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Risk factors associated with fatal injuries in Thoroughbred racehorses competing in flat racing in the United States and Canada

Stamatis P. GeorgopoulosWeipers Centre Equine Hospital, School of Veterinary Medicine, College of Medical, Veterinary.

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 MEng, MSc
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Tim D.H. ParkinLife Sciences, University of Glasgow, Glasgow G61 1QH, Scotland.

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Abstract

OBJECTIVE To identify risk factors associated with fatal injuries in Thoroughbred racehorses in the United States and Canada.

DESIGN Retrospective study.

ANIMALS 1,891,483 race starts by 154,527 Thoroughbred racehorses at 89 racetracks in the United States and Canada from 2009 to 2013.

PROCEDURES Data were extracted from the Equine Injury Database, which contained information for 93.9% of all official flat racing events in the United States and Canada during the 5-year observation period. Forty-four possible risk factors were evaluated by univariate then multivariable logistic regression to identify those that were significantly associated with fatal injury (death or euthanasia of a horse within 3 days after sustaining an injury during a race).

RESULTS 3,572 race starts ended with a fatal injury, resulting in a period incidence rate of 1.9 fatal injuries/1,000 race starts. Twenty-two risk factors were significantly associated with fatal injury. Risk of fatal injury was greater for stallions than for mares and geldings and increased as the number of previous nonfatal injuries and race withdrawals and level of competitiveness (eg, horse's winning percentage and race purse) of the horse or race increased.

CONCLUSIONS AND CLINICAL RELEVANCE Results identified several risk factors associated with fatal injuries in Thoroughbred racehorses. This information can be used as a guideline for the identification of racehorses at high risk of sustaining a fatal injury and in the design and implementation of preventative measures to minimize the number of fatal injuries sustained by horses competing in flat racing in the United States and Canada.

Abstract

OBJECTIVE To identify risk factors associated with fatal injuries in Thoroughbred racehorses in the United States and Canada.

DESIGN Retrospective study.

ANIMALS 1,891,483 race starts by 154,527 Thoroughbred racehorses at 89 racetracks in the United States and Canada from 2009 to 2013.

PROCEDURES Data were extracted from the Equine Injury Database, which contained information for 93.9% of all official flat racing events in the United States and Canada during the 5-year observation period. Forty-four possible risk factors were evaluated by univariate then multivariable logistic regression to identify those that were significantly associated with fatal injury (death or euthanasia of a horse within 3 days after sustaining an injury during a race).

RESULTS 3,572 race starts ended with a fatal injury, resulting in a period incidence rate of 1.9 fatal injuries/1,000 race starts. Twenty-two risk factors were significantly associated with fatal injury. Risk of fatal injury was greater for stallions than for mares and geldings and increased as the number of previous nonfatal injuries and race withdrawals and level of competitiveness (eg, horse's winning percentage and race purse) of the horse or race increased.

CONCLUSIONS AND CLINICAL RELEVANCE Results identified several risk factors associated with fatal injuries in Thoroughbred racehorses. This information can be used as a guideline for the identification of racehorses at high risk of sustaining a fatal injury and in the design and implementation of preventative measures to minimize the number of fatal injuries sustained by horses competing in flat racing in the United States and Canada.

Limited research was conducted to quantify risk factors associated with race injuries in racehorses until the 1990s. Injuries were generally attributed to bad racetrack conditions, typically on the basis of opinion or hearsay rather than objective facts.1 Furthermore, associations between various risk factors and injuries were usually evaluated with only univariable analyses, and the use of multivariable techniques that can account for confounding was largely neglected. Age,2–6 sex,2,3,7,8 race distance,4,5,7,9–12 racetrack surface type and conditions,4,6,10,11,13 race type,3–6,12,14 field size (number of horses in race),10,11 season,5 prior racing history,4,5,7,9–11 distance galloped during training,5,9,13,15–18 exercise history,11,13,16,18 prerace inspection by regulatory veterinarians,19 and horseshoe characteristics20,21 have been associated with fatal injuries in Thoroughbred racehorses in previous studies.

The Jockey Club was established in New York in 1894.22 Its mission is to improve Thoroughbred breeding and racing primarily in the United States, Canada, and Puerto Rico.22 In 2008, The Jockey Club initiated the EID.22 The purpose of the EID is to identify the frequency, types, and outcomes of racing injuries in Thoroughbred racehorses competing in flat racing in a standardized format so that valid statistics can be generated in the hope that factors associated with specific injuries can be identified and appropriate measures implemented to prevent such injuries and improve the safety of Thoroughbred racing.22 The EID contains information for most Thoroughbred races that take place in the United States and Canada and serves as a near-census collection of available data. Despite differences in geography and Thoroughbred racing practices among countries and racing jurisdictions, we believe that the EID database can be used to identify risk factors associated with fatal injuries, and that information can be used to improve racing conditions in the United States and Canada as well as other Thoroughbred racing jurisdictions.

