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The Movement of West Nile Virus in North AmericaThe Movement of West Nile Virus in North America
UC BerkeleyUC Berkeley
October 1, 2003October 1, 2003
Lyle R. Petersen, M.D., M.P.H..Lyle R. Petersen, M.D., M.P.H..
Division of Vector-Borne Infectious DiseasesDivision of Vector-Borne Infectious Diseases
Fort Collins, ColoradoFort Collins, Colorado
Photo of Cx. quinquefasciatus courtesy of Dr. Stephen Higgs, UTMB/Galveston
Order of Topics
• What is it?• What is different now?• How did it get here?• How it is spreading?• How big is the problem?• What will happen next?• How do we prevent it?
West Nile Virus: BackgroundWest Nile Virus: Background
• RNA virus• Family Flaviviridae, Genus
Flavivirus• Member Japanese encephalitis
serocomplex• Closely related to SLE virus
• Isolation in 1937, West Nile district of Uganda
West Nile Virus:West Nile Virus:ApproximateApproximate Geographic Range Before Geographic Range Before
19991999
West Nile Virus Before and After Mid-1990s
• Before• Mild self-limited febrile illness• Central nervous system involvement rare• Periodic outbreaks
• After• Outbreaks associated with severe CNS illness• Some outbreaks associated with avian mortality• First recognition in North America (New York City)
in 1999
West Nile Outbreaks
• Israel 1941, 1951-1954, 1957, 1980, 2000
• France 1962, 2000• South Africa 1974• Romania 1996• Italy 1998• Russia 1999• United States 1999-2003
Arrival in North
America
Madagascar
Uganda 1937
Central African Republic2
Egypt 1951France 1962 South Africa 1974
NE United States
Israel 98/99/2000
India
Kunjin
Romania 1996Italy 1998Volgograd 1999
1
LINEAGEClade
Eg 101
U.S./Israel
Europe/Russia
India
Kunjin
Phylogenetic Tree of West Nile
Viruses
Israel 2000
An exotic arbovirus in NYC was surprising and unexpected:
Should it have been?
International Animal ImportationsInto New York
August 1998 – July 1999 N = 2,873,144
none1 – 60006001 - 2000020001 - 5000050001 - 350000350001 -
Animals
Top Ten Countries of OriginPassengers arriving into NY Airports
July 1998 – June 1999N = 4,850,090
Country Passengers PercentDOMINICAN REPUBLIC 539,740 11%UNITED KINGDOM 456,540 9%CANADA 350,510 7%FRANCE 312,400 6%MEXICO 310,330 6%ITALY 215,040 4%JAPAN 177,810 4%BRAZIL 157,220 3%ISRAEL 153,540 3%BAHAMAS 143,390 3%GERMANY 129,690 3%BERMUDA 114,750 2%
West Nile Virus Transmission CycleMosquito vector
Incidental infections
Bird reservoir hosts
Incidental infections
West NileVirus
West NileVirus
Incidental hostsHumansHorses
Other mammals
Other mosquito vectors*Culex salinariusCx.nigripalpus
Ochlerotatus sollicitansOc. taeniorhynchus
Aedes vexansAe albopictus
* With possible epidemic potential
?
?
Amplifying hostsPasserine birds
Enzootic/epizootic(amplifying) vectors Culex pipiens Cx. restuans Cx. quinquefasciatus Cx. tarsalis
West Nile Virus Amplification CycleWest Nile Virus Amplification Cycle
West Nile Virus Ecology in North America,as of Fall 2003
• 43 mosquito species• 170 native and captive avian species
• Mortality varies greatly by species• Very high viremias in some species• Bird-to-bird transmission documented
• Virus in feces and oral swabs• Oral infection possible
• 18 other species (e.g., cats, dogs, squirrels, chipmunks, harbor seal, alligator, bats, reindeer)
WNV Surveillance
• Dead birds (especially crows)• Human surveillance• Mosquitoes• Horse surveillance• Live captive sentinels (e.g. chickens)• Live wild birds• Zoo animals
