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

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.

Phenotypic characterisation of cell populations in the brains of horses experimentally infected with West Nile virus

“…It should be mentioned that although the MAC387 marker is routinely used in staining of macrophage populations, this nonspecific antibody also reacts with polymorphonuclear cells. This is consistent with data indicating that occasional macrophages [1] and neutrophils [24] are found in WNV-infected horses. Conversely, rodents and humans have been shown to have increased macrophage counts in response to WNV infection [21,31,34].…”

“…Phenotypic characterization of the cells within WNV lesions in the natural host is limited to a few human and equine case studies in which immunolabeling was used to characterise inflammatory cell response [1,3,4,[19][20][21][22][23][24] subjectively. The vast majority of WNV characterisation of brain and/or CNS lesions resultant from WNV infection has occurred in experimental rodent models.…”

Sindbis and Middelburg Old World Alphaviruses Associated with Neurologic Disease in Horses, South Africa

“…In those 2 horses, lesions of meningoencephalitis were visible by histopathology but were nonspecific and could not be ascribed to either virus. Immunohistochemistry (IHC) for WNV in equine cases is reportedly a poor diagnostic tool ( 13 ), although IHC for SINV and MIDV needs to be established. In the third fatal case, SINV was detected in blood; neurologic samples were unavailable for testing.…”

Pathological findings in African buffaloes (Syncerus caffer) in South Africa

“…While the source of viral encephalitis was not determined in the buffaloes in this series, the lesions resembled those in domestic horses in South Africa as a result of West Nile virus infection (Williams 2014 ). Serological evidence of alpha and flavivirus infections is recorded in various African wild animals (Kading et al 2013 ).…”