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Sarcocystis neurona probably causes little pathology in the appropriate intermediate host, however, CNS lesions in the horse are often extensive. Lesions may be microscopic to several centimeters wide. The brainstem and spinal cord are affected most often. Microscopically, lesions are characterized by focal to diffuse areas of nonsuppurative inflammation and necrosis. Grey and/or white matter are affected. Organisms have been found in neurons, leucocytes, and vascular endothelium although they tend to develop most often in neurons.