Marginal ancestral contributions to atrial fibrillation in the Standardbred racehorse: Comparison of cases and controls

Paroxysmal Atrial Fibrillation in Horses: Pathophysiology, Diagnostics and Clinical Aspects

Atrial fibrillation (AF) is the most common arrhythmia in horses causing poor performance. As in humans, the condition can be intermittent in nature, known as paroxysmal atrial fibrillation (pAF). This review covers the literature relating to pAF in horses and includes references to the human literature to compare pathophysiology, clinical presentation, diagnostic tools and treatment. The arrhythmia is diagnosed by auscultation and electrocardiography (ECG), and clinical signs can vary from sudden loss of racing performance to reduced fitness or no signs at all. If left untreated, pAF may promote electrical, functional and structural remodeling of the myocardium, thus creating a substrate that is able to maintain the arrhythmia, which over time may progress into permanent AF. Long-term ECG monitoring is essential for diagnosing the condition and fully understanding the duration and frequency of pAF episodes. The potential to adapt human cardiac monitoring systems and computational ECG analysis is therefore of interest and may benefit future diagnostic tools in equine medicine.

Analysis of Genetic Diversity in the American Standardbred Horse Utilizing Short Tandem Repeats and Single Nucleotide Polymorphisms

American Standardbreds were developed as a harness racing horse breed. The United States Trotting Association closed the studbook in 1973 and implemented a book size cap in 2009. This study aimed to investigate genetic diversity in the American Standardbred after the studbook cap was introduced using short tandem repeats (STRs) and single-nucleotide polymorphisms (SNPs). Sixteen STRs from horses foaled from 2010 to 2015 and their sires and dams (n = 50 621) were utilized to examine allelic richness (A_r), expected heterozygosity (H_E), observed heterozygosity (H_O), unbiased heterozygosity (H_U), inbreeding coefficient (F_IS), and fixation index (F_ST). These analyses found that trotting and pacing sires were less genetically diverse than dams (H_EP_Bonferroni = 0.029 and 6.3 × 10⁻⁵, respectively) and their offspring (A_rP_Bonferroni = 0.034 and 6.9 × 10^-6, respectively), and pacing offspring were significantly less diverse than their dams (H_EP_Bonferroni = 2 × 10^-3). Inbreeding coefficients for trotters (F_IS = −0.014) and pacers (F_IS = −0.012) suggest that breeding practices have maintained diversity. Moderate levels of genetic differentiation (0.066 < F_ST < 0.11) were found between pacing and trotting groups. Additionally, 10 of the most prolific trotting sires and their male offspring (n = 84) were genotyped on the 670K Axiom Equine HD Array. H_O values higher than H_E (P < 0.001), low inbreeding coefficients (mean F = −0.064), and mean F_ROH = 21% indicate relatively high levels of diversity in this cohort, further supporting the STR data. However, in contrast, H_O values were higher for trotting sires (0.41) than their offspring (0.36). This observation warrants further monitoring of diversity over time. These data provide an updated foundation of diversity indices for further, long-term analysis in the breed.

Fundamentals of arrhythmogenic mechanisms and treatment strategies for equine atrial fibrillation

Atrial fibrillation (AF) is the most common pathological arrhythmia in horses. Although it is not usually a life-threatening condition on its own, it can cause poor performance and make the horse unsafe to ride. It is a complex multifactorial disease influenced by both genetic and environmental factors including exercise training, comorbidities or ageing. The interactions between all these factors in horses are still not completely understood and the pathophysiology of AF remains poorly defined. Exciting progress has been recently made in equine cardiac electrophysiology in terms of diagnosis and documentation methods such as cardiac mapping, implantable electrocardiogram (ECG) recording devices or computer-based ECG analysis that will hopefully improve our understanding of this disease. The available pharmaceutical and electrophysiological treatments have good efficacy and lead to a good prognosis for AF, but recurrence is a frequent issue that veterinarians have to face. This review aims to summarise our current understanding of equine cardiac electrophysiology and pathophysiology of equine AF while providing an overview of the mechanism of action for currently available treatments for equine AF.

Heritability estimates of atrial fibrillation in Thoroughbred racehorses in Hong Kong and Australia

INTRODUCTION

Atrial fibrillation (AF) is the most common performance-limiting arrhythmia of racehorses. A genetic contribution has been suggested in Standardbred racehorses but has not been investigated in Thoroughbreds. The objective of this study was to determine the heritability of AF in Thoroughbred racehorses.

ANIMALS, MATERIALS AND METHODS

Horses racing between 2007 and 2019 in Hong Kong and Australia that had AF detected postrace via auscultation (n = 463 cases) were compared with five randomly selected racing contemporaries (n = 2,315 controls). The ASReml-R programme was used to fit an animal model to the AF data to estimate heritability using the entire data set and a subset of horses (n = 106 cases) that had electrocardiographic confirmation of AF. Variance components were estimated assuming AF was normally distributed and on the logit-transformed scale. The risk of producing AF-affected offspring was calculated using Fisher's exact test for stallions that sired ≥10 individuals in the case-control population.

