Musculoskeletal injuries in Standardbred racehorses on Prince Edward Island

Abstract

A study of musculoskeletal injuries in Standardbred racehorses on Prince Edward Island was conducted to determine the incidence and risk factors for injury. Monthly information was collected from 10 trainers and 153 Standardbred horses over a 12-month period. Seventeen new injuries were identified in pacers, representing a horse level incidence risk of 23% and an incidence rate of injury of 2.19 per horse-month at risk. Forty-one percent of the injuries occurred during a race, 53% during training and 6% during jogging. Fourteen horses had experienced a previous injury and 3 of these horses experienced a new injury during the study period. Potential risk factors for injury were the speed at which the horse was trained, previous injury, and the conformational fault of offset knees.

Introduction

Musculoskeletal injuries are the leading factor causing poor performance and retirement of racehorses. Standardbred racing at a specific gait (trotting or pacing) while pulling a cart around an oval track during training and racing has the potential to cause excessive strain on specific anatomical areas of the limbs (1). This can cause acute or chronic, accumulative, degenerative musculoskeletal injury (2). Injuries are primarily responsible for lost training days and fewer races entered, and contribute to the inability of some horses to start racing at the age of 2 y (3,4). Injuries cause inflammation and mechanical stimulation of nociceptors in the skin, subcutis, fascia, muscle, and periosteum, resulting in pain (5,6). The pain and discomfort from these injuries raise ethical concerns regarding the use of horses in sport, particularly horseracing, as some of the types of injuries that occur are related to the activities associated with racing. If the horses were not used for this purpose, these injuries may not occur to the same extent (4). To maximize performance, horses require a regular exercise regimen to develop the capacity, efficiency, and coordination of their musculoskeletal, respiratory, and cardiovascular systems (7–10). Risk factors for injuries are often a combination of factors including age, gender, conformation, body condition score (BCS), trainer, training schedule, track conditions, and racing (2,11).

There has been limited research on the risk factors for injuries in Standardbred racehorses during training and racing. Comparatively, many studies have investigated Thoroughbred racing which has a considerably higher rate of catastrophic and non-catastrophic injuries compared to harness racing (3). For example, catastrophic injuries in Thoroughbred racehorses in California occurred at a rate of 1.8 injuries per 1000 starts, whereas non-catastrophic injuries occurred at a rate of 36.8 per 1000 starts (12). One study in Standardbreds in Ontario reported that the average mortality rate was 0.631 per 1000 starts and 36% of the 975 deaths resulted from musculoskeletal injuries (13).

This study investigated injuries in Standardbred racehorses on Prince Edward Island (PEI). A better understanding of the types of injuries and the potential factors that could affect their prevalence could be used by trainers and veterinarians to minimize the risk of injury.

Materials and methods

A study of musculoskeletal injuries in a population of 153 Standardbred racehorses on PEI was undertaken. During the 12-month study, 799 individual horse assessments were conducted on these horses. The horses were stabled at a racetrack in either Charlottetown (n = 105), Summerside (n = 11), or at a training center (n = 37). Each horse was studied for a mean of 4.4 mo [range: 1 to 12 mo, standard deviation (SD): 2.34 mo]. A total of 10 trainers were recruited with 8 trainers participating at the start of the project and 2 more trainers starting at month 9. Each trainer had between 1 and 30 horses enrolled in the study and was visited each month during the study to obtain data on their horses. Horses of all ages and sexes were enrolled and followed during their training and racing over a 1-year period from May 2017 to April 2018. An individual assessment was defined as a data collection event for an individual horse. The study was an open design with horses entering and leaving the study based on their time spent with the trainer.

