Epidemiology of shivering (shivers) in horses

Reasons for perfoming study: Investigating the epidemiology of shivering in horses. Objectives: The purpose of this study was to characterise the signalment, clinical signs and management factors associated with shivering (also known as shivers), a relatively rare, poorly defined movement disorder in horses. Study design: Web-based case series survey and case-control study. Methods: A Web-based survey was used to obtain information from owners, worldwide, who suspected that their horse had shivering. Survey respondents were asked to answer standardised questions and to provide a video of the horse. Authors reviewed the surveys and videos, and horses were diagnosed with shivering if they displayed normal forward walking, with difficulty during manual lifting of the hoof and backward walking due to hyperflexion or hyperextension of the pelvic limbs. Cases confirmed by video were designated 'confirmed shivering', while those with compatible clinical signs but lacking video confirmation were designated 'suspected shivering'. Owners of confirmed shivering horses were asked to provide information on 2 horses without signs of shivering (control group). Results: Three hundred and five surveys and 70 videos were received; 27 horses were confirmed shivering (50 controls), 67 were suspected shivering and the rest had a variety of other movement disorders. Suspected shivering horses resembled confirmed shivering cases, except that the suspected shivering group contained fewer draught breeds and fewer horses with exercise intolerance. Confirmed shivering signs often began at <5 years of age and progressed in 74% of cases. Owner-reported additional clinical signs in confirmed cases included muscle twitching (85%), muscle atrophy (44%), reduced strength (33%) and exercise intolerance (33%). Shivering horses were significantly taller (confirmed shivering, mean ∼173cm; control horses, ∼163cm) with a higher male:female ratio (confirmed shivering, 3.2:1 vs. control, 1.7:1). No potential triggering factors or effective treatments were reported. Conclusions: Shivering is a chronic, often gradually progressive movement disorder that usually begins before 7 years of age and has a higher prevalence in tall male horses.