Characterization of non-lethal West Nile Virus (WNV) infection in horses: Subclinical pathology and innate immune response.

Most natural West Nile virus (WNV) infections in humans and horses are subclinical or sub-lethal and non-encephalitic. Yet, the main focus of WNV research remains on the pathogenesis of encephalitic disease, mainly conducted in mouse models. We characterized host responses during subclinical WNV infection in horses and compared outcomes with those obtained in a novel rabbit model of subclinical WNV infection (Suen et al. 2015. Pathogens, 4: 529). Experimental infection of 10 horses with the newly emerging WNV-strain, WNVNSW2011, did not result in neurological disease in any animal but transcriptional upregulation of both type I and II interferon (IFN) was seen in peripheral blood leukocytes prior to or at the time of viremia. Likewise, transcript upregulation for IFNs, TNFα, IL1ß, CXCL10, TLRs, and MyD88 was detected in lymphoid tissues, while IFNα, CXCL10, TLR3, ISG15 and IRF7 mRNA was upregulated in brains with histopathological evidence of mild encephalitis, but absence of detectable viral RNA or antigen. These responses were reproduced in the New Zealand White rabbits (Oryctolagus cuniculus) experimentally infected with WNVNSW2011, by intradermal footpad inoculation. Kinetics of the anti-WNV antibody response was similar in horses and rabbits, which for both species may be explained by the early IFN and cytokine responses evident in circulating leukocytes and lymphoid organs. Given the similarities to the majority of equine infection outcomes, immunocompetent rabbits appear to represent a valuable small-animal model for investigating aspects of non-lethal WNV infections, notably mechanisms involved in abrogating morbidity.

Kinetics of the West Nile virus induced transcripts of selected cytokines and Toll-like receptors in equine peripheral blood mononuclear cells.

West Nile virus (WNV) is one of the most common causes of epidemic viral encephalitis in horses worldwide. Peripheral blood mononuclear cells (PBMCs) are amongst the first to encounter the virus following a mosquito bite. This study aimed to elucidate the transcription kinetics of cytokine, Toll-like receptor (TLRs) and TLRs-associated genes following WNV challenge of equine PBMCs. PBMCs were challenged with an Australian strain of WNV (WNVNSW2011) and transcriptomes were quantified at 2, 6, 12 and 24 h post-infection (pi) using qRT-PCR. Type I and II interferons (IFNα, ß and γ) mRNA transcription increased following WNV exposure, as did the transcripts for IL1α, IL1ß, IL6, IL8, and IL22, but with slightly varying kinetics. TLR1, 3, 5, 7-9 transcripts were also upregulated in equine PBMCsin response to WNV challenge, as were those for MyD88, NF-κB, TRAF3, STAT1 and 2, IRF3 and 7, ISG15, as well as SOCS1 and 3 compared to the control cells. Expression of selected genes in the draining lymph node, spleen and brain (medulla oblongata) of experimentally infected horses was also assessed and transcription of most of these genes was also upregulated here. Although qRT-PCR detected higher viral RNA at 24 h pi compared to 6 h pi, the virus did not replicate productively in equine PBMCs. The up-regulation of gene-transcription for selected cytokines, IFNs, TLRs and TLRs-associated molecules suggests their involvement in virus recognition and control of WNV infection in the horse.