surveillance & environmental health west nile virus seroprevalence: results of enhanced...
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Surveillance & Environmental Health
West Nile Virus Seroprevalence:Results of Enhanced Surveillance Program
Goals of the sero-survey
• Compliment to regular clinical data
• Provides baseline infection rate and change over time
• Identifies risk factors of WNv infection
• Lessons learned can be transferred to new arthropod borne viruses
Vectors:Mosquitoes
Reservoir, Introductory &
Amplifying Hosts:Birds
Dead-end Hosts:Humans &
other vertebrates
West Nile virus Transmission Cycle
Illness in Humans
►Majority experience very mild or no symptoms
►WNv Non-neurological Syndrome: Flu like symptoms: fever, headache, body aches, rash
►WNv Neurological Syndrome: Encephalitis, meningitis, acute flaccid paralysis, often with long term effects
1937: Virus first identified in the West Nile province of Uganda.
1990s: more virulent strain emerged in North Africa; outbreaks in Europe and the Middle East (e.g., Romania, Russia, Israel)
1999: WNv found in New York, USA
2000: Surveillance for WNv begins in Canada (birds, mosquitoes, humans, horses)
2001: first detected in Ontario, Canada in birds and mosquitoes
2002: human cases detected in Ontario and Quebec
2003: first human cases in MB, SK, and AB
West Nile virus history
Distribution in the US: 2006
National distribution 2002 Source: West Nile Virus Monitor -
PHAC
National distribution 2003 Source: West Nile Virus Monitor -
PHAC
National distribution 2004 Source: West Nile Virus Monitor -
PHAC
National distribution 2005 Source: West Nile Virus Monitor -
PHAC
National distribution 2006 Source: West Nile Virus Monitor -
PHAC
National distribution 2007 Source: West Nile Virus Monitor -
PHAC
Foothills
Grassland
Parkland
Rocky Mountain
Natural Regions
Health Region Boundaries
Boreal Forest
Canadian Shield
West Nile virus Geographic Distribution of Clinical Cases
Human Cases 1 - 2
3 - 6
7 - 12
13 - 24
25 - 61
PURPOSE • To estimate how many Albertans have been
exposed to WNv
• To assess knowledge, attitudes and personal protective behaviours
METHODS• Short telephone KAB survey
• Blood requisition sent to participants
• Blood screened in AB Provincial Lab of Microbiology, then positives confirmed at the National Lab of Microbiology in Winnipeg, MB.
PARTICIPATION RATE 2004
Laboratory requisitions mailed
3,780
Blood samples received at Provincial Lab
2,518
Response rate 67%
PARTICIPATION RATE 2007
Laboratory requisitions mailed
3,243
Blood samples received at Provincial Lab
1,955
Response rate 60%
Blood Work
Test Name Format Interpretation
WNv IgM(Immunoglobulin M)
Enzyme Immunoassay (EIA) for WNv IgM in serum
• Relatively specific for WNv• Persists >9months in at least 2/3 of cases
WNv IgG(Immunoglobulin G)
EIA for WNv IgG in serum
• Cannot differentiate WNv from other flaviviruses
• Rising IgG levels suggest recent flavivirus infection/vaccination
PRNT: Plaque Reduction Neutralization Titres
Measures ability of serum to block live WNv
• High specificity for WNv ‘Gold Standard ‘ • Takes 4-8 weeks to perform test.
Seroprevalence:
Year: 2004 SeroprevalencePoint estimate (95% CI)
UrbanRest of Alberta 0
Palliser Region 0.77% (0.28, 1.41)
RuralRest of Alberta 0.84% (0.20, 1.82)
Palliser Region 4.56% (2.79, 6.77)
Alberta (total) 0.31% (0.12, 0.58)
Year: 2007
UrbanRest of Alberta 1.00% (0.00, 2.50)
Palliser Region 1.76% (0.73, 2.79)
RuralRest of Alberta 0.74% (0.00, 1.61)
Palliser Region 4.16% (2.31, 6.01)
Alberta (total) 1.52% (0.86, 2.18)
Seroprevalence:
2004 • ~ 6900 Albertans infected with WNv
• ~1/26 infected became clinical cases in 2003
• ~1/142 infected developed severe illness in 2003
2007• ~34,247 Albertans infected with WNv
• ~1/856 infected became clinical cases in 2006
• ~1/1631 infected developed severe illness in 2006
Note: these numbers relate to the year previous the study because we tested for seropositivity that occurred prior to the study year
Conclusions• Seroprevalence appears to have increased between the two study
periods
• Estimates of the absolute numbers of seropositive Albertans has increased due to the occurrence of urban positive samples
• The proportion of seropositive cases that became clinical appears to have reduced since the virus first appeared in the province
• The proportion of seropositive cases that become clinical is very low
• Future sero-surveys will be needed to corroborate these findings
Contact Information
Surveillance and Environmental HealthPublic Health DivisionAlberta Health and WellnessP O Box 1360 Station MainEdmonton, AB T5J 2N3CANADA
Telephone: 1.780.427.4518Facsimile: 1.780.427.1470E-mail: [email protected] Internet: www.health.gov.ab.ca