The purpose of the study reported here was to identify risk factors associated with fatal injuries in Thoroughbred racehorses competing in flat racing in the United States and Canada. Specific factors such as age, sex, race distance, racetrack surface type and conditions, race type, field size, and prior racing history that were associated with fatal injuries in racehorses in previous studies2–21 were evaluated by use of multivariable analysis.

Materials and Methods

Data collection

Data were obtained from the EID for the 5-year period from 2009 through 2013. Race start data were provided by The Jockey Club and included information for all racetracks that voluntarily contributed data to the EID during the years of interest. The race starts reported to the EID represented the starts for 93.9% of all official Thoroughbred racing events in the United States and Canada during the 5-year observation period. Injury reports were submitted to the EID by veterinarians at the participating racetracks. The EID database also contained information for approximately 11,000 anonymized trainers and 3,000 anonymized jockeys associated with the recorded races, which enabled us to analyze trainer- and jockey-related risk factors. Because the EID did not contain information regarding extent of training exercise, we used the number of races started by a horse prior to the race of interest as a proxy for cumulative training exercise. Also, the EID did not contain information regarding prerace inspection by regulatory veterinarians; therefore, we used the number of times a horse had been withdrawn from a race (scratched) prior to the race of interest as the proxy for prerace regulatory inspection.

Data analysis

The unit of analysis for the study was race start. For each race start, horses that died or were euthanized within 3 days after sustaining an injury during a race (ie, fatal injury) were classified as cases and all other horses were classified as controls. All official race starts were used to calculate historical information for each horse, but only the 1,891,483 official starts from the racetracks that voluntarily reported injuries to the EID were used in the regression analyses. The outcome of interest, or outcome variable, for all regression analyses was fatal injury. Forty-four plausible risk factors, or independent variables, for fatal injury were selected for consideration in our analysis. Initially, the respective associations between each risk factor and the outcome variable were assessed by use of univariate logistic regression. Only horses that had been competing in flat racing for at least 6 months were included in the analyses to assess risk factors that summarized historical racing information prior to the race of interest; this facilitated the assessment of those relationships because data for horses that had only begun racing within the preceding 6 months were excluded from those analyses.

Risk factors with values of P < 0.20 on univariate analysis were eligible for inclusion in a multivariable logistic regression model. A threshold of P < 0.20 was chosen to prevent exclusion of a potentially significant risk factor that only becomes evident when a confounder has been controlled for in a multivariable analysis.23 An automated stepwise selection process was used to build the multivariable model. Potential confounders were identified by use of a forward bidirectional elimination approach and assessment of the AIC. The AICs for competing models were compared, and the model with the lowest AIC was preferred.24 Only risk factors with values of P < 0.05 were retained in the final model.

The potential effect of horse in the data analyses was evaluated by creating a mixed-effects model that included horse as a random effect. Results were nearly identical (< 0.01 change in ORs and no meaningful changes in P values) to results obtained with models that did not include random effects. Therefore, only results for models based on race starts without inclusion of horse as a random effect are reported.

Results

Between 2009 and 2013, Thoroughbred flat races took place at 144 racetracks, of which 37 had < 1,000 race starts recorded during the observation period. Collectively, data were extracted from the EID for 154,527 Thoroughbred racehorses that competed in 234,577 races at 89 racetracks. Those horses accounted for 1,891,483 race starts, of which 3,572 resulted in a fatal injury; thus, there were 1.9 fatal injuries/1,000 race starts. Univariate logistic regression results of the respective associations between the selected risk factors and the incidence of fatal injury for all horses (Table 1) and only those horses that had been competing in flat racing for ≥ 6 months (Table 2) were summarized.

Table 1—

Results of univariate logistic regression for assessment of risk factors associated with fatal injuries in Thoroughbred racehorses competing in flat racing in the United States and Canada during the 5-year period from 2009 to 2013.

