Pathology of fatal lineage 1 and 2 West Nile virus infections in horses in South Africa.

This study examines the impact and symptoms of two strains of West Nile virus (WNV) in horses in South Africa, with significant focus on how the disease appears and affects the central nervous system (CNS) of the animal.

Overview of the Study

• The research focuses on the pathological effects of West Nile virus lineage 1 and lineage 2 in horses, a subject which has gained importance since the virus was reported in South African horses in 2007.
• These cases show severe neurological signs and the researchers aimed to describe the physical and microscopic characteristics of the diseases in several horses.
• The diagnosis of WNV was confirmed by real-time reverse-transcriptase polymerase chain reaction (RT-PCR) testing of CNS tissue in the horses.

Pathologies Observed

• The research describes significant variation in the types, severity, and distribution of lesions in the CNS across the infected horses. These variations could indicate possible multiple routes of viral entry into the CNS.
• The gross physical lesions observed were non-specific, often resembling those caused by African Horse Sickness (AHS).
• Lineage 2-infected horses showed similar histopathology to the polioencephalomyelitis observed in Northern Hemisphere lineage 1 and recent Hungarian lineage 2 cases.
• Lineage 1 lesions were comparable to the milder lineage 2 cases.

Immunity Response and Presence of Virus in CNS

• The central nervous systems of all studied horses were tested and found to be negative for various other pathogens, ensuring that the observed effects were due to WNV.
• Immunohistochemistry (IHC) on sections of the CNS with notable lesions was performed and resulted in finding only two antigen-positive cells in the olfactory cortex of one case.
• The presence of different types of immune cells – T-lymphocytes, B-lymphocytes, plasma cells, and macrophage-monocytes – was also confirmed in the CNS, implying an active immune response to the infection.