Descriptive epidemiology of fracture,tendon and suspensory ligament injuries in National Hunt racehorses in training

Reasons for performing study: While fractures and tendon injuries are known to be important diseases in National Hunt (NH) racehorses during racing, there are no accurate estimates of their incidence in NH training yards. Objectives: To estimate the incidence of fractures and tendon and suspensory ligament injuries (TLIs) in NH racehorses in training; to describe the injuries incurred and to compare injury incidence rates by horse age, trainer, gender and background (ex‐flat vs. ex‐store horses). Methods: Cohort data were collected from 14 UK NH training yards for 2 racing seasons. Daily exercise regimens and details of fractures and TLIs occurring in study horses were recorded. Results: Data were gathered from 1223 horses that spent 9466 months at risk of injury. The fracture incidence rate was 1.1/100 horse months and varied significantly by trainer (P<0.001) but not by gender, age or background. The pelvis and third metacarpal bone (MCIII) were the most common fracture sites, although this varied between racing and training. The TLI incidence rate was 1.9/100 horse months and varied significantly by trainer (P = 0.05) and age (P<0.001) but not by gender or background. However, exstore horses were significantly more likely to have a TLI on the racecourse than ex‐flat horses (P = 0.01). Superficial digital flexor injuries accounted for 89% of all TLIs, the remainder being suspensory ligament injuries. Conclusions and potential relevance: Fractures and TLIs are important causes of morbidity and mortality in NH racehorses in training in England. This study provides accurate estimates of their incidence in this population and provides a baseline against which to monitor the effect of future interventions.     

Descriptive epidemiology of fractures occurring in British Thoroughbred racehorses in training

REASONS FOR PERFORMING STUDY: Musculoskeletal injury is the major cause of days lost from training and wastage in Thoroughbred racehorses. Little scientific information is available on the majority of injuries occurring in training. OBJECTIVES: To estimate the incidence of fractures in British racehorses in training and describe the occurrence of different fracture types and bones involved. METHODS: Thirteen UK racehorse trainers participated in a prospective study, providing data on horses in their care for 2 years. Details on horses, their daily exercise and fracture occurrence were recorded. RESULTS: A total of 1178 horses provided 12,893 months at risk. Nontraumatic fracture incidence was 1.15/100 horse months (95% CI = 0.98, 1.35) and 78% of fractures occurred during training. A wide variety of fracture types and bones were involved, although at least 57% were stress fractures. Pelvic and tibial stress injuries accounted for 28% of fractures diagnosed. CONCLUSIONS: It is important to study injuries in training as well as in racing. The number of stress fractures suggests that training regimes for young Thoroughbreds could often be improved to create a more robust skeleton, able to withstand injury. POTENTIAL RELEVANCE: Studying injuries in racehorses in training can provide a scientific basis for the design of safer training regimes.     

Characteristics of Thoroughbred and Quarter Horse racehorses that sustained a complete scapular fracture

Reasons for performing study: To determine if scapular fractures occur in racehorses with distinctive characteristics. Objectives: To test the hypothesis that Thoroughbred (TB) and Quarter Horse (QH) racehorses with a scapular fracture have similar characteristics that are different from those of their respective racetrack populations. Methods: Necropsy findings, case details, last race information and career earnings for TB and QH racehorses that had a scapular fracture in California between 1990 and 2008 were retrospectively compared between breeds. Horse signalment, career earnings, career starts and race characteristics were obtained for all California racehorses. Comparisons were made between affected horses, other racehorses that died, and all horses that raced, in California during the 19 year period. Results: Seventy‐three TB and 28 QH racehorses had a similar, complete comminuted scapular fracture with an articular component, and right forelimb predilection. The QHs had a higher incidence of scapular fracture incurred during racing than TBs (0.98 vs. 0.39/1000 starters). The TB and QH incident rates for musculoskeletal deaths incurred racing were 20.5 and 17.5/1000 starters, respectively; however, a greater proportion of TB musculoskeletal deaths occurred training (40% vs. 8%). Horses with a scapular fracture were more likely to be male and aged 2 or ≥5 years than the racetrack population. Most affected QHs (64%) were 2‐year‐olds; most TBs (74%) were aged ≥3 years. Scapular fractures occurred more commonly during racing in QHs (70%) than TBs (44%). Race‐related scapular fracture was more likely to occur in a Maiden race than in a non‐Maiden race. Horses with a scapular fracture had fewer career starts than the racetrack population. Conclusions and potential relevance: Despite breed differences for signalment and exercise distances, both breeds incur a complete scapular fracture that is more likely to occur in the right scapula of young and older, male racehorses, early in their race career or after few races. Quarter Horses sustain a catastrophic scapular fracture more frequently than TBs.     

