Neurologic Disease in a Horse
History. A client calls and asks you to look at a 4-month-old Arab filly. The owners have had her since birth, and she has always seemed a little clumsy compared with other foals.
They think she is getting worse, however, and say she stumbles in the field. She falls over at limes when she is playing with the other foals, and she seems very stiff, almost stabbing at the ground when she is walking.Clinical Examination. The filly is bright and alert. Her temperature, pulse, and respiration are normal. Abnormalities are limited to your neurological examination. She is weak (paresis) in both the hind and the front limbs (grade II), with the hind limbs being worse (grade III). When you assess her conscious proprioception (ataxia), she is also greatly delayed (grade III hind limbs, grade II front limbs). When she walks, the filly seems to slap at the ground (hypermetria), and she drags her toes forward across the ground. You detect no other neurological deficits.
Comment. This filly has equine degenerative myelo- encephalopathy. An antemortem diagnosis is difficult. Exclusion of other causes is important. Serum vitamin E levels are often, but not exclusively, low. ∕∖ definitive diagnosis is made at necropsy.
The pathogenesis of the disease is not clear, but risk factors include diets low in vitamin E, use of insecticides, keeping animals on dirt lots, and exposure to wood preservatives. On histopathology, significant changes occur in the medulla and spinal cord. There is diffuse neuronal degeneration of the white matter. Astrocytosis and Iipofuscin-Iike pigment accumulate in affected areas. Demyelination is marked.
Animals with this disease have loss of functional neurons as well as the myelin sheath that surrounds them. As a result, the ability to conduct impulses is greatly affected. Clinically, this affects the animal’s ability to respond to external stimuli as well as initiate conscious responses.
Treatment. Supportive treatment is the only therapy that can be given. Keeping horses on green pasture has been shown to be somewhat protective. Supplementing with vitamin E can improve some horses’ condition and slow the progression of disease. There are some familial tendencies in Arabs, Appaloosas, Thoroughbreds, and paso finos.