Femoral asymmetry in the Thoroughbred racehorse

OBJECTIVE: To investigate the occurrence of geometrical asymmetries in the macro-architecture of left and right femurs from Thoroughbred racehorses previously used in competitive training and racing in New South Wales, Australia. METHODS: Detailed postmortem measurements were made of 37 characteristics of left and right femurs from eleven Thoroughbred racehorses euthanased for reasons unrelated to the study. Measurements focused on articulating surfaces and sites of attachment of muscles and ligaments known to be associated with hindlimb locomotion. RESULTS: Five measurements were significantly larger in left compared to right femurs (P < 0.05). The regions showing significant differences between left and right limbs were proximal cranial and overhead medio-lateral widths, greater trochanter depth, depth of the fovea in the femoral head and distal inter-epicondylar width. CONCLUSION: The left-right differences in femoral morphology were associated with sites of muscle and ligament attachment known to be involved with hindlimb function in negotiating turns. These differences may be the result of selection pressure for racing performance on curved race tracks and/or adaptations related to asymmetrical loading of the outside hindlimb associated with repeated negotiation of turns on such tracks.     

The epidemiology of Thoroughbred racehorse injuries

Musculoskeletal injuries have a huge, adverse effect on the Thoroughbred racehorse industry. Although fatal musculoskeletal injuries have a low prevalence, many lameness conditions have a high prevalence. Many of the common lameness conditions in racehorses occur over weeks to months, giving ample time to intervene. Injuries may be prevented by including management practices to minimize low hoof heel angle, incorporation of more frequent, shorter high speed works or races in exercise regimes, avoiding excessive accumulation of high speed distances over short periods of time, recognizing and rehabilitating horses with mild injuries and avoiding the use of high toe grabs. It appears important to achieve uniformity of racing surfaces and for the spectrum of environmental conditions “seen” by the horse’s musculoskeletal system. Key to tracking the prevalence of injuries and the success (or lack of success) of interventions is a comprehensive method to manage racehorse inventory, horse location, and horse movement.     

Pleuropneumonia as a sequela of myelography and general anaesthesia in a Thoroughbred colt

A 3-year-old Thoroughbred colt was presented to the University Veterinary Centre Camden for evaluation of ataxia. The horse was anaesthetised to facilitate cervical radiography and myelographic examination of the spinal cord. Recovery from anaesthesia was uneventful. Five days after general anaesthesia the horse re-presented with pleuropneumonia. It was euthanased 24 hours after presentation on humane grounds. Necropsy revealed severe tracheal erosion over the middle third of the ventral surface of the trachea, pleuropneumonia and narrowing of the cervical cord between C4 and C6. It is postulated that extension and flexion of the neck during myelography resulted in movement of the endotracheal tube cuff, causing the tracheal lesion and predisposing the colt to pleuropneumonia. Severe tracheal lesions and pleuropneumonia have not been reported as sequela of equine myelography, and should be considered as possible complications following repeated cervical manipulation during myelography in the horse.     

Lameness diagnosis and treatment in the Thoroughbred racehorse

Thoroughbred racehorses are susceptible to any and all of the commonly described conditions that affect soundness in the horse. This discussion does not include every conceivable ailment or orthopedic condition that afflicts these animals. Rather, it confines itself to a few specific conditions of the racing Thoroughbred that are either unique to their particular endeavor or so overwhelmingly important to a successful racing career that they must, of necessity, be discussed.     

