Narcolepsy is a neurological and rapid eye movement (REM) sleep disorder. It involves the sudden onset of sleep during the daytime. [1] The condition occurs chronically and throughout life in affected horses unless treatment is provided. [2]

Two types of narcolepsies are recognized in horses: one occurring in young foals (neonatal narcolepsy) and the other affecting adult horses. [2]

Common signs of narcolepsy include excessive drowsiness or sleepiness, sudden lowering of the head, buckling at the knees, and stumbling. In some cases, a loss of muscle control (cataplexy) may occur and result in collapse. [3]

Sleep deprivation is sometimes referred to as “narcolepsy” in horses. Although they have similar signs, sleep deprivation and narcolepsy are separate conditions with different causes. [4]

Narcolepsy is considered rare in horses. Episodes that may appear as narcolepsy are more commonly due to sleep deprivation caused by underlying medical conditions or environmental stressors. [4]

What is Equine Narcolepsy?

Sleep disorders, including narcolepsy, are recognized in many different species of mammals – including horses – but are not fully understood.

Narcolepsy is a neurological condition that results in extreme sleepiness and rapid eye movements that occur during attacks of sleep. [5]

Episodes of narcolepsy commonly take place during periods of inactivity.  Narcolepsy may also be triggered by environmental and emotional stimuli, including nursing, eating and drinking as well as turn-out to pasture, saddling, being startled, or being led.

During narcoleptic episodes, affected horses may exhibit temporary cataplexy (loss of muscle control). However, not all horses with narcolepsy exhibit cataplexy. [3]

The underlying cause of narcolepsy has yet to be identified in horses, although genetics appear to be involved. As well, neurotransmitter imbalances, in particular, neurotransmitters that regulate sleep are believed to play a role. [5]

A narcolepsy diagnosis can be made based on physical examination, clinical signs, health history, and the exclusion of sleep deprivation and other medical conditions that can cause collapse.

The only recognized treatment for narcolepsy is the drug Imipramine. [2]

Affected Horses

Narcolepsy can be present in horses at birth or sometimes appears within a few weeks of being born. The condition can also occur spontaneously in adulthood.

This sleep disorder can occur in any horse but appears to affect specific breeds, including Lipizzaners, Miniature Horses, Shetland ponies, and Suffolks more often than others.

Narcolepsy-like episodes have been reported in horses with pituitary pars intermedia dysfunction (PPID), sometimes called Equine Cushing’s Disease. [6]

Prevalence

Narcolepsy is considered uncommon in horses. [2] The percentage of horses affected by the condition is unknown.

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Causes of Narcolepsy

The exact causes of narcolepsy in horses are poorly understood. Potential causal factors involve alterations in neurotransmitter levels and genetics.

Alterations in Neurotransmitters

Narcolepsy is believed to occur due to alterations in or imbalances in the neurotransmitters that regulate sleep. [5]

Sleep is controlled by complex biological networks in the brain’s hypothalamus that rely on the activities of chemical signalling molecules (neurotransmitters). [5] Neurotransmitters involved in regulating the sleep-wake cycle include acetylcholine, norepinephrine, dopamine, and serotonin among others. [5]

Equine narcolepsy may also involve a deficiency or impairment in orexin (hypocretin) signalling. Orexin is a neuropeptide produced in the brain’s hypothalamus that regulates wakefulness and sleep cycles. [7]

Low orexin levels have been identified in humans with narcolepsy and cataplexy. Researchers propose that a loss of neurons capable of producing orexin may result in daytime sleepiness.[7]

Hypocretins are another group of chemical messengers (neuropeptides) produced in the hypothalamus of the brain. These chemicals transmit signals between neurons (nerve cells) and are also involved in regulating sleep and wakefulness. [1][2][3][8][9][10]

Some horses affected by narcolepsy have decreased levels of hypocretin-1 in samples of their cerebrospinal fluid (CSF), although not all do. [1][2][3][8][9][10]

Genetics

Genetic and non-genetic factors are believed to contribute to the development of narcolepsy in humans. [11] In genetically related narcoleptic canines, the condition is caused by mutations in the hypocretin receptor 2 gene. [11]

