Q: I’ve heard that dogs—particularly older ones—can have strokes, and I was wondering whether this problem occurs in horses. Also, do horses have seizures? Name withheld upon request

A: Horses certainly do have seizures, but rarely do they have true strokes as we know them in people.

A stroke is an injury to a portion of the brain caused when the blood supply is reduced or cut off by a blockage in an artery. In people, strokes usually occur after the arteries have become clogged with fatty plaques—a process that doesn’t happen in horses. On rare occasions, in both people and horses, these blockages can be caused by some other event, such as a clot from a distant infection or even a worm parasite that is “off course” and travels to the brain via the bloodstream.

Seizures are caused when abnormal, synchronous electrical discharges in the forebrain neurons lead to spontaneous, convulsive, involuntary muscle movements. This “short-circuiting” of brain cells tends to begin and end abruptly, and the episode may be brief.

If the seizure begins by affecting only one side of the head or body, it can be referred to as a focal seizure. But usually, the muscle contractions are spread throughout the whole body and the horse falls to the ground; this is called a generalized seizure.

There may be a pre-seizure phase or aura lasting for seconds to minutes when the animal acts distracted and usually is restless. The first outward sign of the seizure itself is muscle spasms, which often proceed to repetitive muscle contractions with the patient thrashing rhythmically. A post-seizure phase of unresponsiveness, restlessness and temporary blindness may last for minutes to hours, occasionally longer. More than one seizure constitutes epilepsy.

Seizures may be caused by almost anything that damages brain cells and makes the neurons misfire. Possibilities include certain neurological diseases or consuming toxic substances. Sometimes there are changes in the brain, such as encephalitis, that can be seen on postmortem examination. Other times, the inciting change that has occurred is functional and not visible, even under the microscope. Epilepsy is usually not inherited in horses.

G. Joe Mayhew, BVSc, PhD
Massey University
Palmerston North, New Zealand