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West Nile Virus
Epidemiology of the North American outbreak
Mira J. Leslie
State Public Health Veterinarian
Washington State Department of Health
Learning Objective
• West Nile virus is established in North
America. The magnitude of the ongoing
WNV outbreak is extraordinary and our
knowledge is incomplete.
• Definitions:
 Arbovirus = arthropod-borne virus
Epizootic = epidemic in animals
Washington State
Department of Health
Do you remember when?
Encephalitis- NYC 1999
• SLE cases reported in
Queens
• Active ph surveillance
identified additional
cases of encephalitis
• Causative agent
unknown in many
cases
Washington State
Department of Health
Bronx Zoo
• Zoo collection birds and birds in
community (crows) dying.
• Veterinary pathologist pursued testing.
• West Nile virus isolated for the first
time in Western Hemisphere.
Washington State
Department of Health
Initial outbreak
West Nile Virus in NYC
1999:
62 human cases
7 deaths
Washington State
Department of Health
- © 2000 Rushton Young
Washington State
Department of Health
West Nile virus: history
•June 10, 323BC: Alexander the Great died in
Babylon. (www.cdc.gov/ncidod/EID/vol9no12/030288.htm)
• 1937: First identified
(human illness) West Nile
district of Uganda.
Epidemic/Epizootic West Nile Virus
• 1937- 1990’s: Mild disease/
meningoencephalitis Middle
East and Africa
1937 – first case identification in Uganda
1950-75
1994 - 2001
Washington State
Department of Health
WorldWNV.ppt
• 1996 – 1999: Severe
neurologic disease in
Romania, Israel, Russia
Washington State
Department of Health
West Nile Virus- the organism
2000-2004
Davis CT, Beasley DWC, Guzman H, Raj P, D’Anton M, Novak RJ, et al. Genetic
variation among temporally and geographically distinct West Nile virus
isolates, United States, 2001, 2002. Emerg Infect Dis 2003 Nov
Washington State
Department of Health
History of Arbovirus Outbreaks in
North America
Year
Virus
# neuro cases
(approx)
# deaths
(approx)
CFR
1933
SLE
1400
280
20%
1975
SLE
2000
170
8%
2003
WN
2866
264
9%
CDC data
Human Encephalitis in the Yakima Valley, Washington, 1942
1942 was fourth year of summer outbreaks – peak in August
28 cases Western Equine and St. Louis encephalitis types, 2 fatal (7.2%)
44 strains of WEE and 5 strains of SLE virus isolated from mosquitoes
JAMA August 18, 1945
JAMA
August 18, 1945
Washington State
Department of Health
Distribution of WNV activity, 1999-2003
States with WNV 4
States with WNV (44 + DC)
1999
States with WNV (12 + DC)
2002
2000
States with WNV (46 + DC)
States with WNV (27 + DC)
2003
2001
Washington State
Department of Health
West Nile Virus, Human Cases
Reported WNV Disease Cases in Humans,
United States, 1999-2003
2002 epicenter
Year
# Cases
Onset Date Range
1999
62
2 AUG – 24 SEP
2000
21
20 JUL – 27 SEP
2001
66
13 JUL – 7 DEC
2002
4,156
19 MAY – 19 DEC
2003
9,862
28 MAR – 3 DEC
2003 epicenter
Washington State
Department of Health
West Nile Virus in the US
How did it get here?
Possible Pathways of Introduction
• Infected human host
• Human-transported vertebrate host
 Legal
 Illegal
• Human-transported vector(s)
• Storm-transported vertebrate host (bird)
• Intentional introduction (terrorist event)not
Washington State
Department of Health
West Nile virus – transmission cycle
Vector mosquito
Incidental hosts
Reservoir host -birds
Washington State
Department of Health
Mosquitoes: WNV vectors
• 37 species of native mosquitoes have
tested positive in the U.S. 1999-2003.
• Some mosquitoes are more competent
vectors than others. Environmental
factors including climate play a role.
• Culex species including Culex tarsalis in
the west are principal arbovirus vectors.
Washington State
Department of Health
Birds: WNV reservoir hosts
• More than 225 species have
been reported to be infected
with WNV.
• Crows and jays predominate
with a high fatality rate.
• Variable experiences:
Chicago vs. Connecticut.
