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6 : West Nile Virus
Category: Clinical Microbiology
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This chapter highlights many aspects of the West Nile virus (WNV) that is an emerging pathogen, including its epidemiology in the Americas, new clinical syndromes, new modes of transmission and their impacts on public health, and progress in the development of therapeutics and vaccines. Virology, entomology, ecology, and pathogenesis are discussed only to the extent required to provide background for the main topics. Strains of WNV can be divided into two genetic lineages by phylogenetic analysis of the complete genome sequence or of the E protein gene sequence. The major mosquito vector in Africa and the Middle East is Culex univittatus, with Culex picilipes, Culex neavei, Culex decens, Aedes albocephalus, or Mimomyia spp. important in some areas. Further evidence that WNV could produce severe neurological disease was obtained in the early 1950s during experiments with WNV as an experimental cancer therapy. Advanced age is the most important risk factor for death. Surveillance data indicate that case fatality rates for persons with WNV neuroinvasive disease increase from 1% among persons under 40 years of age to 36% among persons 90 years old and older. Reduction of vector populations by public mosquito control programs is another mainstay of WNV prevention in North America. New clinical syndromes and new modes of transmission were identified, including transmission by transfused blood. This led to universal WNV screening of donated blood in the United States and Canada.
Known geographic distribution of the JEV serogroup viruses of medical importance before 1999. The distribution of WNV in the United States from 1999 through 2005 is shown in Color Plate 2.
WNV transmission cycle. In North America, C. pipiens, C. restuans, C. quinquefasciatus, and C. tarsalis are the main maintenance and amplification vectors. Culex salinarius and some mosquitoes of the genera Aedes and Ochlerotatus may be important vectors to humans in some areas.
Numbers of reported cases of WNV neuroinvasive disease by week of symptom onset, 2002 to 2004.
Reported numbers of cases of WNV infection, neuroinvasive disease, and death, United States, 1999 to 2005
Symptoms reported among patients with WNV infection by location, year, and hospitalization status