1. Professor of Epidemiology College of Veterinary Medicine
Predictive Modeling of West Nile Virus Outbreaks Using Remotely-Sensed Data
Dr. Michael Ward
Professor of Epidemiology
College of Veterinary Medicine
Texas A&M University
James Steele Conference on Diseases in Nature Transmissible to Man, Austin, 11 June 2007

2. James Schuermann
Zoonosis Control Group
Texas Department of State Health Services, Austin TX
Linda Highfield
Department of Veterinary Integrative Biosciences
Texas A&M University, College Station TX
partial funding provided by the
Texas Equine Research Advisory Committee

3. Outline
Background
Methods
Results
Discussion
Conclusions

4. 1. Background

6. West Nile Virus family Flaviviridae genus Flavivirus
Japanese Encephalitis serocomplex, includes:
Japanese encephalitis
Murray Valley encephalitis
St. Louis encephalitis
Kunjin
antigenically, all closely related

7. WNV History first occurrence in U.S.: 1999 ( Bronx Zoo, New York )
by 2001: extension of range to include Florida
2002: large equine epidemic
by 2003: 46 states, 7 Canadian provinces, 5 Mexican states
only states WNV not detected:
Alaska, Hawaii

8. WNV Life Cycle Vector Mosquito Reservoir Wild birds Dead end host
Horses and humans

9. WNV Mosquito Vectors biological and mechanical vectors
14 species identified
Culex spp. most likely in the U.S.
breed in standing water
Cx. pipiens, quiquefasciatus, tarsalis
Aedes spp. may spread disease to horses
breed in locations where water will be present

10. WNV Avian Reservoirs responsible for distribution
>110 species of birds
most susceptible species include American crows, fish jays, blue jays
game species (wild ducks, geese, pheasants, turkeys, pigeons, doves)
raptors (owls, hawks, eagles)

12. First indicators of WNV activity
% counties
dead bird 62
equine case 29
human case 4
infected mosquito pool 3
sentinel bird seroconversion
0.8
seropositive wild-caught bird
0.2

13. WNV Surveillance Programs
avian mortality surveillance tracking system
mosquito trapping and testing
testing wild birds, sentinel chickens, horses and humans with neurologic disease
forecasting systems: environmental variables
temperature
precipitation
remotely-sensed data

14. 2. Methods

15. reported cases of equine WNV encephalomyelitis: 2002, 2003 and 2004
time series of case reports, 2-week window
image data: 2-week 1km2 resolution rasters of the Normalized Difference Vegetation Index (NDVI)
mean NDVI for each 2-week period
periods with versus without reported cases
autoregressive model: NDVI as a predictor of equine WNV cases (scaled,  transform)

16. What is the NDVI? Advanced Very High Resolution
Radiometer (AVHRR) sensor,
NOAA polar-orbiting satellite
Normalized Difference Vegetation Index:
visible and near-infrared data
daily observations  biweekly 1km2 resolution raster based on daily maximum observed NDVI value
resulting 1x1 km pixel represents maximum scaled NDVI value during each 2 weeks of the study period

19. 3. Results

21. correlation, number of cases reported versus NDVI: 45%
2-week periods N
mean
(95% CI)
WNV cases reported 45
0.4390
(0.4219, )
WNV cases not reported 33
0.3962
(0.3730, )
(P<0.001)
correlation, number of cases reported versus NDVI: 45%

22. cases = – 0.9102 + 8.5762 (casesweeks 1–2) – 5.6137 (casesweeks 3–4)
(NDVIweeks 1–2) – (NDVIweeks 3–4)
no. observed versus predicted cases highly correlated (rSP 83%, P<0.001)

23. mean difference, observed versus predicted cases, P= 0.973

24. 4. Discussion

25. Prevention and Control
reduce exposure
indoor housing, repellants?
mosquito control
larvicides, adulticides, environment
vaccination
killed or recombinant canarypox-vectored
2 doses, 3-6 weeks apart; annual booster

26. Forecasting Systems anticipate increases in risk identify “hotspots”
optimize control strategies
increased awareness
identify “hotspots”
sentinel warning for zoonotic disease

27. 5. Conclusion

28. remotely-sensed data:
availability
low-cost
coverage
could be used to:
enhanced WNV surveillance
provide early warning of increased risk
identify hotspots
warn of potential zoonotic transmission of WNV