Lower respiratory tract disease in Thoroughbred racehorses: analysis of endoscopic data from a UK training yard

REASON FOR PERFORMING STUDY: There has been much research directed at potential causative agents and the epidemiology of lower respiratory tract disease (LRTD), but few reports of the clinical progression and outcome. OBJECTIVES: To define clinical features of LRTD in racehorses, including association with age and average duration of disease, through the analysis of endoscopic data. METHODS: A retrospective analysis of records from a single training yard of endoscopic examinations undertaken over a 2-year period was performed. Horses were subjected to regular endoscopic examination for a variety of reasons, and all horses placed on antibiotic therapy for lower respiratory tract disease were rescoped following treatment. Data analysed included a 0-8 tracheal mucus score based on visible endoscopic mucus and gross tracheal lavage turbidity, as well as age and treatment duration and type. RESULTS: A total of 522 endoscopic examinations undertaken on 123 horses and tracking 169 episodes of lower respiratory tract disease were recorded. Mean duration of disease episode was 15.5 days (median = 11 days, range = 4-61 days). Horses age 2 years were significantly more likely than those age > or=3 years to have at least one episode of respiratory disease (P<0.001). There was a direct association between initial tracheal mucus score and rescope score after treatment. CONCLUSIONS: Lower respiratory tract disease was more common in 2-year-olds than in older horses. Affected horses had endoscopic evidence of increased tracheal mucus accumulation for an average of 15.5 days per episode, a considerably shorter period than that suggested by previous studies. POTENTIAL RELEVANCE: Endoscopic examinations permit longitudinal tracking of lower respiratory tract disease in Thoroughbred racehorses. A prolonged duration of disease, sometimes extending for months, can be expected in a small proportion of cases, some of which appear to be refractory to treatment. There is a clear need for evidence-based analysis of treatment regimes to assist clinicians in decision making when managing disease in both individual and group situations.     

Bone fatigue and its implications for injuries in racehorses

Musculoskeletal injuries are a common cause of lost training days and wastage in racehorses. Many bone injuries are a consequence of repeated high loading during fast work, resulting in chronic damage accumulation and material fatigue of bone. The highest joint loads occur in the fetlock, which is also the most common site of subchondral bone injury in racehorses. Microcracks in the subchondral bone at sites where intra‐articular fractures and palmar osteochondral disease occur are similar to the fatigue damage detected experimentally after repeated loading of bone. Fatigue is a process that has undergone much study in material science in order to avoid catastrophic failure of engineering structures. The term ‘fatigue life’ refers to the numbers of cycles of loading that can be sustained before failure occurs. Fatigue life decreases exponentially with increasing load. This is important in horses as loads within the limb increase with increasing speed. Bone adapts to increased loading by modelling to maintain the strains within the bone at a safe level. Bone also repairs fatigued matrix through remodelling. Fatigue injuries develop when microdamage accumulates faster than remodelling can repair. Remodelling of the equine metacarpus is reduced during race training and accelerated during rest periods. The first phase of remodelling is bone resorption, which weakens the bone through increased porosity. A bone that is porous following a rest period may fail earlier than a fully adapted bone. Maximising bone adaptation is an important part of training young racehorses. However, even well‐adapted bones accumulate microdamage and require ongoing remodelling. If remodelling inhibition at the extremes of training is unavoidable then the duration of exposure to high‐speed work needs to be limited and appropriate rest periods instituted. Further research is warranted to elucidate the effect of fast‐speed work and rest on bone damage accumulation and repair.     

Size and geometry of apical sesamoid fracture fragments as a determinant of prognosis in Thoroughbred racehorses

Reason for performing study: Analysis was performed to examine a method for refining the preoperative prognosis for horses that had surgery to remove apical fractures of the proximal sesamoid bones (PSBs). Objectives: To determine if: 1) there was a difference in size or configuration of apical fractures between the different anatomical locations of the PSBs, which have been shown to affect the prognosis; and 2) the size or configuration could predict the prognosis for racehorses with these fractures. Methods: The study included 110 weanlings and yearlings and 56 training racehorses that underwent surgery to remove apical PSB fractures. Radiographs of the fractures were used for measurement of the abaxial and axial proportion and the abaxial to axial ratio, and race records were used to determine average earnings per start (AEPS) and total post operative starts. Analysis of variance and regression statistics were used to compare the fragment sizes between the specific PSBs on each of the limbs and compare size and configuration of the fractures to prognosis. Results: There was a significantly larger abaxial to axial ratio (more transverse fracture) for the forelimb medial sesamoids than for all other sesamoids in untrained racehorses (P = 0.03). There were no other significant differences in size. There was no relationship between fracture size or configuration and AEPS nor total post operative starts. Conclusions: Apical fractures in weanlings and yearlings tend to be more transverse in the forelimb medial PSBs than the other PSBs. Apical fracture size and geometry does not determine prognosis for apical sesamoid fractures. Potential relevance: Horses that undergo surgery to remove larger apical fractures of the PSBs do not have a worse outcome than those horses with smaller fractures.     

