Characterization of Virulent West Nile Virus Kunjin Strain, Australia, 2011

Melinda J Frost,Jody Hobson-Peters,Stephen L Doggett,Rodney Davis,Kathleen E Arzey,Paul Hick,Xingnian Gu,Andrew Read,John Haniotis,Roy A Hall,Jing Zhang,Alexander A Khromykh,Fiona J May,Natalie A Prow,Deborah Finlaison,Richard C Russell,Peter D Kirkland,Christine Hornitzky,Judith H Edmonds

doi:10.3201/eid1805.111720

Abstract

To determine the cause of an unprecedented outbreak of encephalitis among horses in New South Wales, Australia, in 2011, we performed genomic sequencing of viruses isolated from affected horses and mosquitoes. Results showed that most of the cases were caused by a variant West Nile virus (WNV) strain, WNV(NSW2011), that is most closely related to WNV Kunjin (WNV(KUN)), the indigenous WNV strain in Australia. Studies in mouse models for WNV pathogenesis showed that WNV(NSW2011) is substantially more neuroinvasive than the prototype WNV(KUN) strain. In WNV(NSW2011), this apparent increase in virulence over that of the prototype strain correlated with at least 2 known markers of WNV virulence that are not found in WNV(KUN). Additional studies are needed to determine the relationship of the WNV(NSW2011) strain to currently and previously circulating WNV(KUN) strains and to confirm the cause of the increased virulence of this emerging WNV strain.

Highlights

Viruses were isolated from 4 of these samples; each showed distinct cytopathology in BHK21 and Vero cells. reverse transcription PCR (rRT-PCR) of the culture fluids and immunoperoxidase staining of the cells with pan–flavivirus-reactive and West Nile virus (WNV)-specific monoclonal antibody (mAb) confirmed the isolation of a West Nile–like virus
Eight isolates of WNVNSW2011 were isolated from Culex annulirostris Skuse mosquitoes during the 2011 vector season
It is estimated that at least 1,000 horses were affected during an unprecedented outbreak of encephalitis in southeastern Australia during 2011

Summary

Introduction

In Australia, infection with WNVKUN has been detected intermittently in horses in the Southeast, but reports of encephalitis caused by this virus are rare [3]. In 2011, an outbreak of encephalitis occurred among horses in NSW. To analyze this strain of WNVKUN, we conducted genomic sequencing, antigenic profiling, in vitro growth kinetics, and mouse virulence studies on virus isolates from diseased animals and mosquitoes

Methods

Results

Conclusion