If you have ever witnessed a horse having a so-called "episodic event," you aren't likely to soon forget it, particularly if the event occurred while the horse was under saddle. Episodic events are defined here as unusual or unexpected, usually brief (spanning seconds to minutes) behavioral events. Between attacks, affected horses and foals appear totally normal.

At the 11th Congress of the World Equine Veterinary Association, held September 24-27, 2009 in Guarujá, Sao Paulo, Brazil, I.G. Joe Mayhew, BVSc, FRCVS, PhD, Dipl ACVIM, ECVN, from Massey University in New Zealand, relayed the current body of knowledge regarding sleep disorders and seizures and epilepsy in both foals and horses.

"Fainting" has been described in Suffolk, Shetland, Fell, Warmblood, and Miniature Horse foals beginning at a few weeks of age. Affected foals show daytime sleepiness and episodes of partial or total cataplexy (i.e., flaccid paralysis and lack of normal limb reflexes). Adult-onset, inherited narcolepsy has not been reported in horses; however, sporadic idiopathic hypersomnia, or sleep attacks, occur in a variety of breeds. This latter condition is characterized by drowsiness, hanging of the head, buckling of knees, and in some animals, sudden and total collapse to the ground, usually with full recovery in seconds.

In both adults and foals, the sleep attacks appear to be lifelong conditions, but can sometimes be treated with the antidepressant drug imipramine administered via injection two to three times daily. According to Mayhew, such treatment is usually not recommended if the patients aren't injuring themselves during their falling episodes.

In neonatal foals, seizures are generally indicative of sepsis and/or hypoxic or ischemic encephalopathy (i.e., dummy or barker foals). Adolescent foals, particularly Arabians, can suffer from an inherited form of adolescent epilepsy that is both treatable and resolves over time.

Like narcolepsy, epilepsy manifested by repeated generalized seizures with no apparent underlying disease process has not been reported in adult horses. Instead, generalized seizures in adults appear to be most commonly caused by:

  • Viral encephalitis;
  • Hepatoencephalopathy;
  • Leukoencephalomalacia (a disease of the central nervous system) due to Fusarium sp. mycotoxicosis;
  • Other toxicities;
  • Brain abscesses, and
  • Equine protozoal myeloencephalitis (EPM).

 Anticonvulsant therapy is indicated in affected horses with cluster seizures or status epilepticus (more than one seizure per month), or if the patient injuries itself and the animal's owner will not consider euthanasia. Phenobarbital alone or in combination with potassium bromide is the treatment of choice. Guidelines for administration are included in Mayhew's book Large Animal Neurology (Wiley-Blackwell, UK, December 2009).

Mayhew cautioned that these animals are unsafe to ride until they have remained seizure-free while off of medication for six months.

Mayhew also advised, "24-hour video recording can be a powerful tool to capture episodes of sleep and seizures and help the veterinarian in making an accurate diagnosis and prognosis."

About the Author

Stacey Oke, DVM, MSc

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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