Risk factorControlCaseOR (95% CI)P value
Sex
 Mare or gelding1,648,276 (87)2,986 (84)Referent
 Stallion239,635 (13)586 (16)1.35 (1.24–1.48)< 0.001
Age (y)4.516 (4.514–4.519)4.596 (4.546–4.646)1.034 (1.013–1.056)0.002
Age at first race start (y)3.428 (3.426–3.429)3.514 (3.473–3.554)1.055 (1.029–1.082)< 0.001
Country
 Canada155,510 (8)188 (5)Referent
 United States1,732,401 (92)3,384 (95)1.616 (1.395–1.872)< 0.001
On vet list
 No1,877,766 (99)3,531 (99)Referent
 Yes10,145 (1)41 (1)2.149 (1.579–2.926)< 0.001
First race start
 No1,740,519 (92)3,375 (94)Referent
 Yes147,392 (8)197 (6)0.689 (0.597–0.796)< 0.001
Field size (No. of horses)8.521 (8.519–8.524)8.548 (8.486–8.611)1.007 (0.990–1.024)0.404
Finish percentage of previous race (%)54.047 (54.025–54.069)52.470 (51.963–52.977)0.993 (0.991–0.995)< 0.001
No. of months in racing13.254 (13.237–13.274)13.178 (12.780–13.576)0.100 (0.997–1.002)0.711
No. of previous injuries0.0242 (0.0240–0.0244)0.0414 (0.0343–0.048)1.612 (1.388–1.872)< 0.001
No. of previous scratches1.246 (1.243–1.248)1.369 (1.304–1.434)1.030 (1.015–1.046)< 0.001
Odds at start of race17.571 (17.541–17.602)15.266 (14.642–15.890)0.994 (0.992–0.996)< 0.001
Odds rank in race4.761 (4.757–4.765)4.375 (4.287–4.463)0.946 (0.934–0.958)< 0.001
Starting (post) position4.761 (4.757–4.765)4.801 (4.712–4.890)1.006 (0.993–1018)0.374
Low purse race (≤ $7,500)
 No1,585,978 (84)3,029 (85)Referent
 Yes301,933 (16)543 (15)0.942 (0.859–1.032)0.197
Purse ($)23,905 (23,920–23,989)21,011 (19,467–22,555)0.998 (0.997–0.999)0.001
Change in purse since previous race
 No change851,954 (45)1,528 (43)Referent
 Decrease438,357 (23)959 (27)1.220 (1.125–1.322)< 0.001
 Increase382,306 (20)693 (19)1.011 (0.924–1.106)0.817
 Sharp decrease116,107 (6)223 (6)1.071 (0.930–1.233)0.340
 Sharp increase99,187 (5)169 (5)0.950 (0.810–1.114)0.527
Race distance (fur)6.703 (6.701–6.705)6.559 (6.515–6.603)0.919 (0.896–0.943)< 0.001
First start with new trainer
 No1,710,891 (91)3,155 (88)Referent
 Yes177,020 (9)417 (12)1.277 (1.153–1.415)< 0.001
Training with first trainer
 No826,388 (44)1,737 (49)1.216 (1.134–1.298)< 0.001
 Yes1,061,523 (56)1,835 (51)Referent
Time with same trainer (mo)6.368 (6.356–6.379)5.542 (5.300–5.784)0.986 (0.982–0.991)< 0.001
No. of days between race starts since change of trainer3.144 (3.085–3.204)1.979 (0.526–3.432)0.999 (0.998–1.000)0.097
First start with new jockey
 No914,758 (48)1,639 (46)Referent
 Yes973,153 (52)1,933 (54)1.109 (1.038–1.184)0.002
Time with same jockey (mo)0.845 (0.842–0.848)0.785 (0.726–0.844)0.985 (0.968–1.002)0.083
Racetrack surface
 Synthetic235,878 (12)289 (8)Referent
 Off-dirt*248,168 (13)528 (15)1.737 (1.504–2.005)< 0.001
 Dirt1,152,404 (61)2,350 (66)1.664 (1.473–1.881)< 0.001
 Turf251,461 (13)405 (11)1.315 (1.130–1.529)< 0.001

Values for controls and cases were the mean (95% CI) or the number (%) of horses for all race starts. Data were obtained from the EID. A fatal injury was defined as an injury sustained during a race that resulted in the euthanasia or death of the horse within 3 days. There were 1,891,483 official race starts by 154,527 horses; 3,572 starts resulted in a fatal injury (cases), all other starts were considered controls. On vet list refers to horses that were placed by a veterinarian on the vet list and entered a race while on the list. Finish percentage in previous race was calculated as the finish position of a horse/field size of the previous race (ie, a horse that finished fourth in a field of 8 would have a finish percentage of 50%, whereas a horse that finished eighth in a field of 8 would have a finish percentage of 100%). Within a variable and category (control or case), percentages may not total 100 because of rounding.

Dirt in any nonfast (sloppy or muddy) condition.

— = Not applicable. CI = Confidence interval.

Table 2—

Results of univariate logistic regression for assessment of risk factors associated with fatal injuries for the horses of Table 1 that had been racing for ≥ 6 months.

