Irregular hearts and performance horses

Electrocardiograms from different types of exercise in Eventing horses with and without cardiac signs

BACKGROUND

Exercising arrhythmias can be clinically irrelevant or associated with poor performance, collapse and sudden cardiac death.

OBJECTIVES

To test if readable exercising ECGs can be recorded by grooms or riders and to describe arrhythmias in ECGs from different types of exercise in Eventing horses and investigate associations with type of workout, the presence of previous cardiac signs and intensity of exercise.

STUDY DESIGN

Cohort study.

METHODS

Single lead exercising ECGs were obtained by riders or grooms during training and competition from a convenience sample of horses in training for Eventing competitions. Arrhythmias were described, and associations between different arrhythmia categories and variables that described the horse and the workouts were sought.

RESULTS

There were 1002 ECGs from 62 horses (median [range] 7 [2-97] ECGs/horse) evaluated and 737 workouts (73.6%) were >95% readable and included in the analysis. There were arrhythmias in 250 (33.9%) of the workouts, complex arrhythmias in 13 (1.8%) and the number of premature complexes was median (range) 0 (0-118). Peak heart rate and duration of exercise were associated with the number of premature complexes, the presence of arrhythmias and complex arrhythmias and were colinear with the type of exercise. Having previous signs of cardiac disease and the type of workout were associated with higher odds of having arrhythmias.

CONCLUSIONS

Monitoring the rhythm of equine athletes with ECGs obtained by riders and transmitted to an online cloud was feasible. Arrhythmias were frequent, and complex arrhythmias were rare. The presence of cardiac signs, type of exercise and peak heart rate were associated with the presence of arrhythmias. None of the horses developed poor performance or collapse attributed to cardiac disease. The arrhythmias that should be concerning for equine veterinarians need further definition.

Histological evaluation of cardiac remodelling in equine athletes

Approximately 1-2 per 100,000 young athletes die from sudden cardiac death (SCD) and extreme exercise may be associated with myocardial scar and arrhythmias. Racehorses have a high prevalence of atrial fibrillation (AF) and SCD but the presence of myocardial scar and inflammation has not been evaluated. Cardiac tissues from the left (LAA) and right (RAA) atrial appendages, left ventricular anterior (LVAPM) and posterior (LVPPM) papillary muscles, and right side of the interventricular septum (IVS-R) were harvested from racehorses with sudden cardiac death (SCD, n = 16) or other fatal injuries (OFI, n = 17), constituting the athletic group (ATH, n = 33), and compared to sedentary horses (SED, n = 10). Horses in the ATH group had myocyte hypertrophy at all sites; increased fibrosis at all sites other than the LAA; increased fibroblast infiltration but a reduction in the overall extracellular matrix (ECM) volume in the RAA, LVAPM, and IVS-R compared to SED horses. In this horse model, athletic conditioning was associated with myocyte hypertrophy and a reduction in ECM. There was an excess of fibrocyte infiltration and focal fibrosis that was not present in non-athletic horses, raising the possibility of an exercise-induced pro-fibrotic substrate.

Heritability of sudden cardiac death in Thoroughbred racehorses

BACKGROUND

Sudden cardiac death (SCD) is an important cause of exercise-associated fatalities in Thoroughbred racehorses. Equine deaths share similarities with fatalities in human athletes that result from inherited cardiac disease. Whilst genetic causes have been postulated in horses, these have not been confirmed and heritability of SCD has not previously been estimated in Thoroughbred racehorses.

OBJECTIVES

To determine the heritability of SCD in a sample population of Thoroughbred racehorses.

STUDY DESIGN

Retrospective case-control study.

METHODS

Steward and post-mortem reports of Thoroughbred racehorses in Australia between 2007 and 2020 were reviewed to identify horses with SCD. Control horses were randomly selected from races in which SCD occurred or from races on the date of the case fatality. A five-generation integrated pedigree chart was collected for each horse. Estimates of heritability were obtained using an animal model in the ASReml-R program with variance components estimated assuming SCD was normally distributed, and on the logit transformed scale. Inbreeding coefficients were calculated and the risk of producing SCD-affected progeny was calculated for stallions that sired ≥5 individuals in the case-control population.

