Animal viral diseases and global change: bluetongue and West Nile fever as paradigms

Jiménez-Clavero, Miguel Á

West Nile virus is endemic in large parts of Africa, Australia, and India, and more recently (as discussed below) arrived to North America, where since then became endemic. In Europe, North Africa, and the Middle East the virus has produced occasional outbreaks in areas close to river basins and large wetlands where the presence of vectors (mosquitoes) and reservoirs (birds) provide the optimal conditions for the maintenance of the viral cycle. Short distance spread of the virus to neighboring territories occurs most likely by birds (not necessarily migratory) acting as carriers. The virus can occasionally be spread to long distances by wild bird migrations (Malkinson et al., 2002) , although this is not likely a frequent event (Sotelo et al., 2011b) , neither it explains all transcontinental translocations of the virus, and significantly, it does not explain the arrival of WNV to North America. Long distance geographic dispersal of WNV by migrating birds is a hypothesis based mainly on circumstantial evidence, such as the discovery of an infected flock of storks in Israel in summer 1998 on their migration back to Africa from central Europe (Malkinson et al., 2002) . This hypothesis is supported by molecular phylogenetic studies between isolates from recent outbreaks in Europe and isolates from central Africa, suggesting that birds that migrate between continents may act carrying the virus . However, although translocation of WNV (and also other flaviviruses alike, for instance Usutu virus) by bird migration is likely to occur, its frequency does not seem to be as high as to explain every WNV outbreak found in Europe. On the contrary, recent phylogenetic evidence supports that all WNV strains isolated in the western Mediterranean area since 1996 are a monophyletic group arising from a single common ancestor, a strain which could have arrived to this area in 1996 or even earlier, and since then it has been maintained in endemic circulation, evolving and spreading throughout the area (Sotelo et al., 2011b) . Finally, one must not assume that the flow of WNV between Europe and Africa operates only in one direction (i.e., from Africa to Europe). The example previously mentioned on migrating storks (Malkinson et al., 2002) as well as specific studies on bird migrations and risk of introduction of pathogens (Jourdain et al., 2007) show that WNV can be translocated from Europe to Africa.