Pathologic and immunohistochemical findings in naturally occurring West Nile Virus infection in horses

The pathologic and peroxidase immunohistochemical features of West Nile fla vivirus (WNV) infection were compared in four horses from the northeastern United States and six horses from central Italy. In all 10 animals, there w ere mild to severe polioencephalomyelitis with small T lymphocyte and lesse r macrophage perivascular infiltrate, multifocal glial nodules, neutrophils , and occasional neuronophagia. Perivascular hemorrhages, also noted macros copically in two animals, were observed in 50% of the horses. In the four A merican horses, lesions extended from the basal nuclei through the brain st em and to the sacral spinal cord and were more severe than the lesions obse rved in the six Italian horses, which had moderate to severe lesions mainly in the thoracolumbar spinal cord and mild rhombencephalic lesions. WNV ant igen was scant and was identified within the cytoplasm of a few neurons, fi bers, glial cells, and macrophages. WNV infection in horses is characterize d by lesions with little associated antigen when compared with WNV infectio n in birds and some fatal human infections and with other important viral e ncephalitides of horses, such as alphavirus infections and rabies.