Download 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
Electron micrograph of West Nile virus
isolated from the brain of a crow in New York
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
WNV Surveillance
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
Surveillance for WNV
Crows/Corvids
Veterinary (Horses)
Mosquitoes
Humans
All types employ both active and passive
surveillance activities to some degree.
WNV Activity in the U.S. - 2002
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
WNV Infection in Birds
most birds are simply found
dead
clinical signs may include
incoordination, weakness,
tremors, abnormal head posture
some are asymptomatic
Susceptible Corvid Species
Common raven
Blue jay
American crow
WNV Activity in Birds - 2001
WNV activity in Michigan - 2001
585 birds
submitted
244 birds
tested
38 counties
participated
65 positive
birds,
in 10 counties
WNV Positive Birds 2002
WNV Activity in Michigan - 2002
Bird Surveillance
766 birds
submitted
588 birds tested
80 counties
participated
73 of 83 counties
with positive birds
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
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
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)
Environmental Determinants
Competent vector and host must occur
together with virus
Temperature conditions affect
transmission
Appropriate mosquito habitat
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?
Positive Equine WNV Counties 2002
329
Equine
Cases