Lineage 1 and 2 Strains of Encephalitic West Nile Virus, Central Europe

Two different West Nile virus (WNV) strains caused lethal encephalitis in a flock of geese and a goshawk in southeastern Hungary in 2003 and 2004, respectively.During the outbreak in geese, 14 confirmed human cases of WNV encephalitis and meningitis were reported in the same area.Sequencing of complete genomes of both WNV strains and phylogenetic analyses showed that the goosederived strain exhibits closest genetic relationship to strains isolated in 1998 in Israel and to the strain that emerged in 1999 in the United States.WNV derived from the goshawk showed the highest identity to WNV strains of lineage 2 isolated in central Africa.The same strain reemerged in 2005 in the same location, which suggests that the virus may have overwintered in Europe.The emergence of an exotic WNV strain in Hungary emphasizes the role of migrating birds in introducing new viruses to Europe. G eographically, West Nile virus (WNV) is the mostwidespread member of the Japanese encephalitis virus (JEV) complex within the genus Flavivirus and the family Flaviviridae.The first strain (B 956) was isolated from a human patient in the West Nile district of Uganda in 1937; later the virus was also detected in several mosquito species, horses, humans, and other hosts in Africa, Europe, Asia, and Australia (where it has been named Kunjin virus) (1-3).WNV was introduced into the United States in 1999, and it spread quickly over large parts of North America and reached Mexico (4-7).The clinical impact of WNV varies in different regions.In the Old World, WNV causes relatively mild infections with influenzalike symptoms or no apparent disease (2); encephalitis and fatalities in the human population, horses, or poultry are spasmodic (3,8,9).In the New World, WNV exhibits increased virulence among the local wild bird populations and causes more frequent severe central nervous system symptoms and deaths in humans and horses (6,10).Although exactly how WNV was introduced into New York is unclear, phylogenetic comparison of the viral nucleic acid sequences has shown a close relationship between the American WNV isolates and strains isolated from encephalitic geese and storks in Israel in 1998 (11-13).Experimental infections of rodents indicated that the neurovirulence of WNV correlates with its genotype, and the North American strains are highly neurovirulent for mice (14).WNV shows relatively high levels of sequence diversity.Comprehensive studies on the phylogenetic relatedness of WNV strains show that they form at least 2 main lineages (15-17).Lineage 1 is composed of WNV strains from different geographic regions, and it is subdivided into at least 3 clades.Clade A contains strains from Europe, Africa, the Middle East, and America; clade B represents the Australian (Kunjin) strains; and clade C contains Indian WNV isolates.Lineage 2 contains the B 956 prototype strain and other strains isolated so far exclusively in sub-Saharan Africa and Madagascar.In addition to the 2 major WNV lineages, we recently proposed 2 lineages for viruses that exhibited considerable genetic differences to the known WNV lineages: lineage 3 consists of a virus strain isolated from Culex pipiens mosquitoes at the Czech Republic/Austria border (named Rabensburg virus), and lineage 4 consists of a unique virus isolated in the Caucasus.These 2 viruses, however, may also be considered independent flaviviruses within the JEV complex (18).