Ever-Elusive EPM

Charlie (not his real name) had been a patient at our clinic on several occasions. He was always gentle and cooperative as we tried to determine what was causing his curious collection of clinical signs. He had tolerated everything from the obvious poking and prodding to neck radiographs and endoscopic exams. His visits generally centered on his odd gait and some neck stiffness. He had been treated for equine protozoal myeloencephalitis (EPM), although we had trouble confirming whether he had EPM or if antibodies against Sarcocystis neurona--EPM's causative organism--were from being vaccinated against EPM.

At the most recent presentation his owner reported Charlie had been losing weight and was having difficulty swallowing. He had an ulcer on his tongue and around one of his premolars. He had wastage (atrophy) of the chewing muscles on the left side of his face. When we inserted an endoscope or a nasogastric tube, Charlie could swallow, so there was hope. His tongue seemed strong, but there was the left-side atrophy and his muzzle was deviated toward the right. This meant he was suffering from left side facial nerve paralysis.

Charlie also had a heart murmur and was increasingly ataxic (incoordinated), more so in his hind limbs. His gait was reasonably symmetrical, but he was slightly weaker to the left side. Earlier neck radiographs showed some arthritis in his neck. It seemed as if he had a multi-focal disease (occurring in a number of areas) combined with cranial nerve deficits and ataxia, but we still weren't sure what was causing it.

We did an MRI to look at his brain, brain stem, and cranial nerves. We couldn't see definitive lesions that might be causing his clinical signs, although on the MRI there appeared to be asymmetry of the brain stem cranial nerve nuclei of cranial nerves V and VII. We ruled out other muscle and neurologic disorders with biopsies and blood tests.

Charlie received various antibiotics and anti-inflammatory drugs. When we tested his cerebrospinal fluid again for EPM, it was still a weak positive. It was clear that prior EPM vaccination had not protected this horse, and having been vaccinated made him more difficult to diagnose. Earlier long-term and large-dose ponazuril (Marquis) treatment had been used, but it failed to clear the S. neurona, assuming this was causing his nerve deficits. We started him on nitazoxinide (Navigator) at a half dose to minimize the potential for stomach upset. We offered him a variety of feeds because a high-fat diet is recommended to reduce gastrointestinal problems, but we weren't very successful in getting him eating.

Nine days after admission, Charlie became noticeably depressed and developed aspiration pneumonia, an often-fatal complication. We began intravenous antimicrobials and fluid therapy. We placed an indwelling nasogastric tube to increase his feed intake.

The next evening Charlie staggered to the left side and fell. He was unable to rise, and we tried to get him up with a sling twice. We bedded the stall comfortably and allowed him to lie quietly. He passed away a few hours later with no signs of struggling or pain.

Charlie had two gallons of fluid in his lungs, and his spleen was adhered to the body wall, probably because of a previous surgery. We found S. neurona in the brainstem nuclei of several of his cranial nerves. This is what caused Charlie's jaw muscle atrophy, muzzle deviation, ataxia, and difficulties moving food to the back of his throat.

We told you this story to remind you of the continuing importance of EPM and its many complicating factors. EPM is difficult to diagnose, and when the horse has been vaccinated it is even more difficult. Although we have come an incredibly long way in understanding this disease, there are affected horses that slip through our fingers because we either cannot get a clear diagnosis or we're unable to treat effectively. Research continues at veterinary schools around the country, and we encourage you to support that research and take steps to prevent EPM in your horses (see page 48).

Stephen M. Reed, DVM, Dipl. ACVIM, and Peter Morresey, BVSc, MACVSc, Dipl. ACT, ACVIM, are part of the Internal Medicine department at Rood & Riddle Equine Hospital in Lexington, Ky. Stephanie L. Church compiled this report.

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