Risk factorControlCaseOR (95% CI)P value
Sex
 Mare or gelding1,078,244 (90)1,945 (86)Referent
 Stallion117,862 (10)311 (14)1.463 (1.298–1.649)< 0.001
Age (y)4.942 (4.940 to 4.945)5.001 (4.940 to 5.062)1.028 (0.999–1.057)0.055
Age at first race start (y)3.328 (3.326 to 3.330)3.410 (3.362 to 3.457)1.061 (1.025–1.097)0.001
Country
 Canada94,696 (8)105 (5)Referent
 United States1,101,410 (92)2,151 (95)1.761 (1.448–2.143)< 0.001
On vet list
 No1,188,425 (99)2,229 (99)Referent
 Yes7,681 (1)27 (1)1.874 (1.281–2.741)0.001
Field size (No. of horses)8.394 (8.390 to 8.397)8.465 (8.388 to 8.541)1.019 (0.998–1.042)0.079
Finish percentage in previous race (%)53.205 (53.183 to 53.227)51.418 (50.925 to 51.912)0.988 (0.984–0.991)< 0.001
No. of months in racing19.658 (19.639 to 19.678)19.376 (18.911 to 19.840)0.998 (0.994–1.001)0.223
No. of previous injuries0.0357 (0.0353 to 0.0361)0.0603 (0.0497 to 0.0709)1.600 (1.368–1.871)< 0.001
No. of previous scratches1.775 (1.773 to 1.779)1.915 (1.822 to 2.007)1.027 (1.009–1.044)0.003
No. of race starts by horse
 < 30 d prior to race0.837 (0.835 to 0.838)0.758 (0.731 to 0.785)0.845 (0.795–0.899)< 0.001
 30–60 d prior to race0.966 (0.965 to 0.967)0.100 (0.969 to 1.031)1.053 (1.001–1.108)0.047
 61–90 d prior to race0.868 (0.867 to 0.870)0.954 (0.922 to 0.987)1.134 (1.080–1.192)< 0.001
 91–180 d prior to race2.198 (2.195 to 2.201)2.502 (2.432 to 2.572)1.098 (1.073–1.123)< 0.001
No. of race starts by trainer
 < 30 d prior to race19.757 (19.717 to 19.797)19.778 (18.923 to 20.634)1.000 (0.998–1.002)0.963
 30–60 d prior to race17.160 (17.122 to 17.197)17.204 (16.398 to 18.010)1.000 (0.998–1.002)0.920
 61–90 d prior to race16.090 (16.052 to 16.128)16.059 (15.231 to 16.888)1.000 (0.998–1.002)0.946
 91–180 d prior to race42.091 (41.984 to 42.197)42.476 (40.179 to 44.772)1.000 (0.999–1.001)0.759
No. of trainer wins
 < 30 d prior to race3.061 (3.052 to 3.070)3.075 (2.879 to 3.271)1.001 (0.992–1.009)0.898
 30–60 d prior to race2.684 (2.676 to 2.692)2.740 (2.558 to 2.921)1.002 (0.994–1.011)0.574
 61–90 d prior to race2.528 (2.520 to 2.537)2.580 (2.392 to 2.769)1.002 (0.994–1.011)0.602
 91–180 d prior to race6.644 (6.621 to 6.667)6.840 (6.329 to 7.350)1.001 (0.998–1.004)0.469
Winning percentage
 < 30 d prior to race0.0858 (0.0854 to 0.0862)0.0926 (0.0810 to 0.1042)1.096 (0.944–1.272)0.228
 30–60 d prior to race0.0945 (0.0941 to 0.0949)0.1234 (0.1109 to 0.1359)1.423 (1.242–1.630)< 0.001
 61–90 d prior to race0.0854 (0.0850 to 0.0858)0.1097 (0.0979 to 0.1215)1.385 (1.201–1.598)< 0.001
 91–180 d prior to race0.0972 (0.0968 to 0.0976)0.1208 (0.1118 to 0.1298)1.642 (1.374–1.962)< 0.001
Mean speed during previous race (m/s)16.210 (16.208 to 16.213)16.259 (16.197 to 16.321)1.033 (0.995–1.072)0.086
Mean change in speed from previous race (m/s)−0.006 (−0.009 to −0.003)0.020 (−0.066 to 0.107)1.008 (0.985–1.032)0.482
Time since previous start (d)43.639 (43.517 to 43.762)41.843 (38.902 to 44.784)0.9995 (0.9989–1.0002)0.212
Odds at start of race16.947 (16.910 to 16.984)14.347 (13.610 to 15.084)0.993 (0.990–0.995)< 0.001
Odds rank in race4.691 (4.686 to 4.696)4.291 (4.183 to 4.400)0.942 (0.927–0.958)< 0.001
Starting (post) position4.698 (4.693 to 4.703)4.773 (4.662 to 4.884)1.011 (0.995–1.027)0.178
Low purse race (≤ $7,500)
 No982,380 (82)1,893 (84)Referent
 Yes213,726 (18)363 (16)0.881 (0.788–0.986)0.028
Purse ($)22,683 (22,574 to 22,792)19,014 (17,989 to 20,038)0.997 (0.995–0.998)< 0.001
Change in purse since previous race
 No change492,517 (41)905 (40)Referent
 Decrease299,774 (25)641 (28)1.164 (1.052–1.288)0.003
 Increase263,620 (22)471 (21)0.972 (0.870–1.087)0.622
 Sharp decrease77,497 (6)141 (6)0.990 (0.829–1.183)0.913
 Sharp increase62,698 (5)98 (4)0.851 (0.690–1.048)0.129
Race distance (fur)6.792 (6.790 to 6.794)6.589 (6.533 to 6.644)0.889 (0.861–0.978)< 0.001
First start with new trainer
 No1,063,968 (89)1,938 (86)Referent
 Yes132,138 (11)318 (14)1.321 (1.173–1.488)< 0.001
Training with first trainer
 No739,211 (62)1,523 (68)1.284 (1.176–1.403)< 0.001
 Yes456,895 (38)733 (32)Referent
Time with same trainer (mo)9.057 (9.041 to 9.074)7.647 (7.297 to 7.998)0.981 (0.976–0.986)< 0.001
No. of days between race starts since change of trainer4.162 (4.068 to 4.255)1.932 (−0.346 to 4.209)0.999 (0.998–1.000)0.043
First start with new jockey
 No518,291 (43)946 (42)Referent
 Yes677,815 (57)1,310 (58)1.059 (0.974–1.151)0.180
Time with same jockey (mo)1.050 (1.046 to 1.055)0.930 (0.841 to 1.018)0.978 (0.960–0.997)0.021
Racetrack surface
 Synthetic131,402 (11)150 (7)Referent
 Off-dirt*162,002 (14)353 (16)1.909 (1.577–2.311)< 0.001
 Dirt739,765 (62)1,500 (66)1.776 (1.502–2.101)< 0.001
 Turf162,937 (14)253 (11)1.360 (1.111–1.665)0.003

