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CAB Reviews

A reviews journal covering agriculture, global health, nutrition, natural resources and veterinary science

CAB Review

WNV: emerging threat to the Americas and opportunities for control.


West Nile virus (WNV; family Flaviviridae; genus Flavivirus) is currently the most widely distributed arbovirus in the world. It is maintained in nature in an enzootic cycle between ornithophilic mosquitoes and avian hosts. At least five lineages of WNV have been reported based on nucleotide sequence homology, but lineages 1 and 2 predominate. Lineage 1 WNV was introduced into the eastern USA in 1999, and within 5 years it was established in North America and Canada, the Caribbean, Mexico and Central America, and South America. The hallmark of WNV activity in North America has been the significant morbidity and mortality in a wide range of avian species. Other vertebrates, including humans, have been infected with WNV, but their role in the transmission cycle is less significant because of their generally low levels of viraemia. Symptoms of WNV disease range from non-specific flu-like symptoms to muscle weakness and neuroinvasive disease [West Nile neuroinvasive disease (WNND)]. Approximately 75% of humans infected with WNV develop no or very mild symptoms, 25% develop West Nile fever and one in 150-250 develops WNND. The elderly and immunocompromized individuals are at the greatest risk of developing WNND. Long-term neurologic sequelae have been noted. Effective arbovirus surveillance is essential to predicting human incidence of WNV infection and to planning prevention and control strategies, such as mosquito abatement and dissemination of public warnings. Human vaccines for prevention of WNV are in various stages of development, but currently none are available for human use, even though equine vaccines have been available since 2003.