Humans have been fascinated with narcolepsy for centuries. The sight of an otherwise normal person suddenly lapsing into unexplainable deep sleep was cause for curiosity, if not amusement, even after scientists in the late 1800s found that episodes of sudden muscle weakness (cataplexy) were due to physiological changes in the brain. But can the same thing happen to a horse? Interestingly, many horse owners say that they have heard of a horse or pony with narcolepsy. But most veterinarians, despite being familiar with the condition, never see a true narcoleptic episode during practice.

What Is Narcolepsy?

Narcolepsy in horses is characterized by excessive daytime sleepiness and episodes of cataplexy–the sudden but usually brief loss of muscle tone that results in partial or complete collapse.

According to Frank Andrews, DVM, MS, Dipl. ACVIM, professor of large animal clinical sciences and section chief of large animal medicine at the University of Tennessee, episodes that appear to be narcolepsy can occur with various neurological conditions. In any case of suspected narcolepsy, Andrews stresses the importance of identifying neurologic problems caused by other diseases and conditions.

“True narcolepsy, in my opinion, usually is not associated with another disease,” says Andrews, who has seen about 15 or 20 cases of narcolepsy in his 22 years of practice.

“We attempt to rule out other major diseases such as EPM (equine protozoal myeloencephalitis), pituitary adenoma (equine Cushing’s disease), equine herpesvirus (EHV) encephalitis, West Nile virus (WNV) encephalopathy, epilepsy (seizures), HYPP (hyperkalemic periodic paralysis), liver disease, and collapse associated with the respiratory system or cardiovascular system (arrhythmia or congestive heart failure),” he says. “Sometimes horses with these diseases have narcoleptic-like episodes, but not