English
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 rare in horses. [2] The percentage of horses affected by the condition is unknown.
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 sign