At the initial assessment, the horses were examined to evaluate whether one or more types of conformational faults were present (12). Body condition score (14) was evaluated at the initial assessment, at the midpoint, and at the completion of the study for horses which were enrolled in the full 12 mo of the study. For horses that were enrolled for less than 6 mo, BCS was evaluated at the initial assessment and at the completion of the study unless the horse left the study during the month prior to the visit by the investigator. Trainers were interviewed with a monthly questionnaire that addressed shoeing, training regimen, and race history. All horses in the study were shod, with types of shoes categorized as either steel or not steel (e.g., aluminum) for both the forelimbs and hind limbs. The frequency of being shod was categorized as: the horse was shod less than every 4 wk, or the horse was shod every 4 wk or more. Racing related variables included how frequently the horse was raced and age at first race. The trainers were asked to recall exercise-related injuries that resulted in lameness in each horse during jogging, training, or racing that were sustained during the previous month or whether the horse remained injured from a previous assessment. In all but one case of reported injury, the trainer stated that a veterinarian from the Atlantic Veterinary College (AVC) or another clinic had diagnosed the type and severity of the injury. Other information, such as when the injury occurred, time off from exercise, the track at which the injury occurred, and the potential cause of the injury, was provided by the trainer.

Injury prevalence was recorded as the percentage of horses that were reported to have experienced an injury during the entire study period. The number of individual horse assessments during the study was calculated as the sum of the number of horses that were assessed during each monthly visit. The total time that a horse was recorded as injured was calculated as the sum of the number of assessments during the study period that the horse was still injured. Horse-months at risk was the sum of the number of horses examined that were either jogging, training, and/or racing each month during the 12-month study period and was used to calculate the incidence rate. Injuries included new injuries, injuries that persisted for more than 1 mo, or a reoccurring injury.

Several horse-, training-, and racing-related variables were examined as potential risk factors for injury. Age of the horse and the age at which the horse began training were categorized as 2, 3, and 4 y or older. Gender was classified as female, gelding, or intact male. The gait of the horse was recorded as trotter or pacer. Body condition score was grouped as a binary variable into scores of 3 to 4 or 5 to 6. Each type of conformation fault was grouped into “conformation normal” and “conformational fault present.” Distance jogged per day was grouped into jogged ≤ 6.43 km/d or jogged > 6.43 km/d. Training was categorized as whether the horse was training or not training. The speed during training was grouped into 5 categories: not training, training at ≤ 10.7 m/s, 10.8 to 11.5 m/s, 11.6 to 12.4 m/s, or ≥ 12.5 m/s. The numbers of days off work (i.e., jogging and training) were classified as ≤ 1 d off and ≥ 2 d off. The track where the horse was located and trained was grouped into 3 locations: Charlottetown, Summerside, and training centers that classified multiple tracks into one group. Racing-related variables included frequency of racing and age of first race. The number of times the horse raced in the previous month was recorded and categorized into whether the horse raced or not.

General Estimating Equations models were built to investigate associations of horse, track, and trainer factors with the probability of an injury. An exchangeable correlation structure was used to account for the repeated measurements on each horse. Time (days) that horses were turned out for rest from training was not included in the analysis of days at risk of injury. Only horses that were jogging, training, or racing were included for analysis as they were considered to be at risk of injury. Horses that were injured and were “on rest” due to injury were also included in the analysis to contribute to injury prevalence.

All horses with a reported injury, whether new or reoccurring, were included in the calculation of injury prevalence. If these injuries lasted longer than 1 mo they were reported in subsequent visits. Injury incidence was calculated based on new injuries that occurred during the study period. The time at risk was used to calculate the horse level incidence, which was determined as the number of injury cases observed per 100 horses in 1 mo. The total number of injuries was used to compute the odds ratios (ORs) for potential risk factors. Given the low number of injuries observed, only unconditional associations were evaluated for potential risk factors. All variables with a P-value ≤ 0.10 were considered significant. As a low number of injuries was observed in the study, 2-way interactions were not considered during the model building. Statistics were performed using Stata14 (StataCorp, College Station, Texas, USA).

The project was approved by the UPEI Animal Care Committee and the UPEI Research Ethics Board.

Results

Questionnaires for the first visit took approximately 10 min to complete per individual horse with follow-up visits taking approximately 5 min.

Training program

Horses were jogged 1 to 7 d/wk, with most horses being jogged 4 d/wk and 43% jogged at least 6.4 km/d. In preparation for racing, horses trained 1 to 2 times per wk until they reached racing speed, most commonly 12.9 m/s. At the start of the racing season, many young horses that raced 1 to 2 times per month often had training speeds of 11.6 m/s between races. As horses progressed through the racing season, they seldom trained between races and typically raced once per week. Horses received 1 to 2 d off work each week, frequently the day following racing or training.