There were 1,198,362 official race starts by 101,256 horses that had been racing ≥ 6 months; 2,256 starts resulted in a fatal injury (cases), and all other starts were considered controls. Winning percentage was calculated as the number of wins/number of race starts. See Table 1 for remainder of key.

The final multivariable logistic regression model for all horses included 17 risk factors (Table 3). Horse-related risk factors included sex, age at first race start, whether the race was the horse's first race start, finish percentage (finish position/field size; eg, a horse that finishes fourth in a field of 8 would have a finish percentage of 50%, whereas a horse that finishes eighth in a field of 8 would have a finish percentage of 100%) in previous race, whether the horse has been placed by an association or regulatory veterinarian on the vet list (on vet list), number of previous injuries, number of previous scratches, and odds rank in the race (a horse with an odds rank of 1 is the horse favored to win the race). The risk of a fatal injury was increased for stallions, compared with that for mares and geldings. The risk of a fatal injury also increased if the horse was on the vet list and as its number of previous injuries and scratches increased. Protective horse-related risk factors (factors that decreased the risk of fatal injury) included first race start and odds rank in race (risk decreased as horses become less favored to win). The risk of fatal injury also decreased as the finish percentage in the previous race increased. Racetrack-related risk factors included country where the race took place, racetrack surface, race distance, and race purse. The risk of a fatal injury was increased for horses that raced on racetracks in the United States, compared with racetracks in Canada, and on tracks with a turf, dirt, or off-dirt (dirt in any nonfast [sloppy or muddy] condition) surface, compared with tracks with a synthetic surface. The risk of a fatal injury decreased as the race distance and purse increased. Trainer-related risk factors included time with the same trainer and number of days between races after a change in trainer. Both of those factors were protective; the risk of fatal injury decreased as the number of months the horse was trained by the same trainer increased and as the number of days between races after a change in trainer increased. The only jockey-related risk factor included in the final model was first start with a new jockey; the risk of a fatal injury for a horse in its first race with a new jockey was greater than that for a horse in a race in which it was ridden by a jockey who had ridden it in previous races.

Table 3—

Final multivariable model for the horses of Table 1.