Specific genes associated with narcolepsy are yet to be identified in horses. However, the condition has been observed in related equines, which suggests that genetics play a role. [8][12]

A case report describes three half-sibling Lipizzaner fillies diagnosed with narcolepsy. [8] Episodes of narcolepsy occurred from an early age in each of the foals, with clinical signs including sleepiness, swaying, stumbling, buckling at the knees (carpal joints), and falling down. [2][8]

Another case report described a familial occurrence of narcolepsy in two closely related Miniature horses with a history of excessive sleepiness, depression, and episodes of collapse. [12]

Clinical Signs

The clinical signs of narcolepsy are similar to those associated with sleep deprivation.

Common signs of narcolepsy in horses include placing the head in a lowered position, buckling the knees, and sometimes suddenly falling. [13] Affected horses may attempt to rest their heads or hindquarters on fences or other objects, sway, and stumble. [13]

Diagnosis

Your veterinarian will diagnose narcolepsy by conducting a physical examination, considering health history, completing diagnostic testing and ruling out other neurological diseases. [7]

Narcolepsy is a challenging condition to diagnose because horses may appear normal between narcoleptic episodes, and owners may not be aware of sleep attacks. [7]

There are currently no specific criteria for making a definitive diagnosis of narcolepsy.

Health History and Physical Exam

A veterinarian will assess your horse’s health and behaviour to make a diagnosis of narcolepsy. Information provided by horse owners and handlers is essential for an accurate diagnosis.

Your horse’s breed and any history of narcolepsy (indicating a possible genetic basis) will also be considered during an overall health assessment.

Cataplexy (loss of muscle tone or collapse) is an important diagnostic criterion as it is the most frequently observed symptom of narcolepsy.  [14]

Video monitoring can be helpful to determine if a horse is spending time in a recumbent position, which is necessary to attain REM sleep. If the horse is not seen lying down, they may be experiencing sleep deprivation rather than narcolepsy. [4]

Video monitoring may also provide information on how frequently collapse is occurring in narcoleptic horses. [4]

Your veterinarian will note any unexplained abrasions or scars on the face, front of the fetlocks, and hocks that may be present. These types of wounds occur in horses that are collapsing often. [4]

Ruling Out Other Conditions

A physical examination is critical for ruling out other medical conditions that may be causing collapse and sleepiness. [4]

Potential causes of collapse other than narcolepsy include other neurologic issues, electrolyte imbalances, and musculoskeletal, metabolic, respiratory, and cardiovascular causes. [9] Specific laboratory testing may be necessary to diagnose underlying medical conditions.

Sleep deprivation should be ruled out as a possible cause of collapse. To rule out pain as a source of sleep deprivation, your veterinarian may suggest a trial of pain management medication to see if sleepiness resolves. [9]

Other strategies for addressing potential sleep deprivation include ensuring the horse has a safe, relaxing, and comfortable environment to sleep. [4] Some horses may need a companion or change of herd mates to feel secure enough to attain proper sleep. [4]

Most cases of sleep deprivation can be corrected if the underlying cause is identified and resolved. [4] If the horse is sleep deprived, they may initially sleep for excessive periods of time to increase REM. [4] Sleep routines should normalize within a few days. [4]

Laboratory Testing

Hypocretin-1

Low levels of hypocretin-1 in the cerebrospinal fluid (CSF) are characteristic of narcolepsy in humans and dogs, when it occurs as an inherited genetic condition. [18]

Horses affected by narcolepsy may also have decreased levels of this neuropeptide in samples of their CSF, although not all do. [1][2][3][8][9][10] Therefore, testing hypocretin levels is not a definitive strategy to diagnose narcolepsy. [1][2][3][8][9][10]

Physostigmine and Atropine

The physostigmine salicylate challenge test is a diagnostic tool veterinarians can use to evaluate a horse for narcolepsy. Administering physostigmine salicylate can induce narcoleptic episodes in affected horses within 3 – 10 minutes. [15][7]

There are potentially severe adverse effects of physostigmine salicylate, including colic and diarrhea. [15] This drug is also not a definitive test to diagnose narcolepsy. [13][15]

Atropine sulfate is another medication used to confirm a narcolepsy diagnosis. Administering this drug eliminates and prevents narcoleptic episodes for up to 30 hours.