Racing program

In this study population, 108 horses raced at least once, with 92 horses beginning to race at 2 y of age and 16 horses beginning to race at either 3 or 4 y of age. Reasons for a later start were that the horse sustained an injury or did not meet training expectations as a 2-year-old. The race season in PEI begins in early May and ends in January, although many horses do not race the entire season. Horses commonly raced 3 or 4 times a month with a minimum of 1 and a maximum of 7 races for a single horse during a single month. Horses competed at either the Charlottetown or Summerside racetrack with some horses occasionally racing in New Brunswick or Nova Scotia.

The type of shoes worn varied depending on the horse’s individual requirements and the season. The most common shoe used was a full swedge steel shoe on 73% of the forelimbs and 71.5% of the hind limbs. These shoes were also most common during the winter months with corks inserted to gain traction on ice-covered tracks. Half-round swedge steel shoes were also common, with fewer horses wearing aluminum shoes (12.3% in forelimbs and 2.4% in hind limbs). The frequency of being shod was lower in the winter months when horses were shod every 6 to 8 wk. Horses were shod on average every 4 wk during racing and training season, though some horses were shod every 2 wk. Horses often had their fore and hind hooves shod at different times, as a result of varying shoe wear.

Injuries

Fourteen (9%) horses had been injured prior to the start of the study. New injuries were recorded in 45 of 804 individual assessments, resulting in an assessment level prevalence of 5.6%. Seventeen horses experienced new injuries with an estimated incidence risk of 23% and a rate of 2.19 cases per horse-month at risk or 2.19 cases per 100 horses in a month. Of the injuries, 41% were racing-related, 53% occurred during training, and 6% occurred during jogging. One injury reoccurred 4 mo after the horse was initially injured, and of the 14 horses that were previously injured, 3 experienced new injuries. Three of the injured horses were removed from training and continued jogging after a few days of rest. The total time horses in the study spent injured before returning to training was 549 d.

Type of injury

The types of new injuries recorded are shown in Table 1. Four injuries were classified as “unknown,” as once the horse was injured, it left the trainer’s stable. The time off from training was dictated by the type and severity of the injury and was between 3 and 190 d for mild to moderate injuries, 8 mo for a proximal phalanx fracture with surgical repair and career-ending for 1 horse with a sesamoid fracture (Table 1). Maladaptive syndrome of the 3rd carpal bone was diagnosed by a veterinarian based on clinical examination and radiographs.

Table 1.

The number and type of new injuries recorded over 12 months in Standardbred racehorses on Prince Edward Island (2017 to 2018).

Type of injury	Anatomical location/ type of injury	Severity	Horses with a new injury (n = 17)
Soft tissue (n = 8)	Superficial digital flexor tendon	Mild and moderate	3
	Deep digital flexor tendon	Moderate	1
	Suspensory ligament	Mild and severe	2
	Check ligament	Mild	1
	Undiagnosed	Mild	1
Bone (n = 7)	Sesamoiditis	Mild and moderate	3
	Sesamoid fracture	Severe	2
	P1 fracture	Severe	1
	Maladaptive syndrome	Mild	1
Joint (n = 2)	Middle carpal joint	Severe	1
	Metacarpophalangeal joint	Mild	1

Factors affecting the prevalence of injuries

Individual horse information and incidence of new injuries are listed in Table 2. Conformational faults and the incidence of new injuries are listed in Table 3. New injuries were present exclusively in pacers. Seven trainers had horses with injuries, with 2 trainers having 5 injured horses. Fifty-nine percent of the injured horses experienced an injury in the left forelimb (17.5% in the right forelimb, 17.5% in the left hind limb, and 6% in both hind limbs). No horses experienced an injury in the right hind or both forelimbs.

Table 2.

The total number and percentage of Standardbred racehorses on Prince Edward Island (2017 to 2018), and the number of horses with new injuries and percentage of horses with a new injury in each category.

Horse characteristic	Category	Total number of horses assessed, N = 153 (%)	Total number of horses with new injuries, N = 17 (%)
Age (years)	≤ 2	72 (47%)	8 (47%)
	3	42 (27%)	5 (29%)
	≥ 4	39 (25%)	4 (24%)
Body condition score	3 to 4	44 (29%)	5 (29%)
	5 to 6	109 (71%)	12 (71%)
Gait	Pacer	131 (86%)	17 (100%)
	Trotter	22 (14%)	0
Gender	Female	73 (48%)	11 (65%)
	Gelding	66 (43%)	5 (29%)
	Male	14 (9%)	1 (6%)

Table 3.