•Reported to ArboNET as of 09/03/2003
Year # States # Counties Date Range
1999 4 28 9 AUG – 15 NOV
2000 12** 145 6 FEB – 17 NOV
2001 27** 359 8 APR – 26 DEC
2002 44** 2,531 3 JAN – 19 DEC
2003 45 1,954 1 JAN –29-SEP
States and Counties Reporting WNV Activity, United States, 1999-2003*
** Plus D.C.
Reported WNV Disease Cases in Humans,United States, 1999-2003*
* Reported as of 09/30/2003
Year # Cases # States # Counties Onset Date Range
1999 62 1 6 2 AUG – 24 SEP
2000 21 3 10 20 JUL – 27 SEP
2001 66 10 39 13 JUL – 7 DEC
2002 4,156 39** 740 19 MAY – 19 DEC
2003 5,722 41 730 28 MAR – 26 SEP
** Plus D.C.
Clinical EpidemiologyClinical Epidemiology
• Incubation period 2-14 days• May be longer in immunosuppressed
• Approximately 20% mild febrile illness• <1% severe neurological disease
• Meningitis, encephalitis, AFP• Advanced age primary risk factor for encephalitis• AFP patients may be younger• Immunosuppressive drugs and hematological
malignancies risk factors • Approximate 10% mortality• Long-term morbidity common
~80%Asymptomatic
~20%“West Nile Fever”
<1%CNS
disease
WNV Human Infection “Iceberg” in 2002WNV Human Infection “Iceberg” in 2002
284 fatalities
~ 3300 severe disease
~400,000
asymptomatic
~100,000
mild illness
0200400600800
1000120014001600180020002200
Jan
Feb
Mar
ch
Apr
il
May
June
July
Aug
Sep
t
Oct
Nov
Dec
Month of Onset
# ca
ses
0200400600800
1000120014001600180020002200
Jan
Feb
Mar
ch
Apr
il
May
June
July
Aug
Sep
t
Oct
Nov
Dec
Month of Onset
# ca
ses
Human WNV Illnesses (n=4156),Human WNV Illnesses (n=4156), by Month of Symptom Onset, 2002 by Month of Symptom Onset, 2002
1st Case
May 19
Last Case
Dec 19
0
10
20
30
40
50
60
Age group (yr)
Inci
denc
e pe
r M
illio
n*Human WNV Meningoencephalitis Incidence, Human WNV Meningoencephalitis Incidence,
by Age Group, United States, 2002by Age Group, United States, 2002
* States with human cases
0
5
10
15
20
25
30
35
40
Age group (yr)
Fata
lity
Rat
e (p
erce
nt)
Fatality Rate among Persons with WNV Fatality Rate among Persons with WNV Meningoencephalitis, by Age Group, United Meningoencephalitis, by Age Group, United
States, 2002States, 2002
0
10
20
30
Age group (yr)
Pro
po
rtio
n o
f d
isea
se c
ases
(%
)
WNME WNF
Human WNV Disease Cases, by Age Group and Clinical Category, United States, 2003*
* Reported as of 09/03/2003
Novel Modes of West Nile Virus Transmission, 2002
• Transplanted organs• One donor to four recipients
• Transfused blood• 23 confirmed cases in 2002, many more likely• WNV screening began in July
• Breast milk• One case, infant asymptomatic
• Transplacental transmission • One case, severe outcome to infant
• Percutaneous, occupational exposure
St. Louis Encephalitis
• Related Japanese encephalitis serocomplex flavivirus
• Endemic in the Americas• Similar maintenance vectors• Birds are main hosts• Causes human epidemics• Is SLE a model for the future behavior of WNV?
0
500
1000
1500
2000
2500
1932 1942 1952 1962 1972 1982 1992 2002
Reported and Estimated Number of St. Louis
Encephalitis Cases, U.S., 1932- Sept 12, 2003
0
500
1000
1500
2000
2500
1932 1942 1952 1962 1972 1982 1992 2002
Reported Number of West Nile Meningoencephalitis Cases, U.S., 1932- Sep 30, 2003
DCDC
SLE 1975
DCDC
WNME 2002*
>=10
1 – 9.9
0 – 0.9
Incidence per 106
Reported Incidence Reported Incidence in Humans:in Humans:SLE 1975SLE 1975
vs. vs. WNME 2002WNME 2002
* Reported as of 11/01/2002
West Nile Virus Future in the United States
• Worrisome for several reasons• Prolonged transmission season• Wide geographic range• Many potential amplifying avian hosts• High viremias in birds• Many potential mosquito vectors• Many potential human-biting mosquito species• Persistent infections in areas over 5 years• Urban / suburban / rural transmission
JOIN THE “SWAT TEAM” AGAINST
WEST NILE VIRUS
Prevention
• Repellants• DEET (up to 50% concentration); 10% in
children• Permethrin on clothing and fabrics
• Reduce mosquito breeding sites• Long sleeves and pants• Stay indoors when mosquitoes are biting (dawn
and dusk)• Bug zappers, sonic devices, CO2 devices
(mosquito magnet): no proven efficacy
Habitat elimination
Larviciding
Adulticiding