Treatment

The drug Imipramine (a tricyclic antidepressant) has been used to treat horses with narcolepsy. [2][5] This medication can be given orally and must be administered for life. [2]

According to a case report, therapy with 400 mg imipramine administered orally twice daily eliminated narcoleptic behavior in an affected horse and resulted in more energetic behaviour. [16]

Imipramine acts on the central nervous system to modulate levels of neurotransmitters that affect the sleep-wake cycle. It may be administered prophylactically before riding to prevent horses from collapsing during work.

Dosages must be considered carefully, as adverse effects can occur. [2][5]

Prognosis

The prognosis for narcolepsy varies between horses. Although there is no cure for narcolepsy, some affected horses respond positively to Imipramine treatment. [2][5][10]

Horses usually return to normal behaviour shortly after an attack of narcolepsy. However, horses that experience frequent episodes of narcolepsy and falling can suffer from knee, head, and facial trauma.

There are safety and welfare implications for horses that collapse frequently. In some cases, euthanasia is the most humane option for affected horses if the condition cannot be successfully treated.

Prevention

Because the underlying cause of narcolepsy is unknown, there are no specific preventative measures that can be taken to reduce the risk of this disease.

However, the breeding of horses diagnosed with narcolepsy should be carefully considered, as genetics are strongly implicated in the condition. [8][12]

Narcolepsy vs. Other Sleep Disorders

Sleep disorders in horses are generally poorly understood, but is important to differentiate between narcolepsy and other types of sleep issues.

Narcolepsy involves a sudden urge to fall asleep as initiated by an inappropriate trigger, such as intense emotions or activity. This neurological condition results from the improper triggering of sleep onset during the day.

Other types of sleep disorders involve an excessive amount of sleepiness (hypersomnia) and disordered sleep. [1] Horses affected by sleep disorders not classified as narcolepsy may lie down but not have normal REM sleep. Signs of disordered sleep include poor performance and excessive drowsiness.

The exact causes of hypersomnia in horses are unknown. However, it is believed that non-narcolepsy-related sleep disorders may occur due to underlying diseases such as endocrine (i.e. pituitary pars intermedia dysfunction) or neurologic diseases (i.e. viral encephalitis), brain trauma, or equine protozoal myeloencephalitis (EPM). [4]

Other underlying conditions that could contribute to sleep disorders are yet to be identified. [4]

Narcolepsy vs. Sleep Deprivation

Sleep deprivation is often confused with narcolepsy, but is caused by insufficient and poor-quality sleep. Horses can experience sleep deprivation due to any disruption of their sleep cycle. [17]

Possible signs of sleep deprivation include excessive daytime sleepiness, collapsing episodes (not to be confused with narcolepsy and concurrent cataplexy), and marks and abrasions on the face, fetlocks, and hocks due to falling. [17]

Common causes of sleep deprivation include:

Pain and Discomfort

Horses suffering from medical conditions such as chronic colic, degenerative joint disease, old fractures, or neurologic disease may have difficulty sleeping due to pain. [4]

In some cases, horses may refuse to lie down due to inadequate bedding. This makes it difficult to attain adequate sleep because horses must lie down in a lateral recumbent position to enter REM. [4]

Environmental Stressors

Stressors, including exposure to excessive and constant noise, extreme temperatures, and unfamiliar or unsafe surroundings, may prevent horses from sleeping. [4]

Horses that feel insecure in their surroundings due to the threats posed by predators such as cougars, coyotes, and raccoons may not sleep. [4]

Some horses may not sleep sufficiently due to their rank in the herd. Horses that rank at the bottom of the herd and are stressed by aggressive herd mates may not sleep properly. Dominant horses that are constantly alert may also not sleep normally. [4]

The introduction or removal of horses within a herd can cause stress, resulting in interference with sleep patterns. [4]

Normal Equine Sleeping Behavior

Sleep is essential for all horses, but the amount of time required for adequate rest varies between horses.

As polyphasic sleepers, horses sleep for multiple brief periods (a few minutes at a time) daily for approximately three to five hours in total. Most sleep occurs at night.

