Pathologic and immunohistochemical findings in naturally occuring West Nile virus infection in horses.

This study compared the physical and immunohistochemical characteristics of West Nile Virus infection in horses from the northeastern United States and central Italy, exploring variances in symptoms, lesions and their severity, as well as localisation of the virus within affected tissues. Infections were found to be characterized by mild to severe nervous system inflammation with minimal associated antigen, unlike other viral encephalitides.

Introduction and Methodology

• This study focused on identifying and comparing the pathologic (physical) and peroxidase immunohistochemical (microscopic) features of West Nile flavivirus (WNV) infection in horses.
• Four horses from the northeastern United States and six from central Italy were examined.
• The researchers aimed to identify whether there were common patterns in the WNV infection, or geographic differences in the manifestation of the disease.

Findings

• All horses, regardless of geographic location, experienced mild to severe polioencephalomyelitis, an inflammation of the grey matter in the brain and spinal cord. This was coupled with the presence of T lymphocytes, macrophages around blood vessels, the formation of glial nodules, neutrophils, and occasional neuronophagia, a condition characterized by the destruction of nerve cells.
• Half of the horses had perivascular hemorrhages, or bleeding around blood vessels, observed macroscopically.
• Contradistinctly, the American horses experienced more severe lesions which extended from the basal nuclei through the brain stem and to the sacral spinal cord, whereas the Italian horses displayed moderate to severe lesions primarily in the thoracolumbar spinal cord with mild lesions in the rhombencephalon.
• WNV antigen, the substance which induces an immune response, was minimally identified within the cytoplasm of a few neurons, fibers, glial cells, and macrophages.

Conclusion

• West Nile Virus infection in horses is characterized by specific nervous system lesions with minimal associated antigen.
• This feature distinguishes it from other viral encephalitides of horses, such as alphavirus infections and rabies, and even from WNV infections observed in birds and some fatal human cases, which typically exhibit significant antigen presence.