Emergence of Mosquito-borne Flaviviruses in Central Europe

Abstract:
Although the majority of flaviviruses are endemic in tropical or sub-tropical region, certain species are often detected in Europe as well. In central Europe, the Tick Borne Encephalitis Virus is endemic in several geographic regions. Amongst the mosquito-borne flaviviruses the West Nile virus (WNV) emerged in several European countries within the last 50 years. Outbreaks of WNV encephalitis in humans and horses were reported, and strains were isolated form wild bird and mosquito species too. Recently, new WNV strains were isolated in Central Europe from mosquito vectors and from encephalitic cases of vertebrate host. The aim of this study was to reveal the origin and genetic relatedness of these strains. Another mosquito borne flavivirus, the Usutu virus (USUV) emerged in 2001 in Austria. Previously the virus was detected only in Africa. In Austria the USUV exhibited high pathogenicity to wild birds, especially to the Eurasian Blackbird (Turdus merula). Within the last seven years USUV became resident pathogen in Austria and the infection spreads to the neighbouring countries. The main epidemiological observations of the Austrian USUV outbreak are discussed in this study. The complete genome sequences of Central-European flavivirus strains were determined and submitted to phylogenetic analyses. A WNV strain detected in encephalitic geese in Hungary in 2003 exhibited the closest genetic relationship (~98% nucleotide identity) to WNV strains isolated in Israel and in the United States. Another Hungarian WNV strain was isolated from a goshawk (Accipiter gentilis) in 2004. This virus showed the highest identity rates (~96%) to WNV strains of lineage 2 isolated in Africa. The same strain re-emerged in 2005 in Accipiter species and in a sheep, as well as in 2007 in birds and in a horse. A third WNV strain was isolated from Culex pipens mosquitoes in 1997 in the Czech Republic. This isolate shared only 75% to 77% nt identities with strains of WNV lineages 1 and 2. The USUV strains, which were detected in Austria and Hungary, share >99% identity with each other, and 97% identity with the South-African strain. The virus strains that emerged in Switzerland and Italy in 2006 were nearly identical with the Austrian and Hungarian USUV isolates. The results of the investigations revealed a diversity of flaviviruses in Central Europe and the role of migrating birds as possible carriers of exotic flaviviruses.