Fainting Foals, Sleepy Horses
- Apr 1, 2006
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 true narcolepsy by definition."
Horse owners report a variety of clinical signs that sometimes lead to a diagnosis of narcolepsy. Andrews notes that some horses will show excessive daytime sleepiness in the pasture, but unless the horse falls or drops to its knees, the condition is usually not narcolepsy.
"Horses are usually normal in between episodes unless they have some other disease," says Andrews. "The episodes are usually a very rapid onset of collapse, but may be more subtle and include excessive drowsiness and head droop. The horse may buckle at the fetlocks (ankles) or carpus (knees), and he may be very lethargic and difficult to lead around. These episodes can occur rarely, daily, or several times per day."
The cataplexy that accompanies a narcoleptic episode in humans is often triggered by stress, such as the anticipation of an important social task. Emotional stress, including laughter, fear, or anger, might also precede a human episode. In some cases, the person's knees might buckle, followed by collapse and a temporary paralysis while he remains conscious and awake. Horses seem to respond similarly, with episodes of narcolepsy following common daily events such as grooming, tacking, bathing, brushing, or exercise.
Sudden and complete collapse of a horse during grooming, feeding, leading out of a stall, or putting tack on the horse are the most common signs, according to Andrews. "In my most recent case, the owner just threw the reins up over the head and the horse collapsed," he says. "I have also had horses suddenly collapse while being ridden, which is dangerous and unpredictable."
Signs of Narcolepsy
As in humans, true narcolepsy in horses is characterized by deep sleep or REM (rapid eye movement) sleep that occurs at an unusual time. Horses get most of their sleep on their feet, thanks to a unique physical structure known as the stay apparatus. This system of tendons and ligaments in the hind legs, and to a lesser degree the front legs, locks the limbs in extension, allowing the horse to sleep while he is standing. Even with this ability, horses typically lie down at least once a day to achieve deep sleep. Normal horses that are provided with comfortable, familiar surroundings can obtain sufficient REM sleep during short periods of recumbency.
Katherine Houpt, VMD, PhD, Dipl. ACVB, who studies behavioral physiology at Cornell University's College of Veterinary Medicine, suggests that if a horse is unwilling or unable to lie down to sleep for a length of time due to insufficient bedding, an uncomfortable surface, unfamiliar surroundings, or chronic pain, he might enter REM sleep while standing. The initiation of REM sleep causes him to fall to his knees and might be referred to as pseudo-narcolepsy.
Problems in Minis?
Some people who raise miniature horses notice a higher incidence of what appears to be narcolepsy, often referred to as "fainting foals." Those who have seen this in miniature horses report a common pattern.
Kay Baxter breeds miniature horses at her farm, Baxter's Painted Pasture, in central Illinois. She is also the founder and president of Chance's Rescue, a rescue organization for miniature horses.
"I had a fainting foal born on my farm," says Baxter. "She eventually outgrew it by six months of age. My veterinarian at the time had never seen such a thing and his prognosis was that she would not live. He saw her hours after being born--she would just fall asleep standing up."
Like people who experience narcolepsy, the foal's episodes occurred suddenly and without warning.
"It was just like turning out a light," says Baxter. "One minute she was awake, then her neck and head would drop, her knees would buckle, and she would slowly sink down to the ground."
Baxter says that breeders of miniature horses note another similarity among those animals that exhibit narcoleptic episodes.
"They seem to stress more easily than large horses," says Baxter. "Any kind of restraint around the neck of a mini foal can also bring on an episode of 'fainting.' Many other miniature horse breeders that I know have noted this. It happened with my filly--if I held her around the neck while the vet was treating her, she would immediately fall asleep and sink to the floor."
Baxter's work with the miniature horse rescue puts her in close contact with others who have reported that they have foals that faint.
"I know people who own several ‘cousins' to my filly," says Baxter. "They also were fainting foals. My filly is now three and has not had an episode in two years."
Robin Cole, who operates Picture Perfect Miniatures and Equine Service in Brooksville, Fla., saw a young mini that appeared to have signs of narcolepsy. The foal would "faint" or go limp when stressed.
"I had the occasion to help with a body clipping when he was just a few days old due to heat stress from his baby coat," says Cole. "As I held him in my lap I could feel his heart rate increase, then he would just go limp and unaware. We would stop clipping at that time, let his heart rate recover, then resume. He would regain awareness, then the cycle would start again."
In addition to this mini, Cole had first-hand experience with a horse of her own.
"I had a full-size Quarter Horse filly who experienced episodes of near-narcolepsy, even when I was mounted," says Cole. "During line-ups, after a class had ended, I had to be careful to keep nudging her as she would buckle in the knees or start to lean over like she was going to fall. This would happen in the face of exercise or prolonged periods of standing."
The observations of Baxter and Cole are similar to what D.P. Lunn, BVSc, MS, PhD, MRCVS, Dipl. ACVIM, professor in the department of clinical sciences at Colorado State University's College of Veterinary Medicine, and colleagues found during research at the University of Wisconsin. The 1993 study involved two closely related miniature horses that had a history of excessive sleepiness, depression, and episodes of collapse.
First, researchers collected genealogical data and history of the two subjects. Data from pharmacological testing, electroencephalography, and cerebrospinal fluid analysis were reviewed. In addition to the clinical tests on the two horses, a survey of miniature horse breeders was conducted to locate other animals with signs of narcolepsy.
The survey identified five potential cases of narcolepsy. Each of the five horses showed episodes of collapse at birth or shortly after birth, especially when the foals attempted to nurse or during handling. The researchers concluded that narcolepsy, although rare, does occur distinctly in miniature horses and appears to have a familial tendency.