The number of new injuries recorded for each type of conformation fault in Standardbred racehorses on Prince Edward Island (2017 to 2018).

Type of conformation fault	Category	Total number of horses, N = 153 (%)	Total number of horses with a new injury, N = 17 (%)
Toed out	Absent	51 (33%)	7 (41%)
	Present	102 (67%)	10 (59%)
Back at knee	Absent	147 (96%)	16 (94.1%)
	Present	6 (4%)	1 (5.9%)
Over at knee	Absent	132 (86%)	16 (94%)
	Present	21 (14%)	1 (6%)
Offset knees	Absent	67 (44%)	3 (18%)
	Present	86 (56%)	13 (76%)
Camped under up front	Absent	68 (44%)	10 (59%)
	Present	85 (56%)	7 (41%)
Camped out behind	Absent	117 (76%)	16 (94%)
	Present	36 (24%)	1 (6%)
Base wide	Absent	140 (92%)	16 (94%)
	Present	13 (8%)	1 (6%)
Base narrow	Absent	125 (82%)	14 (82%)
	Present	28 (18%)	3 (18%)
Pastern hoof axis	Absent	40 (26%)	3 (18%)
	Present	113 (74%)	14 (82%)
Low heel	Absent	67 (44%)	7 (41%)
	Present	86 (56%)	10 (59%)
Long toe	Absent	80 (52%)	9 (53%)
	Present	73 (48%)	8 (47%)
Sickle hock	Absent	99 (65%)	12 (71%)
	Present	55 (36%)	5 (29%)
Cow hocked	Absent	91 (59%)	8 (47%)
	Present	62 (41%)	9 (53%)
Pelvic asymmetry	Absent	116 (76%)	11 (65%)
	Present	37 (24%)	6 (35%)

There were 11 risk factors that had an unconditional association with new injury occurrence with a P < 0.2. Due to the small sample size and few injuries recorded during the study period, only predictors with an unconditional association of P ≤ 0.1 were evaluated. The risk factors of interest included: training and racing factors, season, previous injury, and conformation. The conformational fault with the highest associated risk of injury was offset knees (Table 4).

Table 4.

Unconditional associations from logistic regression models between conformational faults and the occurrence of injuries in a population of 153 Standardbred racehorses on Prince Edward Island (2017 to 2018).

Conformation fault	Odds ratio	P-value	95% Confidence interval
Offset knees	4.810	0.007	1.625 to 7.554
Pelvic asymmetry	2.360	0.101	0.846 to 6.620
Camped out behind	0.160	0.074	0.021 to 1.197

All categories of jogging were associated with increased odds of a new injury, with 5 jogging days having the highest odds (OR = 2.70). Horses training at speeds similar or close to racing times of 2:10 min/mile (12.40 m/s) presented the highest risk for injury (OR = 3.90). The risk of having an injury was lower in horses that competed in a race (OR = 0.41). Horses from the Summerside track were more likely to have injuries (OR = 2.58) than horses jogging and training at the Charlottetown track (Table 5). Horses that experienced an injury prior to the study were more likely to experience another injury [OR = 3.29, 95% confidence interval (CI): 1.282 to 8.500, P = 0.013]. The probability that season affected the risk of injury was P = 0.100, with possibly greater risk of injury during the summer and fall (Table 6).

Table 5.

Unconditional associations from logistic regression models between exercise-related factors and track location and the occurrence of injury in a population of 153 Standardbred racehorses on Prince Edward Island (2017 to 2018).

Exercise-related factors	Category	Odds ratio	P-value	95% Confidence interval
Number of jogging days per week	≤ 3	—	0.090
	4	1.910		0.621 to 5.861
	5	2.710		0.845 to 8.849
	≥ 6	1.070		0.285 to 4.047
Is the horse racing	No	—	0.020
	Yes	0.410		0.193 to 0.873
Speed of training (m/s)	Not training	—	0.002
	< 10.8	0.590		0.261 to 1.352
	10.8 to 11.5	0.960		0.258 to 3.603
	11.6 to 12.4	3.350		1.195 to 9.432
	> 12.4	3.890		1.551 to 9.778

Table 6.

