Equine protozoal myeloencephalitis is a neurological disease caused by a tiny parasite. Opossums carry the parasite, and horses contract the disease by eating feed or drinking water contaminated by opossum feces. The EPM symptoms in horses include the three A’s: Asymmetrical ataxia (incoordination) with or without muscle atrophy (degeneration).
Signs of EPM vary from horse to horse, depending on where the protozoa attack the nervous system. Onset of the disease is most common in summer and fall months, and symptoms can develop slowly or present themselves within several days. They can range from mild to severe. Often the first indications are stiffness, asymmetrical gaits and cranial nerve deficits.
Symptoms include ataxia (incoordination), spasticity (stiffness, abnormal gaits or lameness, muscle atrophy, paralysis, difficulty swallowing, head tilt, seizures and collapse, abnormal sweating, loss of sensation and poor balance). Incoordination and weakness are often exacerbated by going up or down slopes or movement when the head is elevated. Cranial nerves control function of the head, so if affected, the horse might have paralysis of the face, problems coordinating actions (chewing, dropping feed, etc.), swallowing or vocalizing.
“I’ve had people report that they noticed when the horse whinnied, it sounded different,” Dr. Kenton Morgan, an equine veterinary specialist for Pfizer Animal Health, says.
A horse suspected of having the disease should be inspected by a veterinarian as soon as possible. The first step is a general examination, followed by a neurological exam. The neurological exam includes assessing the cranial nerve function and working down the body to make sure the sensations are normal. Neurological deficiencies are scored from 0 (none) to 4 (obvious abnormalities). Cranial nerve checks include making sure the nerves that control eye dilation, blinking and the gag reflex work properly. Problems the vet might watch for in the neck include lack of flexibility or abnormal muscling.
Early detection and treatment are key to recovery. Download AQHA's FREE EPM report to start protecting your horse.
The vet will perform a panniculus reflex test, using a blunt object (such as a ballpoint pen) to press on the skin all the way down the backbone. A normal horse will twitch the skin, as if trying to rid himself of a fly. There are also gait assessments (known as proprioceptive tests) that include watching the horse back, circle and move on an incline. The examiner might also test the horse’s balance by pulling his tail while walking. If the horse loses his balance or is unable to resist the pull, this shows neurological damage. At a halt, the vet will also move one of the horse’s legs across his opposing leg to check the horse’s awareness of where his feet are (the horse should return the moved leg to its normal position within 30 seconds).
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