Washington State
Department of Health
Learning about avian
reservoirs of WNV
Most competent reservoirs: Blue jay, Common
grackle, House finch, American crow, House
sparrow
Transmission routes: mosquitoes, ingestion,
and bird-bird contact.
Some birds have persistent infection in many
organs including skin.
Komar et al. Experimental Infection of North American birds with the New York 99 strain
of West Nile Virus, Emerg Infect Dis, Vol 9, No. 3, March 2003
Washington State
Department of Health
Dead birds as early indicators of
WNV activity
Timing of WNV-Positive Dead Bird Collection and Human WNV
Case Onset, By County, United States, 2003*
Regional Differences in WNV Surveillance, 2003*
Region
Area
(hundred mi2)
(A)
# dead birds
tested
(B)
Ratio
(B/A)
East
3,400
11,814
3.5
Central
6,610
5,031
0.8
West
9,301
5,602
0.6
* Reported as of 1/20/2004
Counties Reporting Bird and Human Surveillance (n=725)
Human illness before bird collection (n=188, (26%))
Bird collection before human illness (n=537, (74%))
* Reported as of 1/29/2004
Washington State
Department of Health
U.S. Counties Reporting Equine WNV Disease Cases, 2002*
Horses
12,038 cases
1,678 counties
39 states
• In 2002, an intense epizootic of equine
WNV infection occurred (> 14,500 reported
cases).
• Two new vaccines are licensed for use in
horses.
• Horses do not develop sufficient viremia
to amplify the virus.
Washington State
Department of Health
In addition to humans,bugs, birds,
and horses
•
•
•
•
•
•
•
•
Bats
Squirrels, Chipmunk
Skunk; Rabbit
Goat, sheep, llama,
deer…
Zoo animals
Harbor seal
(Non-human) Primates
Farmed Alligators….
Washington State
Department of Health
West Nile infection, U.S., 2003
• Human illness: 9862
Fatality: 264
• Corvid (crow, jay): 10,200
Other bird: 1866
•
•
•
•
•
Feline: 1
Mosquito pools: 8384
Equine: 5145
Sentinel flocks: 1956
“Other” animals: 48
Canine: 37
Squirrel: 20
Washington State
Department of Health
West Nile virus
WNV activity, U.S.
Jan 1- June 1, 2004:
Cases
States
2
2
Birds
133
15
Mosq.
pools
35
5
Equine
7
3
Flocks
55
3
Humans
Washington State
Department of Health
WNV Seasonality
Human WNV Disease Cases, by Week of Onset,
Northern vs. Southern United States, 2003*
North
South
1200
800
600
400
200
0
22
-M
ar
5-A
19 pr
-A
p
3-M r
ay
17
-M
31 ay
-M
a
14 y
-Ju
28 n
-Ju
n
12
-Ju
26 l
-Ju
9-A l
u
23 g
-A
ug
6-S
e
20 p
-S
ep
4-O
c
18 t
-O
c
1-N t
15 ov
-N
o
29 v
-N
o
13 v
-D
ec
# cases
1000
* Reported as of 1/20/2004
Week ending
Washington State
Department of Health
West Nile virus
Surveillance/Response: Partnerships
•
•
•
•
•
•
Public Health agencies
Health care providers, laboratories
Blood banks
Wildlife agencies and orgs
Zoos
Entomologists, ecologists,
cartographers
• Veterinarians, veterinary laboratories
• Universities
The public, the media, and others
……….
Washington State
Department of Health
Arbonet, CDC
Human cases, 2004
 “Tracking the virus in
real-time”
Secure web-based reporting
 Compiles data on
humans, birds,
mammals, sentinel
flocks, mosquitoes, etc.
 Humans recorded by syndrome and status
 Real-time mappingUSGS
Washington State
Department of Health
Take home messages
West Nile virus:
• Established ecological niche
in North America
• WNV outbreaks are
unpredictable and our
understanding is
rudimentary.
• Management: flexibility,
coordination, partnerships
• Plan surge capacity.
Washington State
Department of Health
Who is this “human”?
QUESTIONS?
Washington State
Department of Health
West Nile virus-important issues
• Laboratory testing
 Public health
labs,commercial labs
 Veterinary labs
 Test development
• Communications
 Health Alert messaging
 Prevention messaging
Strategies and target
audiences
Washington State
Department of Health