Unconditional associations from logistic regression models between season and the occurrence of injury in a population of 153 Standardbred racehorses on Prince Edward Island (2017 to 2018).

Variable	Category	Odds ratio	P-value	95% Confidence interval
Season	Winter	—	0.100
	Spring	0.560		0.187 to 1.684
	Summer	2.770		0.653 to 12.005
	Fall	3.190		0.953 to 10.782

Discussion

The prevalence of injury in Standardbred racehorses on PEI was lower than in some reports for horses used for harness racing (3) but of similar magnitude to the 10% reported in a retrospective trainer-reported survey in New Zealand (15). Differences in reported prevalence could be due to regional differences in training and racing. The trainers who participated in this study may also have been experienced and able to recognize a horse at risk of injury and to modify their training and racing schedule before an injury occurred or became clinically significant. There could also have been problems with the trainers’ recollection of events and difficulties in the recognition of musculoskeletal injuries (15).

As previously reported in the literature, most injuries occurred in the left forelimb and fewer injuries were recorded in the hind limbs (16). More injuries to the left forelimb were likely the result of increased pressure on the left forelimb when racing in a counterclockwise direction. This is supported by a study reporting that the left fore fetlock had a higher temperature than the right fore fetlock when racing in a counterclockwise direction (1,16).

The relative occurrences and types of injuries in the current study were similar to those in a study in Italy (3). Injury to the superficial digital flexor tendon (SDFT) was the primary injury in PEI and was the second most observed soft tissue injury in the Italian study. Injury to the suspensory ligament only occurred in 1.3% of the 153 horses in our study compared to 21% of the 429 horses in the Italian study (3). A study on UK Thoroughbred racehorses reported that injury to the SDFT accounted for 89% of soft tissue injuries in Thoroughbreds (2). This indicates that SDFT injuries are common in both disciplines; however, the incidence has been reported to be as low as 7% in Standardbred racehorses (2,3). Standardbreds have a better prognosis than Thoroughbreds for soft-tissue injuries such as suspensory ligament desmitis and superficial digital flexor tendonitis, and generally, the prognosis for return to performance is higher in pacers than in trotters (17). The occurrence of joint injuries was lower than expected based on the 30 to 40% frequency reported in the literature (11,18). Early medical intervention with intra-articular medications may have reduced inflammation associated with synovitis and accounted for fewer joint injuries.

Injury to tendons such as the SDFT can occur acutely due to a traumatic event; however, many occur as a result of degenerative changes caused by accumulative strain during locomotion. The SDFT and suspensory ligament are especially vulnerable due to the biomechanics of stored kinetic energy which is released as these structures stretch and recoil during each stride taken by the forelimb (19). In one study, lameness in Danish trotters was mostly related to carpal and fetlock problems (20). Injury to the carpal joints can occur during racing due to chronic over-extension of the joint. Factors such as speed of exercise and conformation defects can predispose to this type of injury (21,22). Other studies have shown a higher prevalence of injuries; it was difficult to ascertain why this occurred. The biomechanical differences in gait between pacing, trotting, and galloping may have contributed to the lower prevalence of injuries in this population of Standardbred pacers compared to other studies, as most studies have been performed on trotters and Thoroughbreds. The Italian study on trotters was performed over a 4-year period which would have increased the likelihood of capturing injuries in that population of horses compared to our study in which horses were only studied on average for 4.4 mo. One of the challenges in our study was the mobility of horses within the industry and how frequently they changed trainers and left the study. This study did not capture any injuries in trotters, which may reflect the lower numbers of trotters enrolled in the study or the trotters may have been sounder, had more correct conformation, and, therefore, fewer risk factors.

The sample size and low occurrence of injuries restricted the type of analysis that could be conducted to identify potential risk factors. An increased sample size and multivariable analyses would have been able to account for relationships among potential risk factors. Only guarded inferences can be made from the ORs identified by univariate analysis. Standardbreds with a high body condition score (BCS) can have poorer performance compared to those with a lower score, but no effect on gait has been reported (23), and no significant effect on the risk of musculoskeletal injury was identified in this study. This might have been due to lack of power or confounding.