Post mortem evaluation of palmar osteochondral disease (traumatic osteochondrosis) of the metacarpo/metatarsophalangeal joint in Thoroughbred racehorses

Reasons for performing study: Thoroughbred racehorses are commonly affected by subchondral bone injury, but the exact prevalence and the distribution of palmar/plantar osteochondral disease (POD) lesions are unknown. The relationship between pathologies has not been elucidated, although it is widely accepted that POD is a manifestation of traumatic overload arthrosis. Hypothesis: There is an association between grade of POD and other pathologies affecting the third metacarpal and metatarsal (MC/MTIII) condyles (wear lines, cartilage loss, marginal remodelling, dorsal impact injuries and linear fissures). Objectives: To evaluate the pathology found affecting the distal MC/MTIII condyles of Thoroughbred racehorses at post mortem examination, to describe the prevalence and distribution of POD lesions within a population of racing Thoroughbreds and to determine relationships between pathologies of the distal condyles of the third metacarpal and metatarsal bones. Methods: The metacarpo/metatarsophalangeal joints of 64 Thoroughbred racehorses were examined at routine post mortem examination and graded for third metacarpal and metatarsal condylar pathology. Associations between pathologies were determined. Results: POD had a within horse prevalence of 67%. There was a significant linear relationship between grade of POD and grades of wear lines, cartilage ulceration and dorsal impact injuries. There was a significant relationship, but this was not linear, between grade of POD and grade of linear fissures. Using ordinal logistic regression, compared to condyles with grade 0 or grade 2 linear fissures, condyles with grade 1 linear fissures were found to be more likely to have a lower POD grade. Potential relevance: POD can be considered to be a manifestation of traumatic overload arthrosis, but the role of subchondral bone adaptation is complex and warrants further investigation.     

Poor performance associated with equine gastric ulceration syndrome in four Thoroughbred racehorses

Equine gastric ulceration syndrome (EGUS) is commonly recognised in Thoroughbred racehorses. Although EGUS has previously been associated with reduced athletic performance, no objective studies have been reported. This case report describes 4 racehorses referred for investigation of poor athletic performance where EGUS was the only abnormal finding during a thorough investigation of all body systems. All horses showed considerable improvement in performance following treatment with omeprazole. Therefore, this is the first report in which evidence is presented suggesting a direct link between EGUS and decreased performance, other causes of poor performance having been excluded.     

Descriptive results from a longitudinal study of airway inflammation in British National Hunt racehorses

Reasons for performing study: Rates of airway inflammation in young racehorses decrease with time but it is not clear whether this is associated with increasing age or time exposed to the training environment. The structure of the British National Hunt (NH) population allowed closer examination of this relationship. Objectives: To compare rates of inflammatory airway disease diagnosed by tracheal sampling (trIAD), and its components, in NH racehorses by age and training history and with published rates in young racehorses. Methods: A prospective, longitudinal study was conducted on 5 NH yards over 2 years. Period sample prevalences of nasal discharge, tracheal mucus, airway neutrophilia and trIAD (defined by a combination of tracheal mucus and airway neutrophilia) were estimated and compared between horses with different ages and time in training. Results: Horses new to training had twice the odds of visible tracheal mucus as ex‐flat trained horses (OR 2.0; 95% CI: 1.4–2.8; P<0.001) but no significantly increased odds of airway neutrophilia (OR 1.3; 95% CI: 0.8–1.9; P = 0.3) and inconclusive evidence of increased odds of trIAD (OR 1.8; CI: 0.9–3.5; P = 0.08). However, a lower median time in training was significantly associated with the presence of visible mucus (P<0.001), increased mucus (P = 0.005) and trIAD (P = 0.03). No disease measure varied significantly with age. Conclusions: Tracheal mucus and trIAD, but not neutrophilia detected in tracheal wash samples, were less prevalent in horses that had been exposed to the training environment for longer, explaining previously reported associations with age. Potential relevance: Neutrophil proportion in tracheal wash samples is not as useful a clinical tool as measures of visible tracheal mucus for identifying horses requiring treatment or changes in management. The inclusion of tracheal wash neutrophils in the assessment of equine airways, or at least their relative weighting in definitions of trIAD, should be re‐evaluated.