West Nile Virus
West Nile Virus (WNV) is an infection of birds, horses and humans that can cause encephalitis (inflammation of the brain) or meningitis (inflammation of the lining of the brain and spinal chord). It is spread by the bite of infected mosquitoes.
The disease caused by WNV is known as West Nile Fever or West Nile Encephalitis. The virus is transmitted through the bite of an infected mosquito after a blood meal from an infected bird. If they survive birds carry an infectious form of WNV for 1-4 days, after which they develop life long immunity. Many bird species act as 'maintenance' hosts and provide sources of infection.
Although birds are the main carrier and most remain apparently unaffected, some species are more susceptible to disease, particularly crows, and mass die-offs can occur in those species.
A range of mammals can be affected, but they usually do not have the virus in their blood long enough or in sufficient concentrations to pass the virus on. These species are considered to be 'dead-end hosts' and include horses and humans.
The majority of people who become infected do not suffer from any illness. Around 20% of infected people develop a 'flu like' disease; a small number (less than 1% of the total) suffer serious disease with potentially fatal meningitis.
In Scotland the 1987 Infectious Diseases of Horses Order requires suspect cases of equine encephalitis to be reported to the regional office of the Animal & Plant Health Agency (APHA).
WNV infection has never been identified in horses or humans in Scotland. Recent research has found antibodies against the virus in birds in GB, suggesting past or present infection with WNV.
Historically the virus occurred in Africa, Europe, the Middle East, West and Central Asia. Although it appeared for the first time in the USA as recently as 1999, it has since spread throughout much of the country and is now considered endemic.
Recently (2015) WNV has been found circulating in southern France.