Clinical exercise testing in the normal Thoroughbred racehorse

To evaluate normal cardiorespiratory and metabolic responses of Thoroughbred horses to a standardised treadmill exercise test, we examined 28 horses ranging in age from 1 to 4 years. The group consisted of eight yearlings, eight 2-year-olds and twelve 3 and 4-year-olds. All horses except the yearlings were in training, and either racing or ready to race, at the time of examination. None of the horses had histories of performance problems. On the first day the horses received a full physical examination, resting electrocardiogram, upper respiratory tract endoscopy and either one or two acclimatisation runs on the treadmill. The following day they were given an exercise test on a treadmill inclined at 6 degrees (+10% slope). The test consisted of 3 min at 4 m/sec, 90 sec at 6 m/sec and 60 sec intervals at 8, 10, 11, 12 and 13 m/sec. During the last 15 sec of each step, blood samples were collected for plasma lactate determination, expired respiratory gases were obtained using an open flow mask system for measurement of oxygen uptake, and heart rate was measured using telemetry electrocardiogram. From these measurements, various derived values were calculated, which have been used by others as indices of exercise capacity. These values included: V200 (speed at HR of 200 bpm), VHRmax (speed at which horses reached maximum HR), VO2-200 (oxygen uptake at a HR of 200 bpm), VO2max (maximum oxygen uptake), VLA4 (speed at which horses reached a plasma lactate of 4 mmol/l) and HRLA4 (HR at which horses reached a plasma lactate of 4 mmol/l). The yearlings had significantly lower values than the older age groups for most of the derived values.(ABSTRACT TRUNCATED AT 250 WORDS)     

Placement of a long term tracheal cannula in a Thoroughbred racehorse with bilateral laryngeal dysfunction

A Thoroughbred horse with bilateral laryngeal dysfunction was treated by placement of a long term tracheal cannula in order to restore full athletic function. The horse initially presented with right-sided arytenoid dysfunction that was considered to be due to a congenital malformation of the laryngeal cartilage. This was corrected by a right-sided laryngoplasty and ventriculectomy. The horse re-presented 1 year later with idiopathic left laryngeal hemiplegia. The decision was made to place a long-term tracheal cannula due to the low success rate and high complication rate associated with other surgeries used to correct bilateral arytenoid dysfunction. A three-piece tracheotomy cannula and stopper were positioned under general anaesthesia. The horse rapidly adjusted to the tracheal cannula and no post-surgical complications were encountered. The horse resumed training 4 weeks postoperatively. Due to the rules governing racing in Australia the horse was exported to New Zealand where he continued his racing career. It was concluded that tracheotomy is a straightforward technique associated with minimal complications and can provide an excellent prognosis for return of full airway function in racing horses. It is recognised that many issues need to be considered prior to more general acceptance of this technique, including assessment of animal welfare issues and public perception. In appropriate cases consideration should be given to using long term placement of a tracheotomy cannula as a treatment option to restore maximal airflow in horses with upper airway obstruction.     

Computed tomography of proximal metatarsal and concurrent third tarsal bone fractures in a Thoroughbred racehorse

A 2‐year‐old Thoroughbred filly presented to the William R. Pritchard Veterinary Medical Teaching Hospital at the University of California at Davis for whole body nuclear scintigraphic evaluation after an acute onset of nonweightbearing right hindlimb lameness post race. The use of computed tomography (CT) enabled visualisation of additional pathology that was not initially appreciable using combined imaging modalities of nuclear scintigraphy and digital radiography, which ultimately altered the selected course of treatment. Computed tomography in horses with fractures of the cuboidal bones of the hock can provide valuable additional information regarding fracture configuration and radiographically occult pathology, which may help to guide clinical decisions about treatment.     

Modelling the spread of a viral infection in equine populations managed in Thoroughbred racehorse training yards

A Monte Carlo model that simulates the management life cycle of a horse population in training on a Thoroughbred flat racing yard (i.e. stable) was developed for computer implementation. Each horse was characterised by several state variables. Discrete events at the horse level were triggered stochastically to reflect uncertainty about some input assumptions and heterogeneity of the horse population in a particular yard. This mathematical model was subsequently used to mimic the spread of equine influenza (EI) within a typical yard following the introduction of one or several infectious horses. Different scenarios were simulated to demonstrate the value of strategies for preventing outbreaks of EI. Under typical UK management conditions and vaccination protocols, the model showed that EI would propagate and that the timing of vaccination in connection with the racing season and the arrival of new horses was a critical factor. The predicted outcomes (based on published characteristics of one EI vaccine) suggested that vaccination in mid-December with boosters in June and September was a viable and successful strategy in preventing the spread of EI in a training establishment.     

