Catastrophic musculoskeletal injuries of the distal limb in Thoroughbred racehorses

Abstract

Catastrophic musculoskeletal limb injuries on the track are disconcerting and unsettling, resulting in the immediate ending of a racehorse s career. Numerous studies on the incidence rates and factors influencing injuries and fatalities have been reported in most of the major racing countries around the world. Limited published data on the incidence rates and factors influencing this on South African racetracks is available. The factors which show potential for increasing the risk of injury are age, gender, track surface type and condition, distance of race, number of previous starts and time interval between races. The objective of this study was to report and describe the incidence of the types of catastrophic musculoskeletal injuries (CMIs) of the distal limb leading to immediate euthanasia in racing Thoroughbreds on four Gauteng racetracks and also to identify their associated risk factors. The case material data for this study was divided into two groups. Group one consisted of data from all the horses that sustained a CMI on a Gauteng racetrack within the study time period of 1998-2012 and totalled 114 in number. This data was used for determining the incidence rate of CMIs per year as well as to obtain a more accurate overview of the type of CMIs seen on tracks under study. Group two consisted of 53 racing Thoroughbred cadaver distal forelimbs, from 52 horses that were euthanized due to sustaining a forelimb fracture or rupture of the suspensory apparatus on a Gauteng racetrack. Each limb had five different radiographic views taken from mid metacarpus 3 (MC3) to distal phalanx 3, centred on the fetlock joint. Full dissections were then conducted on each limb noting the extent of the injuries. These limbs had complete histories and were used for determining all the summary statistics and those relating to the interaction of risk factors. (Approved: Animal Ethics Committee, University of Pretoria. Protocol no. V020/13) The results obtained were as follows: using the data from group 1, the most common location for the 114 CMIs in the study period were the proximal sesamoid bones (PSBs), including the suspensory apparatus, and was represented by 55.26% of the cases. Condylar metacarpal/metatarsal fractures were excluded from this category as they were classed as a separate CMI. The second most common location for CMI was the MC3 bone at 13.16% followed closely by the carpal region at 12.28%. Fractures involving phalanx 1, MC3 condyles and tibia were represented respectively by 6.14%, 4.39% and 2.63% of the cases. Pelvic and scapular fractures were equally represented at 1.75%. The least common region for a CMI was the femur at only 1 reported case out of the 114 horses (0.88%). There was only one reported case that failed to classify the affected area of the horse in the report. Because some of the horses in the study population obtained more than one type of CMI, the total percentage is greater than 100. Using the data from group two, which consisted of the 53 cadaver forelimbs, 74% of all the affected limbs were of the left forelimb and 79% were of male gender, and of those 68% were geldings. Risk factors which were identified as having the strongest potential for being significant in affecting a horse s risk for a CMI were the horse s age, gender, distance raced and the jockey s weight. A total of 203 965 starts was recorded over the fourteen year racing period from 1998-2012 for all four racetracks. The racetrack with the greatest amount of starts over the study period was Vaal with 80 861 starts and the racetrack with the least amount of starts for the study period was Gosforth Park totalling 11 768 starts; this lower number of starts was due to the closing of this racecourse at the end of the 2001 racing year. Comparing the results to international studies the following conclusions were made. Worldwide there is a trend for the left forelimb being injured most commonly regardless of the direction in which the racing and training takes place. It is interesting to note that while many studies have identified this phenomenon very little explanation has been provided as to why it occurs. A suggestion is that perhaps horses, like humans, have a preference for one side and therefore take more weight onto it, regardless of racing direction, and thus have more strain placed onto it. Another possibility is that when racing, more weight is placed on the medial aspect of the outside limb i.e. the left limb when racing clockwise. This would provide an explanation as to the finding here in South Africa, where racing takes place in a clockwise direction, but would not explain why, even when racing anti-clockwise, we still see the left forelimb being the most commonly injured limb. The high number of male horses, in particular geldings, maybe due to them generally having longer racing careers than fillies, mares and colts which may be used for breeding purposes, thus having a shorter racing career. The types of CMI in relation to the factors, especially surface type, are generally on par with what has been reported in other countries. A limitation of this study is the relatively low numbers of limbs in group two of the study population. This study lays the foundation for further research into determining the risk factors for CMI in South Africa.