West Nile Virus and Hendra Virus: Can we learn anything from Down Under?

Of recent times, Australia has experienced several outbreaks of equine viral disease. Equine influenza (EI) virus breached quarantine in August 2007, resulting in widespread respiratory disease and disruption to equine industries until containment and eradication of the virus. Australia was declared officially free of EI in June 2008. While Australia is one of few countries that has successfully eradicated EI, the widespread outbreak of an exotic disease was a costly reminder of the potential effect of an introduced infectious pathogen into a susceptible population. Thankfully, while the morbidity rate was high, mortality rates were low. While the EI outbreak caused great concern and media attention, Australia also experiences sporadic and unpredictable outbreaks of endemic equine viral diseases, which may result in considerable animal morbidity and mortality and are zoonotic. Topically, and of greatest importance are Hendra virus and arboviral infections, including two flaviviruses (Kunjin virus, Murray Valley encephalitis virus [MVEV)) and an alphavirus (Ross River virus: RRV).

The equine viral events in Australia over the past 2 decades provide valuable information and opportunity for consideration of possible changes in the infectious disease landscape of the UK, through exotic pathogen introduction, emergence of new pathogens or changes in virulence of existing pathogens.

Recent viral disease events in Australia

Kunjin virus is a strain of West Nile virus (WNVKUN) endemic to Australia and is maintained in bird (reservoir/amplification host)-mosquito (vector) transmission cycles in north-western Australia. Sporadic outbreaks of WNVKUN (or MVEV) infection in humans or seroconversion in sentinel chickens occur in south-east Australia; however until recently, these viruses were rarely associated with equine disease. In the summer and autumn of 2011, a major outbreak of arbovirus-associated neurological disease occurred in 5 states of Australia. A total of 982 cases of arboviral disease were reported, involving WNVKUN, MVEV and RRV, and many affected horses demonstrated signs of encephalomyelitis. Prior to the outbreak, much of south-eastern Australia had higher than average rainfall and flooding: conditions conducive to the development of mosquito and wild bird (e.g. wading birds) populations, rapid viral amplification and spread and spill over of infection into mammalian dead-end hosts. Not surprisingly, the majority of cases were within 10km of water courses (Roche et al. 2013). In addition to climatic conditions, the outbreak was facilitated by a horse population naive to flavivirus infection and the emergence of a new strain of WNVKUN. This new strain, WNVNSW11, is thought to have arisen from point mutations at key locations on the genome which resulted in ncreased virulence and development of neurological disease in horses (Roche et al. 2013).

Hendra virus (HeV) was first isolated in 1994 in a Thoroughbred stable in the suburb of Hendra, Brisbane, when a Thoroughbred trainer and 14 horses succumbed to a rapidly progressive and fatal respiratory/neurological disease. Subsequently, there have been sporadic and unpredictable occurrences of HeV disease in horses. Up to 2010 there were 14 separate incidents, while in 2011 alone there were 18 outbreaks with involvement of 24 horses (Anon 2011). The zoonotic potential of HeV is clear: to date, 7 people have been infected with HeV, of which 4 died (including 2 veterinarians). Flying foxes are the reservoir host for HeV, however mechanisms of transmission within flying fox populations and infection of horses are not clear, although the virus is present in urine, foetal fluids and aborted flying fox pups. An additional epidemiological development of HeV was the detection of antibodies in a dog on a farm that had 3 HeV infected horses: this is the first report of antibody detection in a dog outside of an experimental setting. The biological importance of this finding is difficult to determine, however it does raise the possibility that other species could be involved in the transmission of HeV.

Ongoing uncertainties regarding the occurrence of arboviruses and HeV in Australia remain. Eradication of either arboviruses or HeV is not realistic due to maintenance in the respective wild reservoirs. Recently, a vaccine for protection against HeV in horses has been released and may reduce occurrence of equine (and human) infection, however efficacy in a field setting is unknown currently. Given the established ability of WNV to over-winter in vector/reservoir hosts and emergence of the more virulent WNVNSW11 strain, further outbreaks of flavivirus-associated neurological disease in southern Australia are possible. However, such outbreaks may be difficult to predict and require consideration of complex interactions between host susceptibility, strain virulence, climatic conditions and host and vector population dynamics. Surveillance programmes are in place in several jurisdictions and will serve to detect changes in virus activity.

What are the lessons for the United Kingdom?

The events in Australia may seem remote and of no relevance to the UK, however some important concepts have arisen and are worthy of consideration:
1. Emerging diseases are a major issue in animal populations worldwide and with changes in host populations, virus strains, horse populations and movement and vector movement, incursions of viral disease into the UK remain possible. Such risks are exemplified by the introduction of bluetongue virus into the UK in 2007
2. New strains of endemic viruses may develop and be associated with greater virulence and risk of animal disease
3. Co-ordinated surveillance programmes are required for monitoring for changes in activity of endemic viruses and introduction of exotic pathogens
4. The zoonotic potential of equine pathogens must be considered, and protocols for animal examination and outbreak management be developed.

References

Anon. (2011) Guidelines for veterinarians handling potential Hendra virus infection in horses, Version 4.2. Department of Employment, Economic Development and Innovation. Biosecurity Queensland. http://www.daff.qld.gov.au/documents/Biosecurity_GeneralAnimalHealthPestsAndDiseases/Hendra-GuidelinesForVets.pdf. Accessed 24th April 2013.

Roche, S.E., Wicks, R., Garner, M.G., East, I.J., Paskin, R., Moloney, B.J., Carr, M. and Kirkland, P. (2013) Descriptive overview of the 2011 epidemic of arboviral disease in horses in Australia. Aust Vet. J. 91, 5-13.