Risk factorOR (95% CI)P value
Intercept0.0024 (0.0018–0.0032)< 0.001
Age at first race start (per y)1.076 (1.049–1.104)< 0.001
Sex
 Mare or geldingReferent
 Stallion1.430 (1.305–1.566)< 0.001
Country
 CanadaReferent
 United States1.376 (1.185–1.598)< 0.001
On vet list
 NoReferent
 Yes1.766 (1.292–2.413)< 0.001
First race start
 NoReferent
 Yes0.633 (0.529–0.743)< 0.001
Finish percentage in previous race (per 1%)0.993 (0.991–0.995)< 0.001
No. of previous injuries (per injury)1.550 (1.332–1.804)< 0.001
No. of previous scratches (per scratch)1.029 (1.013–1.046)< 0.001
Odds rank in race (per rank)0.966 (0.953–0.980)< 0.001
Low purse race (≤ $7,500)
 NoReferent
 Yes0.830 (0.752–0.916)< 0.001
Purse (per $1,000)0.998 (0.997–1.000)0.022
Change in purse since previous race
 No change (between −$1,000 and +$1,000)Referent
 Moderate decrease ($1,000–$15, 000 decrease)1.135 (1.042–1.236)0.004
 Moderate increase ($1,000–$15,000 increase)0.941 (0.856–1.035)0.212
 Large decrease (> $15,000 decrease)1.057 (0.912–1.225)0.464
 Large increase (> $15,000 increase)1.034 (0.861–1.242)0.717
Race distance (per furlong)0.916 (0.892–0.940)< 0.001
Time with same trainer (per mo)0.983 (0.979–0.988)< 0.001
No. of days between race starts since change of trainer (per d)0.999 (0.998–1.000)0.014
First start with new jockey
 NoReferent
 Yes1.076 (1.003–1.154)0.042
Racetrack surface
 SyntheticReferent
 Off-dirt*1.582 (1.367–1.830)< 0.001
 Dirt1.524 (1.345–1.726)< 0.001
 Turf1.295 (1.110–1.512)0.001

See Table I for key.

The final multivariable logistic regression model for only those horses that had been competing in flat racing for ≥ 6 months also included 17 risk factors (Table 4). Risk factors included in both final multivariable models (ie, the models for all horses and only horses that had been racing for ≥ 6 months) included age at first race start, sex, country where race took place, on vet list, finish percentage in previous race, number of previous injuries, number of previous scratches, odds rank in race, race purse, race distance, time with same trainer, number of days between races after a change in trainer, and racetrack surface. The respective effects (increase or decrease risk of fatal injuries) of those risk factors were the same for both models. Risk factors included in the final model for horses that had been racing for ≥ 6 months but not in the final model for all horses were number of race starts prior to the race of interest and the winning percentage (number of wins/number of race starts). The risk of a fatal risk injury increased as the winning percentage up to 180 days before the race of interest increased. Interestingly, the risk of a fatal injury decreased as the number of race starts within 30 days before the race of interest increased but increased as the number of race starts within 61 to 180 days before the race of interest increased.

Table 4—

Final multivariable model for the horses of Table 2.

Risk factorOR (95% CI)P value
Intercept0.002 (0.001–0.003)< 0.001
Age at first race start (per 1 y)1.078 (1.041–1. 116)< 0.001
Sex
 Mare or geldingReferent
 Stallion1.544 (1.366–1.744)< 0.001
Country
 CanadaReferent
 United States1.304 (1.066–1.594)0.010
On vet list
 NoReferent
 Yes1.526 (1.038–2.243)0.032
Field size1.060 (1.035–1.085)< 0.001
Finish percentage in previous race (per l%)0.991 (0.987–0.996)< 0.001
No. of previous injuries1.550 (1.323–1.815)< 0.001
No. of previous scratches1.020 (1.002–1.039)0.032
Odds rank in race0.951 (0.934–0.968)< 0.001
Low purse race (≤ $7,500)
 NoReferent
 Yes0.817 (0.725–0.922)0.001
Purse (per $1,000)0.996 (0.994–0.998)0.001
No. of race starts
 < 30 d prior to race0.801 (0.748–0.857)< 0.001
 61–90 d prior to race1.080 (1.023–1.140)0.006
 91–180 d prior to race1.064 (1.038–1.092)< 0.001
Race distance (per fur)0.895 (0.865–0.925)< 0.001
Time with same trainer (per mo)0.985 (0.980–0.990)< 0.001
No. of days between race starts since change of trainer (per d)0.998 (0.997–0.999)0.002
Winning percentage (per l%)
 31–60 d prior to race1.248 (1.084–1.437)0.002
 91–180 d prior to race1.227 (1.007–1.494)0.042
Racetrack surface
 SyntheticReferent
 Off-dirt*1.752 (1.442–2.128)< 0.001
 Dirt1.618 (1.363–1.920)< 0.001
 Turf1.322 (1.075–1.627)0.008

See Table 2 for key.

