I was invited recently by a colleague to evaluate an older mare whose owner reported episodes of collapse while the mare was resting in her stable. These episodes had been increasing in severity over the previous year.

Occasionally the episodes happened while the mare was tied up outside her stable being groomed or while her owner was standing having a chat with another rider. Although the mare was only used for light hacking, her owner was very attached to her and understandably concerned at what was causing the episodes. In the absence of any clinical evidence of other causes of intermittent weakness or collapse such as heart disease or seizures, my colleague had made a presumptive diagnosis of “equine narcolepsy”. I admitted the mare to the clinic and set up a video camera overnight just outside her stable to capture the episodes.

In humans, narcolepsy is described as a neurologic disorder, where the brain fails to regulate the cycle of sleeping and waking. Human sufferers describe excessive sleepiness during the day and episodes of sudden collapse or muscle weakness which may be set off by a strong emotional reaction such as laughter, anger or fear. They also describe vivid hallucinations, nightmares and short episodes of muscle paralysis (inability to move or speak) while falling asleep or waking up.

In horses, a condition characterised by excessive sleepiness and/or sudden weakness or collapse has been recognised for years and is called “equine narcolepsy”. For obvious reasons, the occurrence of nightmares, hallucinations and sleep paralysis have not been confirmed in these horses!

Episodes of “equine narcolepsy” usually occur when the horse is resting in its stable or in the field. The horse begins to look sleepy and the head and neck droop. It may place its legs wide for support. Often the horse buckles on its forelimbs and catches itself before it collapses, standing back up again. Alternatively it may sink on its haunches or buckle over onto the front fetlocks. The episodes last from seconds to minutes. If the horse is moved during the episode it appears really uncoordinated. After the episode, and in between episodes, the horse appears completely normal. Quite often the episodes are not actually observed and owners simply complain of unexplained grazing and wounds of the skin over the front fetlocks.

The sudden weakness or collapse associated with this condition has also been observed in horses that are not resting. Stimulants such as taking the first bite of feed, or mouthful of water, being groomed or being led outside from the stable have induced episodes of collapse in affected horses. It is speculated that these events may cause an emotional response in the horse in a manner akin to the condition in humans. Unfortunately, these episodes of collapse during activity make it dangerous to handle or ride these animals.

It is not known what causes narcolepsy in people but theories include genetic abnormalities or a disease of the immune system leading to abnormal levels of chemicals in the brain. There may be a genetic component to “equine narcolepsy”, with thoroughbreds, standardbreds and Welsh and Shetland ponies among the breeds more commonly affected. Usually genetic disease becomes apparent in the young foal, within the first six months of life.

However, adult onset cases of “equine narcolepsy” are also well recognised. Some experts believe that adult onset “equine narcolepsy” is not true narcolepsy but occurs as a result of sleep deprivation. Horses that do not have regular periods of slow wave sleep where they lie down and rest may become sleep deprived. The ability or willingness to lie down, or get up, may be affected by arthritic pain in the limbs or neck or by behavioural issues such as discomfort or insecurity in their environment or lack of dominance in the herd.

RECORDED

On the video recorder overnight I captured some impressive episodes of profound sleepiness and collapse although the mare never lay down to rest. I also observed, on closer examination of the mare, clinical signs consistent with significant neck and multiple limb arthritis, which may have contributed to sleep deprivation. Questioning of the owner revealed that she never observed the mare lying down to sleep. The mare also had chronic wounds over her front fetlocks as are often described in these cases.

There is no cure for narcolepsy in humans or horses. In humans, lifestyle adjustments minimising stress are recommended along with the strategic use of stimulants and antidepressants. In horses, various antidepressants have been used to attempt to control the signs; however, these drugs have quite severe side effects of colic and are not commonly used. Some believe alleviating the pain associated with arthritis or addressing behavioural issues resulting in sleep deprivation in horses goes a long way to improving the quality of life for these horses. For this mare, a course of anti-inflammatory pain relieving medications and some homeopathic therapy appeared to significantly improve the situation. The owner reported far fewer episodes of sleepiness or collapse and a generally improved demeanour.

Ultimately, horses that display intermittent episodes of weakness and collapse are not safe to be around and certainly not safe to ride and this should remain an overriding concern when these cases are identified. Small management changes might make life easier in the management of valued companion horses with this condition.

Installing rubber padding and removing any loose objects in the stable will minimise the trauma to the horse if it falls. Putting light bandages over the front fetlocks will stop them getting grazed and wounded if the horse collapses. Also quick release knots should be used when tying the horse up to be groomed. This kind of supportive and palliative care may have a place in prolonging the life of a valued companion horse, provided the owner is aware of the risks and takes appropriate care while handling the animal.

Dr Vivienne Duggan MVB PhD Dipl. ACIM & ECEIM is President of Veterinary Ireland Equine Group and is a lecturer in Equine Medicine at the UCD School of Veterinary Medicine.

Email: hq@vetireland.ie

Telephone: 01-4577976