Question: Recently my girlfriend's horse has been diagnosed with Equine Protozoal Myelitis (EPM). He was dragging his hind legs whenever we would ride, and you could hear the scuffing of his hind hooves. We rode him a couple of weeks ago, and he almost fell with her. She called the vet out who took blood samples and said that he may have EPM. We took him to the hospital where they treated him for two weeks, but didn't do a spinal tap. He returned home and is still weak. Is EPM a progressive disease or is it something that just happens quickly? He is stalled 8-10 hours a day and let out to pasture in the evenings. I'm concerned of his instability when we turn him out. Should we be doing something different?
Answer: EPM is a disease where a parasite called a protozoa invades the spinal cord and damages it. This can be a complex disease to work with, both to diagnose correctly and to treat. The diagnosis can be difficult because the symptoms are often slow to come on and are not specific for this condition. This means that the symptoms can look like other diseases. The diagnosis is also difficult because the blood test shows that many if not most horses have been exposed to the parasite, but we know that only a few of the exposed horses get the disease. The spinal tap, which is a painful and expensive procedure involving taking fluid from the spinal column itself, still does not give a conclusive answer, though it may be better than the blood test.
There are a number of other conditions and diseases of horses that can look like EPM and should be considered if your horse has neurologic symptoms. These include West Nile virus, Equine herpesvirus-1 (the neurologic form), Eastern or Western Encephalitis, Wobbler syndrome (usually horses under age 4), EPSM and related diseases, and some less common conditions such as Cauda Equina syndrome.
Here is a discussion of a complete neurological exam and the symptoms of EPM. The most common symptoms are some degree of lack of coordination: perhaps stumbling more than before, slipping easily, bumping into trees on the trail, dragging toes like your horse is, and back or muscle soreness for no apparent reason.
Conditions that can cause similar symptoms and are often not considered in a regular neurologic work-up are Lyme disease and a sprain or damage to the neck or back leading to chiropractic subluxations. A chiropractic subluxation is a fancy word for the loss of normal motion in the vertebra of the spine including the neck, which is essentially a sprain of the spine.
Lyme disease is a fairly new disease in the equine, though the horses and people of Connecticut and nearby areas have had cases for many years. Now it is spreading across the country and the world, though may still be rare in some places. In most cases, Lyme is considered to show signs of joint pain rather than neurological symptoms, however, some of my colleagues and I are recognizing neurologic signs in the horse (neurologic signs in people are not uncommon). Some horses that have not responded to the EPM treatments have responded to the Lyme drugs, so this may be another important disease to check for.
Neck and back injuries can bruise the spinal cord, or the loss of motion in the spine can interfere with the nerve signals. This causes similar signs to EPM, such as stumbling, dragging toes, loss of energy, hitting jumps, turning poorly, and back or neck pain. Neck and back injuries usually do not cause rapid loss of muscle in selective areas or rapidly progressive weakness, especially if the horse shows signs of falling down. A horse may injure his neck or back by falling, but the reason for a severe fall in this case is most likely due to a neurological condition. Very rapid onset of severe symptoms is more likely to point to a bad case of EPM or other infectious disease.
There is a simple test you can perform that lets you know how flexible your horse's neck is. With the carrot stretch you ask your horse to stretch around and take a carrot off his hips and down between his legs (see photos). If your horse has difficulty doing this stretch, you may want to have him evaluated by a chiropractor or osteopath as part of your veterinary work-up. If your horse can do this easily yet has neurologic symptoms, something like EPM is more likely the cause.