A seizure in a horse is an episode of abnormal, uncontrolled electrical activity in the brain that results in involuntary muscle contractions, loss of balance, altered consciousness, or collapse. The horse may paddle its legs, exhibit rhythmic jaw clenching, fall to the ground and thrash, or simply display sudden behavioral changes and ataxia depending on which region of the brain is involved and the seizure’s intensity.
Common causes of equine seizures include hepatic encephalopathy (liver failure elevating ammonia and toxins in the blood), hypocalcemia (low blood calcium, particularly in lactating mares), head trauma, ingestion of neurotoxic plants such as yellow star thistle or water hemlock, and idiopathic epilepsy. In neonates, neonatal maladjustment syndrome (dummy foal syndrome) can produce seizure activity within hours of birth. Infections causing encephalitis (equine herpesvirus neurological form, West Nile virus) are also recognized triggers.
A seizing horse is a safety emergency. Bystanders should clear the area of hard objects and avoid restraint, which can worsen injury; if the horse falls, do not approach until activity has stopped. Post-seizure, the horse typically shows a recovery period (post-ictal phase) of confusion, weakness, or incoordination. Veterinary evaluation is required to identify the underlying cause, as treatment depends entirely on etiology — anticonvulsants, calcium supplementation, liver support, or toxin management depending on what triggered the event. Recurrent seizures warrant neurological workup and may indicate structural brain pathology requiring imaging. Note that sedative overdose is itself a recognized pharmacological trigger and should be confirmed as ruled out before any sedation protocol is adjusted.
Further Reading: Wikipedia’s article on seizures explains the underlying electrical mechanisms and classification system. The Wikipedia entry on hepatic encephalopathy details the ammonia-driven neurological disruption that is among the more common triggers in adult horses.