Discussion

Results of the present study suggested that the risk for fatal injuries (euthanasia or death of a horse within 3 days after sustaining an injury during a race) in Thoroughbred racehorses that competed in flat racing in the United States and Canada during the 5-year period of 2009 to 2013 was increased for horses that were examined by a veterinarian immediately before a race (on vet list) or had sustained a previous injury. Racing a horse that was on the vet list was one of the risk factors with the highest magnitude of effect in the present study. Additionally, the risk that a horse would sustain a fatal injury increased approximately 3% for each scratch (withdrawal from a race). Those findings concur with the results of another study19 in which horses identified as being at risk by regulatory veterinarians during prerace examinations were 5 to 14 times as likely to sustain a musculoskeletal injury or injury of the suspensory or SDFT as horses that were identified as not at risk by regulatory veterinarians during prerace examinations.

In the present study, the risk of a fatal injury was greater for stallions than for mares and geldings. Results of a case-control study25 of deceased Thoroughbred racehorses in California indicate that stallions are more likely than mares and geldings to fracture the proximal sesamoid bones of the forelimb. Greater than 80% of the fatal injuries assessed in the present study were fractures. Stallions are also at greater risk of sustaining a catastrophic musculoskeletal injury,3 nonfatal SDFT injury,26 or indeed any form of fatal injury,2 compared with mares and geldings. Interestingly, results of a case-control study27 involving Thoroughbred racehorses in Australia suggest that sex is not significantly associated with serious musculoskeletal injures. However, the investigators of that study27 compared the incidence of serious musculoskeletal injures between males and females; it is possible the results were confounded, or biased, by the inclusion of geldings with stallions in the male category.

Successful racehorses appeared to be at an increased risk of fatal injury in the present study. Races with a purse < $7,500 appeared to be protective against fatal injury, although the risk of fatal injury appeared to decrease as the race purse increased. Races with a low (< $7,500) purse might have been a proxy for the quality of racehorses or lack of competitiveness in those races, which may explain their apparent protective effect against fatal injury. Another indicator that successful and competitive horses are at increased risk of fatal injury is the apparent protective effect of the odds rank; the risk of fatal injury decreased as the odds rank for a horse increased (ie, as the horse became less favored to win). In the case-control study27 of Thoroughbred racehorses in Australia, the risk of serious musculoskeletal injury also increased as the competitiveness or race purse increased.

In the present study, the risk of a fatal injury decreased as the number of race starts for a horse within 30 days prior to the race of interest increased, but increased as the number of race starts for a horse increased within 61 to 90 and 91 to 180 days before the race of interest. This finding suggested that the risk of fatal injury increases for horses with a > 1-month break between races, and is consistent with results of another study19 in which the risk of a horse sustaining a nonfatal SDFT injury increased 8-fold after a > 2-month break in racing. Results of another study28 also indicate that the risk of an SDFT or suspensory apparatus injury was greater for horses with no race starts than for horses with multiple race starts. Investigators of a case-control study17 of Thoroughbred racehorses in California reported that the risk of a fatal skeletal injury was increased as the cumulative exercise and race distance during the 2 months prior to the race of interest increased. The number of workouts, or exercise sessions, for racehorses is positively associated with the risk of proximal sesamoid bone fractures.25 The EID did not include information regarding the extent of training or exercise for individual horses evaluated in the present study, but the number of race starts during certain periods (eg, 61 to 90 days and 91 to 180 days prior to the race of interest) could be used as a proxy measure for cumulative exercise, in which case our findings are similar to those of the other studies17,25 in which the risk of injury increased as cumulative exercise increased. Another interesting finding of the present study was that the risk of fatal injury decreases as the time that a horse is trained by the same trainer increases.

Racetrack surface was also associated with the risk of fatal injuries in the Thoroughbred racehorses of the present study. Compared with racetracks with synthetic surfaces, the risk of fatal injury was greatest for racetracks with off-dirt and dirt surfaces followed by racetracks with turf surfaces. That finding was consistent with the results of another study29 in which the race-associated fatality rate for racehorses in California was greatest for dirt tracks followed by turf and then synthetic tracks. In that study,29 the race-associated fatality rate for racetracks with dirt surfaces was significantly greater than that for racetracks with synthetic surfaces but did not differ significantly from that for racetracks with turf surfaces, and the fatality rate for racetracks with turf surfaces did not differ significantly from that for racetracks with synthetic surfaces. The reason we were able to identify a statistical difference between synthetic and turf surfaces in the present study is likely a reflection of the large cohort available for analysis, which provided substantially greater power than the smaller study population in that other study.29 In the present study, as in a previous study,1 the risk of fatal injury on racetracks with dirt surfaces was significantly greater than that for racetracks with turf surfaces but did not differ significantly from that for racetracks with off-dirt surfaces.

