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West Nile Virus

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The virus is transmitted between birds and man, though a wide range of other animal species can also become infected.

West Nile Virus (WNV) can be transmitted to both humans and animals via the bite of an infected mosquito. A mosquito becomes infected by biting wild birds that carry the virus.

The infection is a zoonosis.  In the case of WNV, the virus is transmitted between birds and man, though a wide range of other animal species can also become infected. The mosquito vectors primarily involved are Culex species which are known to occur in some parts of the UK.

Migrating birds are the most likely mechanism of the infection being introduced into the UK. Humans, horses and other animal species are believed to be dead-end hosts, i.e. there is no spread from them to other people or animals.

The virus historically occurs in Africa, Europe, the Middle East, West and Central Asia, and recent outbreaks have occurred in Morocco (1996), Romania (1996), Italy (1998), Russia (1999) and the South of France (2000). However, it appeared for the first time in the USA in 1999. Since WNV was first recorded in the USA in 1999, it has spread throughout much of the country where it is now considered to be endemic.

The horse seems the most susceptible to infection but most cases show the horse showing no obvious signs of disease but becoming seropositive (i.e. positive to the blood test for antibodies to the virus).   The incubation period is 5-15 days and mortality rate in the USA is about 35% of the animals which show signs of disease.

Affected animals develop a fever and often encephalitis (inflammation of the brain). Whilst birds are the main carrier and most remain apparently unaffected, some species are susceptible to disease - especially the crow family. Mass die-offs can occur in these species. Some other animals that can be infected are cats, bats, squirrels, rabbits and dogs (rarely).

If you suspect signs of West Nile Virus (WNV), you must immediately notify your local Animal and Plant Health Agency Office (external link).


In January 2009 the European Medicines Agency authorised West Nile Virus vaccine produced by Fort Dodge, for use in Europe.

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