Long‐term performance of show‐jumping horses and relationship with severity of ataxia and complications associated with myeloencephalopathy caused by equine herpes virus‐1

Abstract Background Equine herpesvirus myeloencephalopathy (EHM) has severe impact on the sport horse population. Objective Study the influence of EHM on the likelihood of affected horses to return to their previous performance and investigate the association of clinical variables with prognosis. Animals Twenty‐six horses positive for equine herpesvirus type 1 (EHV‐1) were admitted to a veterinary teaching hospital (VTH) during a natural EHM outbreak at an international jumping event. Methods Data collected from the VTH, the International Equestrian Federation, and surveys completed by the riders and horse owners were retrospectively analyzed. Results Horses affected by EHM had 68% chance of returning to exercise, and 52.9% were able to achieve their preoutbreak performance level. Horses with an ataxia grade at admission ≥4/5 had an increased fatality rate (P < .05) and 10% chance of reaching their preoutbreak performance level. None of the horses with both vascular and urinary complications returned to their previous performance level. Finally, horses vaccinated against EHV‐1 and those with urinary complications had a 71.4% and 43.7% fatality rate, respectively. Conclusions and Clinical Importance Horses affected by EHM were able to return to their previous performance levels, but certain clinical variables were negatively associated with postoutbreak performance. Ataxia grade upon admission and the development of systemic signs of vasculitis and urinary complications were potential poor prognostic indicators in sport horses. Variables linked to fatality included prior vaccination against EHV‐1, ataxia grade upon admission, and the development of urinary complications.