West nile virus: From Africa to Europe, America, and beyond

Abstract

Since the mid-1990s, West Nile virus (WNV) outbreaks of increased severity first appeared in Africa, then in Europe, and finally in North America. These outbreaks were due to related lineage one viruses of apparent pathogenicity. More recently lineage 2 strains also of apparent increased pathogenicity have caused outbreaks in Europe. Some of the implicated lineage 1 strains and the lineage 2 strains have a mutation in the NS3 helicase gene, which confers increased viral pathogenesis in birds. The recent pattern of sporadic cases and outbreaks of WNV that has emerged in Europe and North America shows no signs of abating. While broad areas of high risk can be identified, the sporadic, local and regional outbreaks that occur within these areas remain elusively unpredictable. Fewer than one percent of persons infected develop neuroinvasive disease, characterized by meningitis, encephalitis, or acute flaccid paralysis. Increasing age is a risk factor for neuroinvasive disease, both in humans and horses. Four WNV vaccines are currently marketed for horses and clinical trials have been conducted for two human vaccines. Treatment is supportive.