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West Nile Virus

West Nile Virus. Kimberly Signs, DVM Michigan Department of Community Health. West Nile Virus. Found in Africa, West Asia, Eastern Europe, and the Middle East Emerging Infectious Disease Flavivirus Family Japanese Encephalitis Group Closely related to St. Louis Encephalitis virus.

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West Nile Virus

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  1. West Nile Virus Kimberly Signs, DVM Michigan Department of Community Health

  2. West Nile Virus • Found in Africa, West Asia, Eastern Europe, and the Middle East • Emerging Infectious Disease • Flavivirus Family • Japanese Encephalitis Group • Closely related to St. Louis Encephalitis virus

  3. Electron micrograph of West Nile virus isolated from the brain of a crow in New York

  4. Pathogenicity of Virus • Previous outbreaks in Africa not associated with significant bird or human morbidity or mortality • NYS 1999 strain essentially the same as that associated with stork and goose mortality in Israel • Different lineages of WNV

  5. WNV Surveillance

  6. Types of Surveillance Activity Used by Public Health: • Passive -rely on receipt of reports • Active -efforts made to receive reports or perform testing • Sentinel –groups of susceptible animals placed and sampled to provide early warning of disease • Special –focused studies

  7. Surveillance for WNV • Crows/Corvids • Veterinary (Horses) • Mosquitoes • Humans All types employ both active and passive surveillance activities to some degree.

  8. WNV Activity in the U.S. - 2002

  9. Western Hemisphere • North America currently only has experience with related SLE-a sporadic illness here • Central and South America-other flaviviruses more prevalent-Dengue, Yellow Fever (vaccine) • Southern spread may be hindered by immunity to these other flaviviruses

  10. WNV Infection in Birds • most birds are simply found dead • clinical signs may include incoordination, weakness, tremors, abnormal head posture • some are asymptomatic

  11. Susceptible Corvid Species Common raven Blue jay American crow

  12. WNV Activity in Birds - 2001

  13. WNV activity in Michigan - 2001 • 585 birds submitted • 244 birds tested • 38 counties participated • 65 positive birds, in 10 counties

  14. WNV Positive Birds 2002

  15. WNV Activity in Michigan - 2002Bird Surveillance • 766 birds submitted • 588 birds tested • 80 counties participated • 73 of 83 counties with positive birds

  16. Bird Ecology • Competence: Passerine birds • WNV found in 162 species of birds to date • Reservoirs need to be abundant and exposed to the infection • Sero-prevalence: 30% in epizootic areas

  17. Transmission in Birds • Mosquitoes: (ticks? biting flies?) • Contact:Experimental w/ crows Virus high-oral and cloacal • Oral:Experimental w/ infected bird and mammal carcasses-birds can have high levels of virus in tissues up to 14 days after viremia is cleared

  18. Vector Ecology • Primarily Culex species involved (bird biters), container breeders • Bridge vectors (bite birds and mammals) • Over-wintering in adults (sewer systems) • Vertical Transmission in some species (+ male mosquitoes)

  19. Environmental Determinants • Competent vector and host must occur together with virus • Temperature conditions affect transmission • Appropriate mosquito habitat

  20. Equine Surveillance • ~15,000 cases in 2002 • Fatality rate 30% • Dead-end host-viremia low and brief • CS: weakness, ataxia, muscle twitching • DX: tissues-VI, PCR serum-IgM ELISA, PRNT • Vaccine: Fully licensed Interferes with PRNT Pregnant mare?

  21. Positive Equine WNV Counties 2002 329 Equine Cases

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