GRF 2nd One Health Summit 2013: Presentation by Edward N. Walker, Michigan State University

1. Ecoepidemiology of West Nile virus
transmission in urban areas:
Processes and Predictions of Disease Outbreaks
Edward D. Walker
Michigan State University

2. Acknowledgements
Michigan State University
• Edward Walker
University of Illinois
• Marilyn Ruiz
• Uriel Kitron
University of Wisconsin
• Tony Goldberg
Ecology of Infectious Diseases Program
080403 - Eco-epidemiology of West Nile Virus
Transmission in Urban Areas
Graduate students: Gabe Hamer, Scott Loss,
Allie Gardner, Bethany Krebs, Christina Newman
Postdoctorals: Gabe Hamer, Luis Chavez, Tavis
Anderson

3. Risk of introduction and spread of vector-borne
and zoonotic disease agents is great but
our predictive capacity is poor.
Hierarchical transition risk analysis of emerging
zoonotic infections such as West Nile virus
offers a conceptualization and algorithm
that may serve:
Probabilistic events at branch points
lead to success or failure depending
upon context, in particular
receptivity of the environment
under invasion. I am unaware of
any application of this
algorithm to emergence of
zoonotic infections but it
could be robust.
Kolar C, Lodge D. Ecological predictions and risk assessment for alien fishes in North America.
Science 2002; 1233-1236.

4. Geographic range expansion and epidemiologic impact of West Nile virus
Study site:
Chicago metropolitan
region

5. Super-spreader bird species (e.g.,
American robin, Turdus americanus)
promote seasonal transmission
Culex pipiens: enzootic and epizootic
vector amongst birds, and importantly
epidemic/bridge vector to humans
West Nile virus: a mosquito-borne
flavivirus in the Japanese encephalitis
virus complex, showing extraordinary
host range and invasibility into
a wide range of suitable environments

6. Chicago metro area
Observation 1: there is spatial
aggregation of human, bird, and
mosquito infections.
Research question: Can this spatial
aggregation be explained by
urban landscape factors and
biological associations?
Ruiz et al. 2004. Environmental and social determinants of human risk during a West Nile virus outbreak in the
greater Chicago area, 2002. International Journal of Health Geographics 3:8-18.

7. Spatial clustering of human cases
in one of 5 classified urban
landscapes:
Spatial clustering of mosquito infection
in these same landscapes in close association
with human cases
Two thirds of all cases in one urban landscape

8. Receptive habitat and risk landscape for West Nile virus: the suburban backyard setting
of post World War II “old suburbs.” WNV is an anthropogenic zoonosis.
Good vegetation structure for bird habitat
and mosquito harborage
Water source: watering lawns
Lots of resident people, including retired
elderly, living close together
Mosquito production sites

9. Observation 2: There is substantial variation in intensity and timing of West Nile virus
amplification amongst years in the Chicago metropolitan area. Some years are high risk years,
others are low risk. Presupposing that this variation relates to human risk of infection annually,
why does this variation occur and can we use it to model human risk of infection?
Amplification: seasonal increase in
infection rate for virus in mosquito
populations.

10. Abnormal Degree Week
Degree Week (DW) difference from average by week
35.00
2012
High risk
WNV
years
DW
Degree-Week
25.00
15.00
2010
2005
2011
2002
2006
5.00
2001
2007
2008
-5.00
2003
2009
2004
-15.00
Low risk
WNV
years
July
Temperature over 22 C (~ 72 F)
accumulated each week
-25.00
16
20
24
28
32
Week
Week of year
36
40
44

11. The model successfully predicts weekly Culex mosquito
WNV infection (MIR) with precipitation and temperature.
25
--------Used to develop model----
20
----Weekly weather predicts MIR-----
15
10
5
0
-5
1
14
27
40
1
14
27
40
1
14
27
40
1
14
27
40
1
14
27
40
1
14
27
40
1
14
27
40
1
14
27
40
1
14
27
40
-10
03
Model2 + AVE
2004
2005
2006
2007
Observed
2008
2009
2011
2010
2012
Number of WNV Cases by Year in
Chicago metropolitan area
2004
2005
2006
2007
2008
2009
2010
2011
2012

12. Number of WNV cases
250
Early
200
150
T1
100
50
0
003
Middle
0.00
5.00
10.00
MIR for T2
2004
15.00
2005
20.00
T2
Late (summer time periods)
T3
The prevalence of human infection increases with increasing
mosquito infection rate (MIR) in early to mid-summer
2006
2007
2008
2009
2010
2011
2012
Higher MIR during T2 - June to mid-July –
is associated with more human illness
(not with T1 or T3 periods)
Number of WNV Cases by Year in
Cook & DuPage counties, Illinois
2004
2005
2006
2007
2008
2009
2010
2011
2012

13. Conclusions
• West Nile virus infections in humans and mosquitoes has marked spatial
and temporal structure in the Chicago metropolitan area.
• Analysis of the urban landscape reveals a strong association of human and
mosquito infection with one of five landscape types, the “post World War
II suburbs.” This green, residential, anthropogenic landscape comprises
the greatest risk for human infection. Abundant bird habitat and copious
mosquito production sites immediately associated with human dwellings
offers an explanation for this association.
• Temporal dynamics of seasonal virus amplification in the mosquito
population can be modeled and predicted based largely on Degree Week
accumulation with base 22 degrees C. The “heat effect” is modulated by
precipitation. More rain means less West Nile virus infection risk.
• Risk of human infection increases with linear rise in mosquito infection in
the time period from early to mid summer when amplification intensifies.
Human risk can be predicted from Degree Week accumulation.
• Can the temporal pattern of mosquito infection be predicted?
Yes, reasonably well. Abiotic factors (climate, weather) operate strongly in
this system. Hot and dry promote transmission and increase risk.