Ataxia is a clinical sign — not a disease — defined as the loss of voluntary coordination of muscle movement. In horses it manifests as stumbling, swaying, crossing of limbs, dragging of toes, or difficulty maintaining balance, particularly on uneven ground or when turning. The severity is graded on a 0-to-5 scale: grade 0 is normal; grade 5 is recumbent and unable to rise.
The underlying cause is always a lesion somewhere in the nervous system’s proprioceptive or motor pathway — the spinal cord, cerebellum, vestibular apparatus, or brainstem. In horses, the most common cause of progressive spinal ataxia is cervical vertebral myelopathy (wobbler syndrome), in which malformed or unstable cervical vertebrae compress the spinal cord. Other causes include equine protozoal myeloencephalitis (EPM), equine herpesvirus myeloencephalopathy (EHV-1), trauma, and toxin ingestion. Each has a distinct distribution of deficits — hindlimb-only ataxia suggests a thoracolumbar lesion; four-limb ataxia with head tremor implicates the cerebellum.
Ataxia is a veterinary emergency when it appears suddenly or progresses rapidly. A horse that cannot coordinate its hindquarters is a danger to itself and to handlers. Diagnosis uses neurological examination, cervical radiographs, cerebrospinal fluid analysis, and Western blot testing for EPM. Management depends entirely on the underlying lesion — EPM is treated with antiprotozoal drugs; wobbler syndrome may require surgical stabilization in severe cases. The prognosis ranges from full recovery (mild EPM caught early) to euthanasia (severe cord compression). Equine anatomy — particularly the cervical skeletal structures — guides the clinician in localizing where in the nervous system the lesion is likely to sit.
Further Reading: The Wikipedia article on ataxia covers the neurological basis of the condition across species; the Merck Veterinary Manual’s section on disorders of the spinal column and cord in horses covers wobbler syndrome and other causes of progressive equine ataxia in clinical detail.