WEST NILE ENCEPHALITIS
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This is a Flavirus disease closely related to the Japanese encephalitis group. Its origin is in Uganda but has now spread to the Middle East, other African countries and 24 European countries and the Americas, where it was first found in 1999. The spread throughout America was very rapid, reaching all of the United States, Canada and Mexico by the 2020.
Service (2008) reported that the virus is mainly an infection of birds, with lethality in crows being very high. Recent experiments show that other bird species will be more efficient reservoir hosts, however. The virus has been isolated from more than 69 mosquito species, but the Culex pipiens complex, Cx. modestus and Cx. univittatus are the most important vectors. There is co-feeding transmission when an uninfected mosquito is feeding on a host very close to an infected mosquito the virus from the infected mosquito passes to the uninfected mosquito, making it a potential vector (Service 2008). Sometimes a mosquito that feeds on both birds and mammals (i.e., a "bridge vector") transfers the infection to humans, horses and other mammals. Mammals are incidental dead end hosts because they do not have sufficient virus titer to infect other mammals.
Symptoms are typically flu-like, but the duration is usually less and the fevers, joint aches, etc. are milder. Nevertheless, being a newly invaded virus it is impossible to determine whether more virulence will be associated with West Nile.
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