SummaryReasons for performing study: Cardiac arrhythmias are a recognised but poorly characterised problem in the Standardbred racehorse. Frequency data could aid the development of cardiac arrhythmia screening programmes. Objectives: To characterise the occurrence of cardiac arrhythmias in Standardbreds prior to racing and in the late post race period using a handheld, noncontinuous recording device. Study design: Prospective, observational study, convenience sampling. Methods: Noncontinuous electrocardiographic recordings were obtained over a 12 week period from Standardbred horses competing at a single racetrack. Electrocardiograms were obtained before racing and between 6 and 29 min after the race using a handheld recording device. Prevalence of arrhythmias was calculated for all horses and overall frequency of arrhythmias was calculated for race starts and poor performers. Univariate logistic regression analysis was used to identify risk factors for cardiac arrhythmias. Results: A total of 8657 electrocardiogram recordings were obtained from 1816 horses. Six horses had atrial fibrillation after racing (prevalence = 0.11%, frequency = 0.14%), one horse had supraventricular tachycardia before racing (prevalence = 0.06%, frequency = 0.02%), and 2 horses had ventricular tachyarrhythmias after racing (prevalence = 0.06%, frequency = 0.05%). The frequency of atrial fibrillation among race starts with poor performance was 1.3-2.0%. Increasing age was a significant risk factor for the presence of atrial premature contractions before racing and atrial fibrillation and ventricular ectopy after racing. Conclusions: Both physiological and pathological cardiac arrhythmias can be detected in apparently healthy Standardbred horses in the prerace and late post race period using noncontinuous recording methods. Future studies should examine cumulative training or racing hours as a risk factor for cardiac arrhythmia. The prevalence and frequency information may be useful for track veterinarians and regulatory personnel following trends in cardiac arrhythmias.

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Background Upper reference limits for cTnI have not been established for healthy Standardbred racehorses. Objectives To establish cTnI upper reference limits for Standardbred racehorses and determine if increases in plasma cTnI concentration can be detected in 1–2 hours after a race. Animals Samples were obtained from 586 apparently healthy Standardbreds aged 2–14 years before racing and from the winners of 144 races 1–2 hours after the end of the race. Methods Prospective, observational study; convenience sampling; assay validation; and reference limits determinations were performed according to ASCVP guidelines. Plasma cardiac troponin I concentrations before racing were determined, potential outliers identified, and the 95th and 99th percentile upper reference limits calculated using nonparametric methods. The correlation between cTnI concentration and age, differences in median cTnI concentrations by subgroups and differences between cTnI concentrations before and after racing in winning horses were determined. Results The 95th and 99th percentile upper reference limits for all horses excluding outliers were < 0.04ng/mL and 0.06 ng/mL. There were no significant differences in cTnI concentrations based on age (P = .06), sex (P = .35), gait (P = .55), or race classification (P = .65) and a weak correlation of cTnI with age (ρ = 0.09, P = .03). There were no significant differences between cTnI concentrations before and after racing in winning horses (P = .70). Conclusions and Clinical Importance Because of lack of standardization across cTnI assays, the reference limits apply only to the Stratus CS immunoassay. Future studies looking at the effects of high intensity, short duration exercise on cTnI should consider sampling more than 2 hours after racing or using an ultrasensitive assay.

Chromosomal aberrations in the horse are known to cause congenital abnormalities, embryonic loss, and infertility. While diagnosed mainly by karyotyping and FISH in the horse, the use of SNP array comparative genome hybridization (SNP-CGH) is becoming increasingly common in human diagnostics. Normalized probe intensities and allelic ratios are used to detect changes in copy number genome-wide. Two horses with suspected chromosomal abnormalities and six horses with FISH-confirmed aberrant karyotypes were chosen for genotyping on the Equine SNP50 array. Karyotyping of the first horse indicated mosaicism for an additional small, acrocentric chromosome, although the identity of the chromosome was unclear. The second case displayed a similar phenotype to human disease caused by a gene deletion and so was chosen for SNP-CGH due to the ability to detect changes at higher resolutions than those achieved with conventional karyotyping. The results of SNP-CGH analysis for the six horses with known chromosomal aberrations agreed completely with previous karyotype and FISH analysis. The first undiagnosed case showed a pattern of altered allelic ratios without a noticeable shift in overall intensity for chromosome 27, consistent with a mosaic trisomy. The second case displayed a more drastic change in both values for chromosome 30, consistent with a complete trisomy. These results indicate that SNP-CGH is a viable method for detection of chromosomal aneuploidies in the horse.