Horse Health

EPM Symptoms in Horses

December 29, 2011

Know the three A’s of EPM symptoms in horses.

EPM Neurological Test

In a neurological test, a normal horse can resist a pull on the tail. Journal photo.

From The American Quarter Horse Journal

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).

EPM Symptoms in Horses

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. 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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If there are signs of EPM, an owner might choose to start treatment without knowing for sure what it is. However, to confirm a diagnosis, a blood and cerebrospinal fluid analysis can be done on the horse. The blood test will only tell if the horse has been exposed to the protozoa, not if he has the disease. A spinal tap, however, will reveal if the protozoa has accessed the nervous system.

Studies estimate that only 40 percent of horses affected by the disease are able to recover fully, although according to Dr. Morgan, up to 60 percent are able to improve to some degree following treatment.

EPM Quick Symptoms Checklist

  • Incoordination (ataxia): stiff, stilted movements (spasticity); abnormal gait or lameness.
  • Incoordination and weakness that worsens when going up or down slopes or when the head is elevated.
  • Muscle atrophy, most noticeable along the topline or in the large muscles of the hindquarters, but sometimes involving the muscles of the face or front limbs.

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  • Paralysis of muscles of the eyes, face or front limbs.
  • Difficulty swallowing.
  • Seizures or collapse.
  • Abnormal sweating.
  • Loss of sensation along the face, neck or body.
  • Head tilt with poor balance; horse may assume a splay-footed stance or lean against stall walls for support.