Conformational faults can cause unequal distribution of forces in the limbs, which may predispose a horse to injury when combined with the physical stress of exercise during training and racing. In one study in Sweden, trotters with “curby or sickle hocks” and an angled fetlock were at greater risk of injury than horses without these faults (24). In the current study, horses with offset knees or pelvic asymmetry were more likely to experience an injury compared to horses with correct conformation. Six of the 43 horses with pelvic asymmetry experienced an injury. One study noted that Standardbreds with pelvic asymmetry more often had joint capsule distension in the left intercarpal, left stifle, and hock joints than horses without this fault (25). The higher percentage of horses with offset knees is likely associated with the restricted population of Standardbreds used in this study. Most of the horses in the study were bred and raised locally; therefore, reduced genetic diversity may have contributed to the higher percentage of offset knees in this population due to a limited number of stallions and mares used for breeding. Misclassification of conformation by the investigator can also occur, and confounding effects caused by the types of shoes, frequency of being shod, and the farrier, could be present. Horses with poor conformation may need corrective shoeing and more frequent shoeing due to excessive shoe wear.

The number of jogging days per week was a potential risk factor for injury, especially when the number of jogging days per week increased from 3 to 5 d. Horses jogging 5 d/wk were likely training or racing and had 1 or 2 d of rest per week. Horses that jogged 6 d/wk had a lower risk of injury, suggesting minimal impact when horses were only jogging and not in heavy training or racing.

The risk of injury likely increased with faster training times as exercise-related factors are known to be risk factors for injury. A New Zealand study reported that 48% of injuries were training related (15), which is supported by this study in which 53% of injuries were related to training. Repeated exercise can result in more articular cartilage erosion and palmar metacarpal arthroses in metacarpophalangeal joints than in control horses (26). Trainers could have impacted several potential risk factors as they dictate training and racing schedules and shoeing practices.

The age at which most horses started racing was 2 y compared to 3 y reported in a survey published in 1985 in which horses that started racing at 2 and 3 to 4 y of age had an active racing career that lasted 5 and 4 y, respectively (27). Racing in the fall months apparently had a higher risk for injury, possibly due to younger horses beginning to race in the summer months and culmination of wear and tear on the musculoskeletal system throughout the racing season. Differences between tracks due to factors such as footing, banking, or maintenance of the track could potentially affect the risk for injury.

Horses with a previous injury are likely to be at a greater risk of injury; however, in some cases, it can be difficult to determine the extent of a previous injury when a full history has not been disclosed to the trainer, which is a common situation with horses competing in claiming races. A horse with a previous injury could have a modified training regimen to decrease training intensity by slowly increasing the speed of training with a longer period between training days. A previous history of injury may also motivate a trainer to be more observant of subclinical signs of inflammation, thereby preventing a more significant injury from developing.

Once risk factors for injury are identified, it is the expectation that trainers can modify training regimens to mitigate some of these risk factors. Exercise-related risk factors have the greatest potential for modification, whereas conformational defects cannot be adjusted. Offset knees were present in 56% of Standardbreds in this study which may be a consideration when purchasing Standardbred yearlings, as they may be at a greater risk of injury.

This study had several limitations, including the few injuries observed, a young population of horses, and a small sample size. It is also possible that trainers did not recall all injuries that occurred within the last 30 d or trainers provided inaccurate information since medical records were not available if the horse was not a patient of the AVC.

Standardbred horse racing has economic and cultural importance in Canada. As the industry has grown, there has been an increase in the amount of wastage. Concern for the welfare of horses increases with a greater prevalence of injuries and associated pain. Injuries in horses that are exercise-related are inevitable; however, they can be mitigated with proper management. An Australian study on exit rates in both Thoroughbred and Standardbred racehorses indicated that 27% left due to injury (28). These horses are no longer useful as racing horses within the industry and determining the cause of injury may provide information on how to lower the wastage seen and improve the overall welfare for these horses. Identifying risk factors for injury as well as early identification of the causes of poor performance could improve the management of these horses, potentially decrease the number of horses exiting the sport, and improve the welfare of those horses remaining in the industry. Further studies are required to identify risk factors for injury.