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West Nile Fever - Things You Should Know

Horse and Vet

In the summer and autumn of 2011, an unprecedented number of cases of neurological disease in horses occurred across the south-east of Australia. A variant West Nile virus (WNV) strain, WNVNSW2011 was identified as the causative agent for most cases. This virulent virus emerged in Australia. WNVNSW2011 is related to the indigenous WNV strain in Australia, Kunjin virus, but is substantially more neuroinvasive.

The clinical signs exhibited by horses infected with WNVNSW2011 were consistent with those described for West Nile fever (WNF). The World Organisation for Animal Health (OIE) provides the following criteria to define the occurrence of WNF:
1. WNV has been isolated from an animal that shows signs consistent with WNF; or
2. viral antigen or viral ribonucleic acid (RNA) specific to WNV has been identified in samples from one or more animals that show clinical signs consistent with WNF, or that is epidemiologically linked to a confirmed or suspected outbreak of WNF; or
3. antibodies to WNV have been identified in an unvaccinated animal that shows clinical signs consistent with WNF, or that is epidemiologically linked to a confirmed or suspected outbreak of WNF.

The experience of 2011, supported by recent research, leads to the conclusion that WNF as defined in the OIE Terrestrial Animal Health Code1 is present in Australia. Based on this information, Australia can no longer claim country freedom from WNF. To-date, there have been no reports of WNF in Australia in species other than horses. The occurrence of the disease in any species, including wild species, is notifiable to the OIE.

Clinical WNF is on Australia’s national list of notifiable diseases. This means there is a legal requirement for anyone who diagnoses WNF to immediately notify their relevant state or territory animal health authority.

I would like to acknowledge the work of Peter Kirkland and histeam at the NSW Department of Primary Industries, Elizabeth Macarthur Agricultural Institute in characterising WNVNSW2011.2

Mark Schipp

Australian Chief Veterinary Officer

Frost MJ, Zhang J, Edmonds JH, et al. Characterization of Virulent West Nile Virus Kunjin Strain, Australia, 2011. Emerg Infect Dis 2012;18:792–800

Australian Veterinary Association

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