West Nile Virus (WNV) is one of the equine arboviral encephalitides and as such, can cause encephalitis (a serious nervous disease) in horses, which can be fatal.
It is caused by a flavivirus, which can infect many bird and mammal species (among which humans and horses).
WNV is spread by insects, generally mosquitoes, which have bitten infected birds (WNV reservoirs). Most birds that carry the virus remain asymptomatic. Severe clinical disease in humans and/or horses has been diagnosed in many American and European countries including the United States, France and Italy. The virus is also present in Africa, Asia and the Middle East. Any suspicion of this notifiable disease must be notified to the National Government and the European commission.
Information on West Nile fever (ECDC)
Information on West Nile Virus (DEFRA)