European experience with the West Nile virus ecology and epidemiology: could it be relevant for the New World?

A review of West Nile virus (WNV) and the epidemiology of West Nile fever (WNF) in Europe is presented. European epidemics of WNF reveal some general features. They usually burst out with full strength in the first year, but few cases are observed in the consecutive 1 to 2 (exceptionally 3) years, whereas smaller epidemics or clusters of cases only last for one season. The outbreaks are associated with high populations of mosquitoes (especially Culex spp.) caused by flooding and subsequent dry and warm weather, or formation of suitable larval breeding habitats. Urban WNF outbreaks associated with Culex pipiens biotype molestus are dangerous. Natural (exoanthropic, sylvatic) foci of WNV characterized by the wild bird-ornithophilic mosquito cycle probably occur in many wetlands of climatically warm and some temperate parts of Europe; these foci remain silent but could activate under circumstances supporting an enhanced virus circulation due to appropriate abiotic (weather) and biotic (increased populations of vector mosquitoes and susceptible avian hosts) factors. It is very probable that WNV strains are transported between sub-Saharan Africa and Europe by migratory birds. The surveillance system for WNF should consist of four main components: (1) monitoring of mosquito populations and their infection rate; (2) wild vertebrate surveys; (3) sentinel birds (domestic ducks rather than chickens); and (4) monitoring of human disease. In the case of persisting high risk of WNF for humans and equids in certain enzootic areas, immunization against WNF should be considered. For that purpose a commercially available, cross-protective vaccine against Japanese encephalitis could be used.