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Poor performance in Standardbred racehorses without lameness issues often involves multiple problems, researchers in Italy report.

Chiara Lo Feudo and her fellow researchers, writing in the Journal of Veterinary Internal Medicine, said poor performance is a common and complex condition of racehorses, usually associated with subclinical disorders.

In addition to musculoskeletal conditions, medical disorders are common, and their identification can be challenging for veterinarians.

In fact, poorly performing horses are mostly normal on physical examination, and the identification of underlying medical diseases often requires dynamic tests. Multiple disorders can affect horses at the same time, and it can be difficult to determine the contribution of each to impaired racing performance.

The study team, with the University of Milan, set out to describe the prevalence and distribution of medical disorders diagnosed in a population of poorly performing Standardbred trotters and their associations with fitness variables measured during a standardized treadmill test.

In addition, the associations between simultaneously encountered diseases was evaluated, and the contribution of each disorder on changes in fitness-related variables in a multifactorial setting was determined.

The medical records of Standardbred trotters referred for poor performance to the Equine Sports Medicine Unit of the Veterinary Teaching Hospital at the University of Milan between 2002 and 2021 were reviewed.

All of the 259 non-lame horses identified were in full training upon admission. The age of the horses varied from 2 to 9years. They were made up of 93 females, 146 stallions and 20 geldings.

All underwent a standard diagnostic evaluation for poor performance, with identification of subclinical causes.

The diagnostic protocol included a resting examination, plasma lactate concentration, a treadmill test with continuous heart monitoring and assessment of fitness variables, creatine kinase activity, a treadmill endoscopy, postexercise tracheobronchoscopy, bronchoalveolar lavage, and gastroscopy.

The prevalence of different disorders was evaluated, including cardiac arrhythmias, exertional myopathies, dynamic upper airway obstructions, exercise-induced pulmonary hemorrhage (bleeding from the lungs), moderate equine asthma and gastric ulcers.

Moderate equine asthma and gastric ulcers were the most common disorders identified, followed by bleeding from the lungs, dynamic upper airway obstructions, cardiac arrhythmias, and exertional myopathies diseases that affect the muscles that control voluntary movement.

Hemosiderin scores were positively correlated with levels of neutrophils, eosinophils, and mast cells in lavage fluid. Increased creatine kinase activity was linked with neutrophilia in lavage fluid, dynamic upper airway obstructions, premature complexes (when the lower chambers of the heart contract before they should), and gastric disease affecting the upper squamous portion of the stomach.

Treadmill velocity at a plasma lactate concentration of 4mmol/L and at a heart rate of 200 beats per minute was negatively affected by neutrophilia in lavage fluid, multiple dynamic upper airway obstructions, exertional myopathies, and squamous gastric disease.

The multifactorial nature of poor performance was confirmed, the researchers said, with moderate equine asthma, dynamic upper airway obstructions, myopathies and equine gastric ulcer syndrome representing the main diseases involved in fitness impairment.

Two or more disorders were simultaneously observed in almost 90% of the horses.

The most commonly diagnosed disorders were gastric ulcer syndrome and moderate equine asthma, followed by bleeding in the lungs, equine glandular gastric disease and dynamic upper airway obstructions.

All disorders, except for premature complexes, were associated with one or more fitness variables.

The greatest impairment of fitness was related to the diagnosis of exertional myopathies, severe and multiple dynamic upper airway obstructions, neutrophilic inflammation of the lower airway, and severe grades of squamous gastric disease.

A definitive diagnosis was reached for 254 of 255 horses, which suggests that the diagnostic protocol performed in our study was comprehensive enough to evaluate the most common subclinical non-orthopedic causes of racing poor performance.

Gastric ulcers were diagnosed in almost all horses, with severe squamous gastric disease (grade 3 or 4) detected in 87% of the horses, whereas the prevalence of glandular gastric disease was approximately 58%.

Respiratory diseases represented an equally common cause of poor performance among the study horses. A diagnosis of mild to moderate asthma was made in 97% of the horses, with the majority of them affected by a mixed inflammatory form.

The authors noted that mild to moderate asthma is universally recognized as one of the most common causes of impaired athletic capacity in horses.

Based on endoscopy, around 60% of the horses had blood in the trachea, but only 17% were positive for exercise-induced pulmonary hemorrhage (EIPH) based on their total hemosiderin score and could therefore be considered bleeders.

Clinically relevant premature complexes were detected in 18% of the horses during treadmill exercise.

Finally, post-exercise creatine-kinase activity was higher than normal in 11% of the horses, whereas clinical myopathies (observed during hospitalization or inferred by history) affected 17% of the horses.

Exertional rhabdomyolysis is reported to have a prevalence of 6% among Standardbred racehorses, which may be higher in our study because of the inclusion of only poorly performing horses.

The researchers said the horses in the study came from different training centers over a period of 20years. Therefore, different training techniques may have influenced the results.

Finally, because our study was performed retrospectively and only included poorly performing Standardbred racehorses, future studies should verify whether or not the identified associations are found in a mixed population, including a control group of well-performing horses.

The study team comprised Lo Feudo, Luca Stucchi, Bianca Conturba, Giovanni Stancari, Enrica Zucca and Francesco Ferrucci, all with the University of Milan.

Lo Feudo, CM, Stucchi, L, Conturba, B, Stancari, G, Zucca, E, Ferrucci, F. Medical causes of poor performance and their associations with fitness in Standardbred racehorses. J Vet Intern Med. 2023; 1- 14. doi:10.1111/jvim.16734

The study, published under a Creative Commons License, can be read here.

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Poor performance in non-lame Standardbreds often involves several ... - Horsetalk