Abstract
West Nile virus (WNV) is an arthropod-borne virus belonging to the Flavivirus genus and Flaviviridae family, which is currently considered a serious public health problem worldwide. The virus, first identified in Africa in 1937, is characterized by at least eight different lineages. WNV lineages 1 (L1) and 2 (L2), often associated with cases of encephalitis in humans and horses, are by far those most widespread in Europe and the Mediterranean basin. These two lineages are endemic in many Italian regions.
In this study, we have tried to uncover the spatial and temporal viral dynamics of the two lineages between Africa and Europe. By integrating epidemiological studies with molecular, phylogenetic, and phylogeographic analyses, we showed that there is a strong viral connection between the two continents, supported by birds crossing international boundaries while migrating through the African-Eurasian flyways. According to the data analysis, WNV L1 clade 1A seems to originate in the 1900s between North and West Africa. It moved forward and backward through Senegal, Morocco, and Western-Mediterranean European countries. WNV L2 strains now circulating in Europe and Africa likely originated between the 18th and the 19th century in South Africa from where it was introduced into Hungary in 2004 and then spread throughout Europe. Our study also gave an insight of the dynamics of the viral circulation in Italy, demonstrating the endemic presence of WNV L1 and L2 in part of Italy supported by resident wild birds and vector competent mosquitoes mainly belonging to the Culex genus, and the existence of a continuous transmission of the two strains between Western-Mediterranean countries, supported by short distance migratory birds. It also showed the presence of genetically stable WNV L2 strains in Italy with continuous circulation throughout the time, and characteristic silent periods observed for WNV L1 in the country, with unnoticed circulation lasting sometimes for more than 10 years thanks to overwintering mechanisms supported by bird-to bird, rodent-to bird, or mosquito-to bird transmission routes.
In conclusion, from this study it clearly appears that WNV L1 and L2 have had different eco-epidemiological and genetic evolutionary features with L1 exhibiting complex behaviour and genomic heterogeneity, while L2 being characterised by homogeneity and less inclination to spread. We stress the crucial importance of an integrated surveillance system between Africa and Europe to monitor both the introductions and circulation of avian emerging zoonotic diseases in the two continents.