Foals, especially neonatal foals, sleep more frequently and for longer periods during the day compared to adult horses. [4]

Although horses can sleep standing up, they must lie down to complete a full sleep cycle and avoid the symptoms of sleep deprivation. [4]

Horses can enter slow-wave sleep (SWS) while standing up, but will lie down during this phase if they feel safe and comfortable. [17] SWS is a deep sleep stage that plays a role in growth, memory, and immune function. [17][18]

Horses must lie down to enter REM sleep, the restorative part of the sleep cycle, since they lose muscle tone in this stage of sleep. This phase comprises 20 to 30 minutes of their total sleep throughout a 24-hour period. [4]

Supporting Sleep Cycles

You can support your horse’s sleep patterns by ensuring they have an appropriate environment to sleep in, including a comfortable spot to lie down without excess light or noise.

Follow these tips to help your horse get a good night of sleep:

  • Turn off barn lights and prevent light pollution in the environment. Studies show that horses exposed to artificial light at nighttime spend significantly less time lying down to sleep. [19]
  • Determine whether your horse sleeps better in a stall or turned out in a paddock. Some horses are better able to relax when they are confined on their own in a stall, while others prefer not to be confined. [19]
  • Identify and address any excess noises in your horse’s environment. Not all sources of ambient noise can be eliminated, but if noisy fans or equipment can be turned off at night, your horse may sleep better.
  • Use deep, comfortable bedding and eliminate excess ground moisture to encourage your horse to lie down.

For more research-backed tips, read our article on Sleep Deprivation in Horses.

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References

  1. Fuchs C. et al. Narcolepsy: Do Adult Horses Really Suffer from a Neurological Disorder or Rather from a Recumbent Sleep Deprivation/Rapid Eye Movement (REM)-Sleep Deficiency? EVJ. 2016.
  2. Desjardins, I. Narcolepsy in horses. Pratique Vétérinaire Equine. 2011.
  3. Bathen-Nöthen, A. et al. Hypocretin Measurement in an Icelandic Foal with Narcolepsy. J Vet Intern Med. 2009. View Summary
  4. Aleman, M. et al. Sleep and Sleep Disorders in Horses. AAEP Proceedings 54. 2008.
  5. Peck, K.E. et al. Pharmacokinetics of imipramine in narcoleptic horses. Am J Vet Res. 2001. View Summary
  6. Bentz, A. Update on Pituitary Pars Intermedia Dysfunction in Horses. Accessed 05/04/23.
  7. Andrews, F. Sommardahl, C. Equis – CNS: narcolepsy / cataplexy. Vetlexicon
  8. Léger D et al. Slow-wave sleep: From the cell to the clinic. Sleep Med Rev. 2018.
  9. Ludvikova, E. et al. Familial narcolepsy in the Lipizzaner horse: a report of three fillies born to the same sire. Veterinary Quarterly. 2012. View Summary
  10. Lunn, D.P. et al. Familial occurrence of narcolepsy in Miniature Horses. Equine Vet J. 1993. View Summary
  11. Chabas D. et al. The genetics of narcolepsy. Annu Rev Genomics Hum Genet. 2003.
  12. Equine sleep and sleep disorders. UC Davis. Accessed 05/01/2023.
  13. Bertone, Joseph J. Sleep and Sleep Disorders in Horses. Equine neurology. 2015.
  14. Pichon, S. Narcolepsy in a South-German cart horse mare – a case report. Praktische Tierarzt. 2011.
  15. Aleman, M. et al. Sleep and Sleep Disorders in Horses. AAEP Proceedings. 2008.
  16. van Nieuwstadt RA, van der Want CJ, Binkhorst GJ. Narcolepsie bij het paard [Narcolepsy in horses]. Tijdschr Diergeneeskd. 1993. View Summary
  17. Belling, Theodore H. Sleep patterns in the horse. Equine Practice. 1990.
  18. C. H. Lyle and J. A. Keen. Episodic collapse in the horse. BEVA. 2010.
  19. Greening, L. et al. The effect of altering routine husbandry factors on sleep duration and memory consolidation in the horse. Appl Anim Behav Sci. 2021.