Abstract:
Since 2007, West Nile virus (WNV) has been reported in South African horses, causing severe
neurological signs. All cases were of lineage 2, except for one case that clustered with lineage
1 viruses. In the present study, gross and microscopic lesions of six South African lineage
2-infected horses and the one lineage 1 case are described. Diagnoses were confirmed by
real-time reverse-transcriptase polymerase chain reaction (RT-PCR) of central nervous system
(CNS) tissue and one by RT-PCR of a brain virus isolate. The CNS of all cases was negative by
RT-PCR or immunohistochemistry (IHC) for African horse sickness (AHS), equine encephalosis
virus, equine herpes viruses 1 and 4, other zoonotic flaviviruses, alphaviruses, and shunivirus,
and either by immunofluorescence or IHC for rabies. Gross visceral lesions were nonspecific but
often mimicked those of AHS. The CNS histopathology of WNV lineage 2 cases resembled the
nonsuppurative polioencephalomyelitis reported in the Northern Hemisphere lineage 1 and
recent Hungarian lineage 2 cases. Occasional meningitis, focal spinal ventral horn poliomalacia,
dorsal and lateral horn poliomyelitis, leucomyelitis, asymmetrical ventral motor spinal neuritis
and frequent olfactory region involvement were also seen. Lineage 2 cases displayed marked
variations in CNS lesion severity, type and distribution, and suggested various viral entry
routes into the CNS, based on findings in experimental mice and hamsters. Lineage 1 lesions
were comparable to the milder lineage 2 cases. West Nile virus IHC on CNS sections with
marked lesions from all cases elicited only two antigen-positive cells in the olfactory cortex
of one case. The presence in the CNS of T-lymphocytes, B-lymphocytes, plasma cells and
macrophage-monocytes was confirmed by cluster of differentiation (CD) 3, CD20, multiple
myeloma oncogene 1 (MUM1) and macrophage (MAC) 387 IHC.
