Q. About two weeks ago my 14-year-old American Warmblood mare that I used for jumpers and preliminary-level eventing randomly collapsed while warming up to jump. She did not trip; it was just more of a drop down and lie there. She lay completely down on her left side, looked to get up, rested her head back down, then finally got up. The episode seemed to last a lifetime, but it was probably only 15-30 seconds. When she got up I walked her around for a bit, then thought to check her gums and heart. Her mucous membranes looked good, and her heart rate was normal with a rate of 36 beats per minute.

Since that episode she has seemed very lethargic and more marish. While working she tires easily and holds her head low. I stop occasionally during workouts to listen to her heart, and it still all seems normal. She does not have any nasal discharge, her appetite and temperature are normal, and so is her respiration and heart rate. I should mention that she is a gray mare, but I cannot find a single melanoma on her body. My vet and I have been trying to figure out what is wrong with my mare, so I thought I would search for some more outside help/input.

Annie, via e-mail

A. Collapse during exercise is certainly a frightening experience for both you and your Warmblood mare. The biggest question is whether the event will reoccur in the future. If a second episode occurs and the cause cannot be identified and controlled, then it would be considered unsafe to ride the mare. Watch the mare's limbs and face for any unusual scratches or swelling to indicate she might have collapsed and then struggled to rise while being turned out or stabled.

There are three major causes of collapse in horses, and these can be difficult to differentiate. The first includes seizures--a horse might have epilepsy, or he might have a mass or lesion in the brain causing periodic abnormal electrical activity.

No matter the cause, a generalized seizure results in loss of consciousness and collapse, which is often associated with random limb and head movement (often rapid and violent) and frequent defecation, urination, or salivation. Sometimes there is not violent movement but just tremors or rapid eye movement. Seizures are usually associated with a pre-seizure phase and a post-seizure phase, during which the horse might appear bewildered, blind, or stagger for a few minutes. Sometimes the post-seizure phase lasts for several hours.

Epilepsy is caused by an electrical or chemical imbalance in the brain and can be treated with medication (horses often require lifelong treatment). This may reduce the frequency of seizures, but it might not eliminate them completely.

To diagnose epilepsy or a brain mass, a veterinarian should perform a thorough neurologic examination on the horse and test his movement, balance (including walking blindfolded), and ability to turn in tight circles and go over obstacles. The practitioner should also test the nerves of the head and face, vision, hearing, and facial muscle strength, and he or she should pay special attention to muscle atrophy, facial symmetry, tone of eyelids, and jaw and ear movement. Ultimately, veterinarians diagnose epilepsy by ruling out structural damage within the brain caused by a tumor, an abscess, or inflammation compressing important brain tissue; CT (computed tomography) or MRI scan (and perhaps also a brain scan or electroencephalogram [EEG]) would be indicated to look for a brain mass. Sometimes these masses are operable, since horses don't have to be able to do calculus and can recover completely, depending on the cause.

Although rare, a brain abscess caused by strangles could create severe clinical signs, but it might be fully correctable after appropriate surgery or antimicrobial therapy. This would require referral to a group of specialists--equine internists who would consult with radiologists, neurologists, neurosurgeons, equine surgeons, and anesthesiologists. These specialists are generally available at university teaching hospitals or large equine referral -practices.

The second reason for collapse is narcolepsy, which is the tendency to fall asleep. Sometimes horses fall asleep and collapse due to "sleep deprivation." These two conditions rarely occur during exercise but, rather, in a quiet stall or during a quiet, pleasurable experience such as being groomed or when the farrier is holding up a leg. Affected horses tend to lower their head and neck and buckle at the knees, literally falling asleep on their feet. They usually wake up as they fall but occasionally collapse onto their knees or noses, causing trauma. Often these horses have back, pelvic, stifle, or hock pain that makes it too uncomfortable for them to lie down and get back up again normally. Although horses can rest standing up, they do need to lie down to sleep properly. Sometimes treating their pain allows them to lie down and sleep, which in turn prevents these periodic -collapses.

The third and most common cause of collapse during exercise would be a heart problem. An equine internist or a veterinary cardiologist with an interest in horses should perform a work-up on the horse to identify any electrical problem affecting the heart rate or rhythm. The problem might be identified by listening to the heart with a stethoscope or performing an electrocardiogram (ECG), but these are only assessments at a single point in time and might only explain why your horse's heart rate seems normal when you listen to it. A veterinarian can help you attach a Holter monitor to record the horse's heart rhythm for at least 24 hours. The horse can even exercise with the monitor in place. You then remove the device and mail it to a cardiologist who reviews the recording for any abnormalities. An occasional abnormal beat is not uncommon, but particular abnormalities or runs of abnormal beats can indicate a serious problem. Veterinarians can also use telemetry to record an ECG in real time during -exercise--while the horse is on a longe line, on a treadmill, or being ridden. Careful listening to the heart may also allow the veterinarian to detect a murmur that should then be investigated via echocardiogram (heart ultrasound). Experienced equine internists or cardiologists perfom this procedure using specialized ultrasound machines that can penetrate 30 cm deep and have Doppler flow, color flow, and computerized cardiac -capabilities.

The best-case scenario for your mare would be that her collapse was a single isolated event caused by a misstep; however, working with your veterinarian and a specialist in equine medicine would be very worthwhile. I would recommend starting with a thorough neurologic and cardiac physical examination with a 24-hour Holter ECG. This probably costs $300-500. Further diagnostics such as ultrasound, CT, and MRI would be more expensive. It would also be worth performing full bloodwork to rule out any infection, inflammation, or liver disease. You can research board-certified equine internal medicine specialists, or you can find them at university teaching hospitals or many equine referral centers.


About the Author

Allison Stewart, BVSc(hons), MS, Dipl. ACVIM, ACVECC

Allison Stewart BVSc(Hons), MS, Dipl. ACVIM-LAIM, ACVECC, is a professor of equine internal medicine at Auburn University’s College of Veterinary Medicine in Alabama. Her research interests include equine critical care, neonate intensive care, neurology, equine fungal infections, sepsis, deafness and vestibular disease, and ultrasound.

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