What should you do if you suspect narcolepsy in your horse? Diagnosing equine narcolepsy begins with a complete history, in which the owner might be asked to provide information on vaccinations, exposure to other equines, and behavior.
"Certainly," says Andrews, "we rule out other diseases through vaccination history. Knowing whether the horse received vaccinations for West Nile virus, equine herpesvirus 1 (EHV-1) encephalopathy, rabies, etc., would be helpful. Rabies could be confused with narcolepsy early in the disease (the first or second day), but rabies is rapidly progressing, with the horse dying in less than 10 days (usually five days). We usually consider rabies in all horses with an acute onset (less than 10 days) of neurologic disease. Rabies can look like any disease."
"We are very cautious with any horse that has neurologic disease or bizarre behavior," he adds. "Again, most horses with true narcolepsy will be normal between episodes and will not have any lasting effects after an episode.
"Exposure to outside horses would be important especially with infectious disease such as EHV-1 encephalopathy," he says. "Unusual behavior would be included in the typical clinical signs of narcolepsy stated above. Usually we ask if the horse just collapsed or if there was a period of anxiousness, followed by a collapsing and loss of consciousness, followed by a period of lethargy. This would more indicate a seizure rather than a narcoleptic attack."
"We also would ask about breed, since Quarter Horses have a predilection to HYPP, which may precipitate an episode similar to narcolepsy," he says. "We would also walk the pasture to determine if there are toxic plants that look like they have been grazed."
Following a complete history, appropriate diagnostic tests are ordered. "Ruling out liver disease is very important, thus a chemistry panel would be helpful (to show changes in liver enzymes)," says Andrews. "A CBC (complete blood count) would rule out anemia or other reasons for weakness. Also included would be a cardiovascular (heart) exam, including auscultation for murmurs or arrhythmias, and in some cases an ECG and echocardiogram. Usually, we'll perform a standing (sedated) or fully anesthetized EEG (electroencephalogram) to determine if there are brain asymmetries associated with organic brain disease such as an abscess or tumor."
"We also obtain cerebrospinal fluid (CSF) to test for WNV and EPM, and do a cell count and protein concentration to determine if encephalitis (inflammation of the brain) is present," he says. "We may also test for EHV-1 encephalitis by doing a titer (levels of antibodies in the blood) on serum and CSF. In smaller horses or foals (under 350 pounds), we can perform a CT scan to evaluate the brain for organic disease such as a brain tumor, hydrocephalus (fluid swelling in the brain), or abscess."
These diagnostic tests are also helpful in ruling out narcolepsy. Sometimes veterinarians get a positive test for EPM or Cushing's disease, and the narcoleptic episodes go away once the horse is treated for those conditions. Thus, successful treatment might be an indicator of narcoleptic episodes due to other causes.
The veterinarian who sees a full-blown narcoleptic episode has a far easier time making a definitive diagnosis.
"If we're lucky enough to see an episode, it's pretty easy to make the diagnosis," states Andrews. "The horse will collapse, then get right up again without any apparent lethargy or problems." He recalls one horse that collapsed on the way to the stall just before anesthesia.
Doug Novick, DVM, an equine veterinarian who practices in the San Francisco Bay area, has seen one case of narcolepsy during his 18 years of practice. In one suspected case, the horse exhibited all of the signs consistent with narcolepsy. However, a blood profile indicated that the horse was suffering from a severe viral infection.
When all other possible causes of sudden collapse are ruled out, Andrews explains, the use of physostigmine might help make a definitive diagnosis. An injection of physostigmine salicylate, a derivative of the Calabar bean, can be administered to induce a narcoleptic episode. Physostigmine is an antidote for certain drugs and deadly plants that produce an anticholinergic syndrome. It is also used to increase bowel motility following surgery.
"If the owner decides to consider this option, we usually take the horse outside in our arena, with good footing and lots of soft ground, and administer the physostigmine to observe the horse collapsing," says Andrews. "Sometime the horse may not show full-blown signs and just get a bit weak or stagger, which is not very spectacular, but enough to make a working diagnosis."
Once a definitive diagnosis is made, several treatment options are available. Although imipramine--a tricyclic antidepressant widely used in human medicine--is the standard treatment for narcolepsy in horses, another tricyclic antidepressant that can be used is amitriptyline. Tricyclic antidepressants work to block the re-uptake of serotonin and norephinephrine. Nick Dodman, BVMS, Dipl. ACVA, ACVB, a professor, section head, and program director in the Animal Behavior Department of Clinical Sciences at Tufts Cummings School of Veterinary Medicine in North Grafton, Mass., believes "it is this catecholamine surge (the block of serotonin and norephinephrine) that buoys the level of consciousness/awareness and prevents the horse literally from falling asleep on his feet." According to Dodman, amitriptyline is slightly more potent than imipramine, so when horses are first introduced to the drug, they should be on colic watch to see how they react to it. Dodman says that he has used amitriptyline 100% successfully on one narcoleptic horse at 300 mg once daily given in a mixture of sweet feed and molasses. "The horse ate it as willing as an aperitif (appetizer)," says Dodman.
Narcoleptic Horse, Now What?
"With the adult onset form of narcolepsy, we give a guarded prognosis, especially in those horses that collapse during exercise (while being ridden)," says Andrews. "They often become dangerous. I currently have a horse on treatment and the horse is doing well, but there is an individual response."
Companion animals often benefit from studies that explore the causes and cures of human disease. Hopefully, ongoing research into human narcolepsy will eventually benefit narcoleptic horses.
About the Author
Sally Colby is a free-lance writer based in Littlestown, Pa.