Spatial coherence and association of
temperature, rainfall and the incidence of
dengue hemorrhagic fever in Thailand
Derek A.T. Cummings, University of Pittsburgh Graduate School of Public Health and
Johns Hopkins Bloomberg School of Public Health
Timothy P. Endy, Walter Reed Army Institute of Research
Ananda Nisalak, Armed Forces Research Institute of Medical Sciences
Donald Burke, University of Pittsburgh Graduate School of Public Health
Research Question
How is climate variation associated with the
incidence of dengue hemorrhagic fever?
Does the dengue season occur at the same
time as the rainy season or hot season
across Thailand?
Effects of Temperature and Rainfall
on Dengue Incidence
Temperature Affects
-Extrinsic incubation period of virus
-Survival rates of mosquito vector
-Biting rates of mosquito
-Fecundity of mosquito vector
Rainfall Affects
-Survival rates of mosquito vector
-Fecundity of mosquito vector
DEG C
34
Watts et al, 1987
O
X
X
X
X
32
O
O
X
X
X
30
O
O
X
X
X
28
O
O
O
X
X
26
O
O
O
O
O
3
7
14
21
24
X = TRANSMISSION, O = NONE
28
DAYS INCUBATION
Source of Incidence Data
Monthly DHF Incidence
Passive Surveillance
72 Provinces
1983-1996
Gathered by Thai Ministry of Public
Health, serologically confirmed to the
extent logistically possible. (~20%)
Roughly 850,000 cases
Normalized Log of Raw Incidence of DHF in Thailand
Rank of Province
Log(Incidence/1000)
N
S
Year
Source of Weather Data
Monthly Mean Temperatures
and Monthly Rainfall for 52 weather
stations in 41 provinces
1983-1996
NOAA Climate Anomaly Monitoring
System
First Hypothesis
The timing of the DHF season is associated
with the timing of the rainy or warm seasons
Spatial Synchrony
of DHF, Temperature
and Rainfall
DHF
Incidence
Temperature
Rainfall
Normalized Log of Raw Incidence of DHF in Thailand
Rank of Province
Log(Incidence/1000)
N
S
Year
Decomposition of Time Series
Using the Empiric Mode Decomposition
Seasonal Mode of DHF Incidence
Phase Differences
Mean Phase Difference of Seasonal Mode of DHF Incidence
vs. Mean Phase Differences of Rainfall
p<0.003
R2=0.2
Temperature
• The timing of changes in ambient
temperature are not associated with the
timing of the dengue season in the same
analysis
Second Hypothesis
Annual DHF incidence is related to annual means
of temperature or annual totals of rainfall
Used a two-stage hierarchical model to generate
pooled estimates of the association between
Annual mean temperature, annual total rainfall
and total annual DHF incidence across the entire
country
Used a two-stage hierarchical model to generate
pooled estimates of the association between
annual mean temperature, annual total rainfall
and total annual DHF incidence across the entire
country
Higher Mean Annual Temperatures are
statistically significantly associated
with higher Annual DHF Incidences
(p<10-4)
Modified by Mean Temperatures (p<0.04)
and Distance from Bangkok (p<10-5)
Beta Coefficients from Linear Regression of
Annual DHF Inc on Temp.
Beta’s from Generalized Linear Model of Annual Incidence
on Temperature vs. Distance from Bangkok
Annual Mean Temperatures
Difficulties with this Analysis
Only monthly data is available. May not be able to detect
association between small changes in the timing of
seasons
Association may be non-linear, with thresholds above or
below which, variation is less important
Seasonal process may not be separable from underlying
intrinsic dynamics
Conclusions
Higher mean annual temperatures are associated with higher
incidences of DHF across Thailand and are modified by mean
temperature and distance from Bangkok
The phase of the dengue seasonal variance in DHF incidence
is associated with the phase of rainfall across Thailand
Average proportion of countrywide DHF cases per month
Relative timing of rainy season and the dengue season regional
Figure 2a. Number of provinces showing highest significant correlation of dengue and rainfall in the indicated lag
month
35
30
Number of provinces
25
20
Thailand
Indonesia
Malaysia
15
Singapore
10
5
0
-6 mos
-5 mos
-4 mos
-3 mos
-2 mos
-1 mos
0 mos
1 mos
Lag (rainfall stationary)
2 mos
3 mos
4 mos
5 mos
6 mos
Next Steps
Examine weekly data available from later years
Examine correlation structure of each of these variables
Thank you