RESULTS

Ninety-three horses with SCD and 465 control horses were identified. Heritability on the underlying scale was 0.15 ± 0.09 (logit animal) and 0.24 ± 0.12 (normal animal). Inbreeding coefficients were not significantly different between groups. Of the 16 first generation sires that appeared ≥5 times in the case-control data set, two sires more frequently produced affected progeny (OR 7.95-10.41).

MAIN LIMITATIONS

Challenges in definitively confirming SCD may lead to misclassification of some cases. Some control horses may have not been exposed to environmental influences of SCD. Case numbers are low and the studied population may not represent the entire Thoroughbred genetic pool.

CONCLUSION

The heritability of SCD in this population was relatively low. However, individual stallions appear more likely to produce affected progeny. Further studies are required to understand the genetic and environmental influences that contribute to disease expression.

Risk Factors for Exercise-Associated Sudden Cardiac Death in Thoroughbred Racehorses

Cardiac arrhythmias resulting in sudden cardiac death (SCD) are an important cause of racehorse fatalities. The objective of this study was to determine risk factors for SCD in Thoroughbreds by evaluating a sample with a policy of mandatory post-mortem following racing or training fatalities. Risk factors were compared between case horses with SCD (n = 57) and control horses with other fatal injury (OFI, n = 188) by univariable and multivariable logistic regression. Survival in years for horses with SCD was compared to OFI using the Kaplan−Meier method with log rank test. The following variables were most important in the multiple logistic model: Horses with SCD were more likely to die during training than during racing, SCD (42/57, 74%) vs. OFI (82/188, 44%; odds ratio [OR], 95% confidence interval [CI], 2.5, 1.2−5.4; p = 0.01), had fewer lifetime starts, median (interquartile range [IQR]), SCD (3.0 [0.0−9.0]) vs. OFI (9.0 [0.0−22.8]; OR, 95% CI, 0.96, 0.9−1.0; p = 0.02 and were less likely to be entire (uncastrated) males, SCD 9/57 (16%) vs. OFI (46/188, 25%; OR, 95% CI, 0.47, 0.1−0.9; p = 0.03). Survival in years (median (IQR)) for horses with SCD was 3.6 (3.1−4.4), which was shorter than OFI (4.5 [3.1−6.0], hazard ratio, 95%CI, 1.6,1.2−2.3; p < 0.001). SCD occurs more commonly in training than racing, which suggests exercise intensity is less important in precipitating this fatality. In this study, SCD occurred early in the careers of affected horses.

Racing-associated fatalities in Norwegian and Swedish harness racehorses: Incidence rates, risk factors, and principal postmortem findings

BACKGROUND

There are no reports on the number of fatalities or causes of death in the Norwegian and Swedish harness racehorses.

OBJECTIVES

The incidence rates (IRs), risk factors, and postmortem findings in horses that died or were euthanized associated with racing between 2014 and 2019 were investigated.

ANIMALS

Thirty-eight Standardbreds and 10 Norwegian-Swedish Coldblooded Trotters died or were euthanized associated with racing. A total of 816 085 race-starts were recorded.

METHODS

Incidence rates were calculated for both countries and horse breeds. Risk factors for sudden death were identified using a case-control logistic model. Postmortem examinations were performed in 43 horses.

RESULTS

The overall fatality IR was 0.059/1000 race-starts. Traumatic injuries accounted for 14.5%, while sudden death for 85.5% of fatalities. Only minor differences between countries and breeds were recorded. The number of starts within the last 30 days increased the risk of sudden death (5 starts odds ratio (OR) 228.80, confidence interval (CI) 10.9-4793). An opposite non-linear effect was observed in number of starts the last 180 days (>10 starts OR 0.12, CI 0.02-0.68). Seven horses were euthanized because of catastrophic injury. Acute circulatory collapse because of suspected cardiac or pulmonary failure or both was recorded in 30 horses, while major hemorrhage after vessel rupture was the primary cause of death in 10 cases. One horse collapsed and died but was not submitted for autopsy.

CONCLUSIONS AND CLINICAL IMPORTANCE

Comparatively low rates of catastrophic orthopedic fatalities were reported, while causes and IR of sudden death were similar to previous studies.