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The clinical signs of Sarcocystis neurona can be quite variable. Variation of the clinical signs are a reflection of the focal, multifocal or diffuse nature of the lesions which occur randomly in the grey and white matter of the brain, brain stem or spinal cord. Usually the physical examination is within normal limits and the horses appear bright and alert, although focal muscle atrophy may be observed. The onset of clinical signs may be gradual, but more typically there are mild signs acutely with sometimes very rapid progression.
The neurological examination often reveals ataxia and incoordination in all four limbs which sometimes shows lateralization or there may be gait abnormalities with only one limb involved. Infection with Sarcocystis neurona can result in brain stem as well as spinal cord signs and often causes damage to the lower motor neuron of spinal cord or cranial nerves leading to muscle atrophy. The muscle atrophy is most common in the quadriceps and gluteal regions in the hind limbs, but also may mimic sweeney, radial paralysis or polyneuritis equi. If the brain stem is involved, atrophy of the temporal-masseter muscles and occasionally the tongue may be evident along with head tilt, facial nerve paralysis and difficulty swallowing. It is important to carefully examine for signs of muscle wasting as well as loss of sensation along the face, neck or body.
A frequent complaint is obscure lameness which can progress to ataxia, spasticity and incoordination of the limbs. Poor coordination and weakness are worsened by walking with the head elevated or walking up or down a slope. The asymmetric nature of the clinical signs is important to recognize as this can be an early indication of EPM. Horses with brain stem involvement often have head tilt, facial paralysis, loss of sensation of the cornea and internal nares as well as dysphagia, circling and acute recumbency. Some horses have a tendency to lean towards the wall of the stall to balance themselves. At least three horses have been observed with seizures as the only clinical signs. There are also reports of narcoleptic-like symptoms in some horses.