a horse’s racing profile and suspect that the horse may be at greater risk for injury. A horse that is returning from a long lay-off may be one such candidate. Some studies have attempted to quantify the risk for injury in a race given particular racing intensity profiles. But the picture is never clear, and confusing combinations of the number of starts over different time periods make it very difficult to interpret these outputs or use them in a prospective manner when assessing the risk for an individual horse before a race.

History of a previous injury is certainly a useful indicator of the risk of fracture, suspensory ligament injury, or superficial digital flexor tendon injury. Multiple studies have revealed that having had a previous injury during racing significantly increases the risk for that structure being injured again, often with more severe consequences. However, it is only in a very few racing jurisdictions that a veterinarian has access to the full clinical records of horses that he or she is trying to assess for risk for injury. In the vast majority of situations, full medical records (and indeed, treatment records) are not available to the racetrack practitioner. More often than not, the only medical records available are those pertaining to injuries incurred during previous appearances on the racecourse. This deficit in data availability and potential source of bias should be addressed in future studies to enable more accurate modeling of the relationship between previous injury and severe breakdown.

Race-level or start-level risk factors that are most frequently examined and quoted include those associated with the racing surface and exercise intensity. Firmer turf, dirt surface, and poorly maintained all-weather tracks are often quoted as surfaces to avoid when there is concern about injury. Numerous studies have confirmed the association with firm turf tracks and the risk for fracture, tendon injury, or fatality in general. Dirt tracks have a consistently higher risk for injury than turf or synthetic tracks in North American racing, where they are commonly used. However, it is all too easy to ascribe the blame for this higher risk on the composition of the racing surface. There are often significant differences in the race profiles and horse types that compete on dirt tracks and all-weather or synthetic tracks, which could contribute to an increased risk for injury by way of horse-level attributes.

There is good evidence relating to the association between increased amounts of high-speed exercise and the risk for several types of musculoskeletal injuries. In a series of papers from researchers using data from the California Horse Racing Board postmortem examination program, more high-speed exercise was associated with an increase in risk for suspensory apparatus failure, condylar fracture of the third metacarpus, and proximal sesamoid bone fracture. These findings are consistent with the hypothesis that horses engaged in this type of exercise are also accumulating subclinical or clinical bone damage that can result in a catastrophic outcome. Structural changes in the distal condyles of MCIII and MTIII of horses in race training have been observed, and more specifically, the subchondral bone of this region undergoes an adaptive response to high-speed exercise (see Chapters 192 and 202).

At the other end of the exercise intensity scale, doing no high-speed work while in training significantly increases the risk for fracture during racing. The bones of the horse doing no high-speed exercise likely have not been optimally adapted to the loads they experience under racing conditions, and they are therefore more likely to fracture. However, it is also possible that the associations between reduced or zero exercise and increased risk for fracture are an example of effect rather than cause. In other words, horses with subclinical injury are unable to train to the same extent as the rest of the population, and it is the subclinical injury itself and not the reduced exercise that increases the likelihood of the severe final outcome when racing. This phenomenon is commonly referred to as the “healthy horse effect” and most frequently manifests as a reduced risk for injury in horses that have raced frequently and recently. In other words, such horses are “able” to race because they are healthy, but this does not imply that racing frequently in a relatively short space of time is in some way causally protective.

Identification of modifiable risk factors is important if epidemiologic studies are to influence the prevention of injury. Previous recommendations have included increasing moisture of the track surface, use of prerace inspections, early detection of subclinical injuries through veterinary examinations, and modifications of exercise speed and intensity.