Pennsylvania Animal Diagnostic Laboratory System
PADLS Conferences Spring 2001 15. WNV in a Horse
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The number of equine cases of clinical West Nile Virus infection in the USA in 2000 was 59, of which 36 survived and 23 (39%) died or were euthanized. Most of the equine cases were along the seaboard of the northeastern part of the country. One case was in Dauphin County in central Pennsylvania, and is the subject of this report.

The horse was a 25-year-old Paso Fino gelding that had onset of clinical signs in September, 2000 and was euthanized 4 days later. There were episodes of walking only backwards and not forwards, with episodes of walking only forwards and not backwards. Head movements were jerky, and there was grinding of the teeth, inability to swallow. Body temperature was between 102 and 103 degrees.

A full range of tissues was examined histologically, including cervical spinal cord. The only significant lesion was moderately severe, diffuse, subacute encephalomyelitis. The lesions were most notable in the brain stem and in the gray matter of the spinal cord. Blood vessels had a thin cuff of lymphocytes and a few macrophages and rare neutrophils. A few to moderately numerous small nodules of microglial cells, macrophages and a few neutrophils were present. A few eosinophilic spheroids and necrotic neurons were associated with the inflammatory foci. In all areas of the white matter of the cervical spinal cord, there were a few distended myelin sheaths and swollen axons. Immunohistochemical staining demonstrated antigens of West Nile virus in phagocytic cells in the inflammatory foci, and in neuronal processes and a few neuronal cell bodies. No West Nile viral antigen could be demonstrated in any of the non-CNS tissues.

The horse was positive for West Nile virus by isolation in tissue culture, RT-PCR test for West Nile viral RNA, IgM capture ELISA and serum neutralization test.

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