RESULTS

Heritability on the underlying scale was 0.064 ± 0.04 (logit animal) and 0.071 ± 0.04 (normal animal) for the entire population and 0.065 ± 0.097 (logit animal) and 0.058 ± 0.11 (normal animal) for electrocardiographic-confirmed AF cases. Of 71 stallions that sired ≥10 individuals, three were more likely to produce affected offspring (odds ratio: 4.05-7.57; p < 0.01). Age (p = 0.991), sex (p = 0.830), and year of birth (p = 0.547) did not contribute to expression.

CONCLUSIONS

Although some stallions were overrepresented amongst affected horses, the heritability of AF in this population of Thoroughbreds was low. Environmental and individual factors contributing to AF development require further investigation.

Incidence, recurrence, and outcome of postrace atrial fibrillation in Thoroughbred horses

Background

Atrial fibrillation (AF) impacts performance and horse and jockey safety. Understanding the outcomes of AF identified postrace will better inform regulatory policy.

Hypothesis/Objectives

To investigate the outcomes after episodes of AF identified postrace and determine whether affected horses are at increased risk of additional episodes compared to the general racing population.

Animals

Total of 4684 Thoroughbred racehorses.

Methods

Race records for Thoroughbred horses racing in Hong Kong from 2007 to 2017 were reviewed. Horses that performed below expectation were examined by cardiac auscultation and ECG. Incidence and recurrence of AF were compared between horses with and without a history of AF and between horses with paroxysmal and persistent episodes using Fisher's exact test.

Results

There were 96 135 race starts during the study. Atrial fibrillation was identified in 4.9% of horses, with an overall incidence of 2.7 episodes per 1000 starts. The incidence of AF in horses after any previous episode (12.8 per 1000 starts) was higher than for horses with no previous episode (2.4 per 1000 starts; odds ratio [OR], 5.3; 95% confidence interval [CI], 3.8‐7.6). Recurrence was seen in 64% of horses previously treated for persistent AF, which was higher than recurrence in horses with paroxysmal AF (23%; OR, 5.9; 95% CI, 1.6‐21.2). Median duration between episodes was 343 days (range, 34‐1065).

Conclusions and Clinical Importance

Thoroughbreds are at increased risk of recurrent AF after both paroxysmal and persistent episodes, but the duration of time between episodes varies widely. These findings support a substantial burden of AF among individual Thoroughbred racehorses.

Atrial fibrillation in horses part 1: Pathophysiology

Atrial fibrillation (AF) is the most common clinically relevant arrhythmia in horses, with a reported prevalence up to 2.5%. The pathophysiology has mainly been investigated in experimental animal models and human medicine, with limited studies in horses. Atrial fibrillation results from the interplay between electrical triggers and a susceptible substrate. Triggers consist of atrial premature depolarizations due to altered automaticity or triggered activity, or local (micro)reentry. The arrhythmia is promoted by atrial myocardial ion channel alterations, Ca²⁺ handling alterations, structural abnormalities, and autonomic nervous system imbalance. Predisposing factors include structural heart disease such as valvular regurgitation resulting in chronic atrial stretch, although many horses show so-called 'lone AF' or idiopathic AF in which no underlying cardiac abnormalities can be detected using routine diagnostic techniques. These horses may have underlying ion channel dysfunction or undiagnosed myocardial (micro)structural alterations. Atrial fibrillation itself results in electrical, contractile and structural remodelling, fostering AF maintenance. Electrical remodelling leads to shortening of the atrial effective refractory period, promoting reentry. Contractile remodelling consists of decreased myocardial contractility, while structural remodelling includes the development of interstitial fibrosis and atrial enlargement. Reverse remodelling occurs after cardioversion to sinus rhythm, but full recovery may take weeks to months depending on duration of AF. The clinical signs of AF depend on the aerobic demands during exercise, ventricular rhythm response and presence of underlying cardiac disease. In horses with so-called 'lone AF', clinical signs are usually absent at rest but during exercise poor performance, exercise-induced pulmonary hemorrhage, respiratory distress, weakness or rarely collapse may develop.

Unraveling the Genetics Behind Equid Cardiac Disease

There have been some advances in understanding the genetic contribution to ventricular septal defects in Arabians, sudden death in racehorses, and atrial fibrillation in racehorses. No genetic analyses have been published for aortic rupture in Friesians or atrioventricular block in donkeys despite strong evidence for a genetic cause. To date, no genetic mutation has been identified for any equid cardiac disease. With the advancement of genetic tools and resources, we are moving closer to discoveries that may explain the heritable basis of inherited equid cardiac disease.