The efficacy of dantrolene sodium in controlling exertional rhabdomyolysis in the Thoroughbred racehorse

REASONS FOR PERFORMING STUDY: Dantrolene sodium (Dantrium) has been used extensively for the treatment of myopathies in man and anecdotal evidence suggests it is of clinical benefit in the control of exercise-induced rhabdomyolysis (ER) in racehorses, although data to support this are currently lacking. OBJECTIVES: To investigate the efficacy of oral dantrolene sodium in controlling ER in a randomised, double-blind, placebo-controlled crossover trial involving 77 Thoroughbred racehorses in Newmarket, UK. METHODS: Horses were treated on 2 occasions 1 week apart, with treatment days coinciding with a return to exercise following 2 days box rest on each occasion. For the first treatment, each horse was randomly selected to receive either 800 mg dantrolene sodium or a colour-matched placebo administered orally 1 h before exercise. This was followed by crossover to the other treatment on the second occasion, with each horse thereby acting as its own control. Degree of ER was assessed using rising serum creatine kinase (CK) levels, by subtracting pre-exercise blood CK levels from those measured in 6 h post exercise blood samples. For each horse, the difference in change between pre- and post exercise CK values between placebo and dantrolene treatments was calculated, with positive values indicating a greater rise with placebo than with dantrolene sodium treatment. RESULTS: The overall mean difference for all horses was +104.8 iu/l and the null hypothesis, that there was no true difference in non-normally distributed post exercise rises in CK values between placebo and dantrolene treatments, was rejected (P = 0.0013) using the nonparametric Wilcoxon signed rank test. Additionally, no horses given dantrolene sodium showed clinical signs of ER, whereas 3 horses given the placebo developed ER following exercise. The incidence of ER in the study was 4% (3/77). CONCLUSIONS: The results confirmed that oral administration of dantrolene sodium, 1 h before exercise, had a statistically significant effect on reducing the difference between pre- and post exercise plasma CK levels compared with a placebo in the same animals, and preventing clinical ER in susceptible individuals. POTENTIAL RELEVANCE: This study suggested that dantrolene sodium is of use in controlling ER in the Thoroughbred racehorse. Further investigation into pre- and post exercise myoplasmic calcium levels and the repeat of the study late in the season when horses receive a much higher energy ration and more strenuous exercise would appear to be warranted.     

Estimation of heritability for Tying-up syndrome in the Thoroughbred racehorse by Gibbs sampling

Tying‐up is a condition that primarily affects the muscles of horses. In this study, the heritability of the Tying‐up syndrome in the Thoroughbred racehorse was estimated by Bayesian analysis with Gibbs sampling based on the threshold model for binary traits. The data used were the clinical data in racehorses diagnosed by veterinarians of the Racehorse Clinics of Japan Racing Association from 2000 to 2003. The health status of the Tying‐up was treated as a binary trait. In the genetic analysis, the effect of changing the amount of the pedigree or inbreeding information on the estimation of heritability was investigated, too. The heritability estimates with non‐zero probability in the posterior densities were approximately 0.16–0.18 in minimum, suggesting that the heritability of the Tying‐up is not zero at least. The posterior density distributions of the heritability estimates were generally more pointed and sharp with using inbreeding coefficients than without using it, suggesting that more stable estimations were obtained when inbreeding coefficients were used. Among the different amounts of pedigree and inbreeding information, the heritabilities obtained with three or four generations of pedigree using inbreeding coefficients seems to be preferable, i.e. heritability of 0.42 or 0.43 for Tying‐up.