title: Role of enhanced vector transmission of a new West Nile virus strain in an outbreak of equine disease in Australia in 2011.
abstract: BACKGROUND
  In 2011, a variant of West Nile virus Kunjin strain (WNVKUN) caused an unprecedented epidemic of neurological disease in horses in southeast Australia, resulting in almost 1,000 cases and a 9% fatality rate. We investigated whether increased fitness of the virus in the primary vector, Culex annulirostris, and another potential vector, Culex australicus, contributed to the widespread nature of the outbreak.
  
  
  METHODS
  Mosquitoes were exposed to infectious blood meals containing either the virus strain responsible for the outbreak, designated WNVKUN2011, or WNVKUN2009, a strain of low virulence that is typical of historical strains of this virus. WNVKUN infection in mosquito samples was detected using a fixed cell culture enzyme immunoassay and a WNVKUN- specific monoclonal antibody. Probit analysis was used to determine mosquito susceptibility to infection. Infection, dissemination and transmission rates for selected days post-exposure were compared using Fisher's exact test. Virus titers in bodies and saliva expectorates were compared using t-tests.
  
  
  RESULTS
  There were few significant differences between the two virus strains in the susceptibility of Cx. annulirostris to infection, the kinetics of virus replication and the ability of this mosquito species to transmit either strain. Both strains were transmitted by Cx. annulirostris for the first time on day 5 post-exposure. The highest transmission rates (proportion of mosquitoes with virus detected in saliva) observed were 68% for WNVKUN2011 on day 12 and 72% for WNVKUN2009 on day 14. On days 12 and 14 post-exposure, significantly more WNVKUN2011 than WNVKUN2009 was expectorated by infected mosquitoes. Infection, dissemination and transmission rates of the two strains were not significantly different in Culex australicus. However, transmission rates and the amount of virus expectorated were significantly lower in Cx. australicus than Cx. annulirostris.
  
  
  CONCLUSIONS
  The higher amount of WNVKUN2011 expectorated by infected mosquitoes may be an indication that this virus strain is transmitted more efficiently by Cx. annulirostris compared to other WNVKUN strains. Combined with other factors, such as a convergence of abundant mosquito and wading bird populations, and mammalian and avian feeding behaviour by Cx. annulirostris, this may have contributed to the scale of the 2011 equine epidemic.
authors.affiliation: Virology, Public and Environmental Health, Forensic and Scientific Services, Department of Health, Queensland Government, Brisbane, QLD, Australia. andrew.vandenhurk@health.qld.gov.au.|Australian Infectious Disease Research Centre, School of Chemistry and Molecular Biosciences, University of Queensland, Brisbane, QLD, Australia. andrew.vandenhurk@health.qld.gov.au.
authors.forename: Andrew F
authors.initials: AF
authors.lastname: van den Hurk
authors.affiliation: Virology, Public and Environmental Health, Forensic and Scientific Services, Department of Health, Queensland Government, Brisbane, QLD, Australia. sonja.hall-mendelin@health.qld.gov.au.
authors.forename: Sonja
authors.initials: S
authors.lastname: Hall-Mendelin
authors.affiliation: Department of Medical Entomology, University of Sydney and Pathology West - ICPMR Westmead, Westmead, NSW, Australia. Cameron.Webb@health.nsw.gov.au.
authors.forename: Cameron E
authors.initials: CE
authors.lastname: Webb
authors.affiliation: Australian Infectious Disease Research Centre, School of Chemistry and Molecular Biosciences, University of Queensland, Brisbane, QLD, Australia. s.tan5@uq.edu.au.
authors.forename: Cindy S E
authors.initials: CS
authors.lastname: Tan
authors.affiliation: Institute of Health and Biomedical Innovation and School of Biomedical Sciences, Queensland University of Technology, Kelvin Grove, QLD, Australia. francesca.frentiu@qut.edu.au.
authors.forename: Francesca D
authors.initials: FD
authors.lastname: Frentiu
authors.affiliation: Australian Infectious Disease Research Centre, School of Chemistry and Molecular Biosciences, University of Queensland, Brisbane, QLD, Australia. n.prow@uq.edu.au.
authors.forename: Natalie A
authors.initials: NA
authors.lastname: Prow
authors.affiliation: Australian Infectious Disease Research Centre, School of Chemistry and Molecular Biosciences, University of Queensland, Brisbane, QLD, Australia. roy.hall@uq.edu.au.
authors.forename: Roy A
authors.initials: RA
authors.lastname: Hall
mesh_terms: D000818:Animals; D001315:Australia; D003465:Culex / Q000502:physiology / Q000821:virology*; D004196:Disease Outbreaks; D006734:Horse Diseases / Q000453:epidemiology / Q000635:transmission* / Q000821:virology; D006736:Horses; D007303:Insect Vectors / Q000502:physiology / Q000821:virology*; D014901:West Nile Fever / Q000453:epidemiology / Q000635:transmission / Q000662:veterinary* / Q000821:virology; D014902:West Nile virus / Q000502:physiology*