The risk of fatal injury decreased as race distance increased and field size decreased in the present study. Results of a case-control study10 of fatal fractures in the distal portion of the limbs of Thoroughbred racehorses in the United Kingdom also indicate a positive association between the risk of fatal injury and field size; however, unlike the present study, there was a positive association between risk of fatal injury and race distance. The reason for the discrepancy in the nature of the association between risk of fatal injury and race distance between that study10 and the present study is most likely associated with differences in distance ranges for flat races in the United Kingdom, compared with the distance ranges for flat races in the United States and Canada. Thoroughbred flat races in the United States and Canada tend to be shorter than those in the United Kingdom. Consequently, races in the United States and Canada are run at a faster pace than races in the United Kingdom, and that fast pace likely contributed to the negative association between risk of fatal injury and race distance observed in this study.

Investigators of multiple studies10,15,17,19,27,28 have reported a positive association between age and injury risk in Thoroughbred racehorses. Although age was eligible for inclusion in the multivariable models of the present study, it was not maintained in the final models, likely because of multicollinearity. Age was highly correlated with age at first race start, first race start, and number of previous injuries; however, correlation does not imply causation and inclusion of age in the models did not substantially improve the information gain.

The incidence rate of fatal injuries in Thoroughbred racehorses competing in flat racing at 89 tracks in the United States and Canada during the 5-year period from 2009 to 2013 was 1.9/1,000 race starts. That incidence rate was fairly consistent with the incidence rates for fatal musculoskeletal injuries in Thoroughbred racehorses reported in other US studies,17,12,30,31 (1.2 to 1.96 fatal or catastrophic injuries/1,000 race starts) and a South African study32 (1.4 catastrophic injuries/1,000 race starts). The incidence rate of fatal and nonfatal injuries in Thoroughbred racehorses was 3.18/1,000 race starts in a study33 that involved 27 racetracks in Minnesota from 1987 to 1992; unfortunately, the number of fatal injuries was not reported separately from the number of nonfatal injuries in that study. Regardless, the incidence rates of fatal injuries in Thoroughbred racehorses in the United States appear to be much higher than those for racehorses in the United Kingdom13,34,35 (0.38 to 0.9 fatal injuries/1,000 race starts) and Australia36,37 (0.3 to 0.44 fatal injuries/1,000 race starts). Moreover, the incidence rates within each jurisdiction, or country, have remained fairly constant since the 1980s.

To our knowledge, the present study was the first to extensively evaluate data obtained from the EID. The data evaluated in the present study represented 93.9% of all official Thoroughbred racing events in the United States and Canada during the 5-year observation period. Therefore, we are confident that our results are unbiased effect estimates for the risk factors evaluated because the data for the approximately 6% of racing events that were not reported to the EID represent a small source of bias.

It is important to note that we did not make any attempt to differentiate the causes of fatal injury in the present study. Risk factors vary among types of fractures,13,25 and it is likely that some of those risk factors were not identified in the present study. The types of injuries sustained and the reason for euthanasia have been accurately reported to the EID only recently. Thus, future analyses will be able to use more specific outcome variables to identify risk factors associated with the most common reasons for euthanasia of Thoroughbred racehorses following race-induced injuries.

We concede that statistical significance does not necessarily translate to clinical significance. Although we identified several risk factors that were significantly associated with fatal injuries in Thoroughbred horses competing in flat racing, it is important to point out that the vast majority (1,887,911/1,891,483 [99.8%]) of race starts evaluated in the present study did not result in a fatal injury. Because of the extremely large number of race starts evaluated and the resulting high statistical power of this study, the magnitude of effect for some of the risk factors was very small, and this should be considered during discussions regarding the selection and implementation of measures expected to have the greatest effect on minimizing the number of horses that sustain fatal injuries during flat races in the United States and Canada. Nevertheless, the results of the present study can be used as a guideline for the identification of racehorses at high risk of sustaining a fatal injury during a race.

Acknowledgments

Supported by an Industry Partnership Doctorate of Philosophy Program provided by The Jockey Club and the University of Glasgow.

Presented as an abstract at the 20th International Conference of Racing Analysts and Veterinarians, Republic of Mauritius, September 2014.

ABBREVIATIONS

AIC

Akaike Information Criterion

EID

Equine Injury Database

SDFT

Superficial digital flexor tendon

References

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Contributor Notes

Address correspondence to Mr. Georgopoulos (s.georgopoulos.1@research.gla.ac.uk).