West Nile virus and Kunjin virus: lessons for the UK

Arboviruses have a great capacity to move long distances as their vertebrate hosts are transported into new areas or their invertebrate vector species expand into new areas. When these viruses move into previously unaffected areas or countries, the consequences can be significant in the mammalian and avian hosts species. The West Nile virus (WNV) outbreak that first appeared in the United States in 1999 [1] and subsequently swept from East to West across the US, killing large numbers of horses, native birds and people [2] is an excellent example of what can happen when a new virus is introduced to a susceptible and naïve ecosystem. Outbreaks of WNV have also been reported in Europe, North and South America as well as the Middle East and Africa.
In Australia in the summer and early autumn of 2011 there was a widespread outbreak of equine neurological disease associated with a variety of endemic arboviruses [3]. Neurological disease was associated with infection with 2 equine flaviviruses, West Nile virus (previously called Kunjin virus) or Murray Valley encephalitis virus. Clinical signs more referable to musculoskeletal disease were associated with infection with Ross River virus. The 2011 Australian arbovirus outbreak was largely a result of dramatic weather conditions, with very heavy rainfall and flooding across much of Eastern Australia in the spring and summer of 2010–11 which had resulted in amplification of mosquito and wild water bird populations. These weather conditions were sufficient to alert local public health officials to the likelihood of an arbovirus outbreak [4]. Prior to the 2011 outbreak, cases of Murray Valley encephalitis, West Nile virus or Ross River virus in horses were very uncommon and were sporadic in nature. During the 2011 outbreak 982 equine cases of neurological or musculoskeletal disease were reported, with a case-fatality rate approaching 10% [3].
The 2011 West Nile (Kunjin) virus cases in Australia represent a re-emergence of an endemic disease from the normal wildlife hosts into an incidental hosts species (horses). In this respect there is no lesson for the UK, as this virus is not endemic in this country. However, in combination with the US outbreak there are important lessons to be learned. The isolates of West Nile from the US outbreak acquired some significant genetic changes during the course of the outbreak that enabled the US strains to replicate to high titres in American native birds [5] and that also reduced the extrinsic incubation period in mosquitos [6,7]. Several of these changes were also detected in virus strains isolated during the 2011 Australian outbreak [8]. These observations suggest that the lack of previous infection with an arbovirus like West Nile virus does not mean that an outbreak cannot occur in the future. Viruses such as West Nile virus can rapidly acquire genetic mutations that alter their pathogenicity in mammalian and avian hosts and unusual or extreme weather conditions can provide the necessary invertebrate hosts to enable an outbreak to occur. Development of contingency plans for the introduction of viruses such as West Nile virus would be prudent.