THE EFFECTS OF CLIMATE VARIABILITY ON THE ABUNDANCE OF WEST NILE VIRUS ...

83 Pages·2012·2.39 MB·English

THE EFFECTS OF CLIMATE VARIABILITY ON THE ABUNDANCE OF
WEST NILE VIRUS VECTORS IN SAN DIEGO COUNTY
_______________
A Thesis
Presented to the
Faculty of
San Diego State University
_______________
In Partial Fulfillment
of the Requirements for the Degree
Master of Public Health
with a Concentration in
Environmental Health
_______________
by
Jose G. Velascosoltero
Summer 2012

iii
Copyright © 2012
by
Jose G. Velascosoltero
All Rights Reserved
iv
DEDICATION
This thesis is dedicated to my beloved mother, may she rest in peace and to my father,
brothers and sisters who have supported me throughout my studies at San Diego State
University.
v
ABSTRACT OF THE THESIS
The Effects of Climate Variability on the Abundance of West Nile
Virus Vectors in San Diego County
by
Jose G. Velascosoltero
Master of Public Health with a Concentration in Environmental
Health
San Diego State University, 2012

The effects that climate change has on the abundance of West Nile virus vectors in
San Diego County was evaluated by correlating winter temperature and precipitation with
mosquito abundance in the spring and summer. Significant associations were observed
between winter minimum monthly averages and spring abundance of C. tarsalis,
C. erythrothorax and C. quinquefasciatus (p<0.05) but not for summer mosquito abundance.
Winter precipitation was also correlated with summer abundance of C. quinquefasciatus and
modestly correlated with C. tarsalis after controlling for summer temperatures (p<0.05).
Monthly temperatures were strongly correlated with mosquito abundance (p<0.01), but
monthly precipitation was negatively correlated with mosquito abundance (p<0.01) when it
was compared with monthly mosquito abundance. These findings suggest that temperature is
the best predictor for monthly mosquito abundance while the lag effect of winter
precipitation is essential in creating breeding sites for C. tarsalis and C. quinquefasciatus
during the summer when San Diego does not generally receive precipitation.
The effects that El Niño Southern Oscillation (ENSO) have on mosquito abundance
were also evaluated comparing abundance data for C. tarsalis and C. erythrothorax from
1994-2011 and abundance data of C. quinquefasciatus from 2007-2011. Abundance of
C. tarsalis and C. erythrothorax were greater during normal weather, followed by El Niño
and La Niña respectively. Significant differences were observed between abundance of
C. tarsalis during normal weather and La Niña (p<0.01) and between normal weather and El
Niño (p<0.01). Abundace of C. erythrothorax was also greater during normal weather versus
La Niña (p<0.05) and between normal weather and El Niño (p<0.05). No significant
differences were observed between abundance of C. quinquefasciatus and ENSO episodes.
Temperature averages were greater during normal weather and were significantly different
from La Niña but not for El Niño (p<0.05). Precipitation was greater during El Niño and was
significantly different than during normal weather (p<0.01) but not during La Niña. The fact
that mosquito abundance was greater during normal weather when the weather was warm
and dry suggest that the long-term climate patterns associated with climate change will
increase the mosquito abundance and the risks of contracting West Nile virus in San Diego
County.
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TABLE OF CONTENTS
PAGE
ABSTRACT ...............................................................................................................................v
LIST OF TABLES ................................................................................................................. viii
LIST OF FIGURES ................................................................................................................. ix
ACKNOWLEDGEMENTS .......................................................................................................x
CHAPTER
1 INTRODUCTION .........................................................................................................1 
Background ..............................................................................................................1 
Purpose/Objectives ..................................................................................................3 
2 LITERATURE REVIEW ..............................................................................................4 
West Nile Virus........................................................................................................4 
West Nile Virus Mode of Transmission ..................................................................5 
West Nile Virus in the United States .......................................................................7 
West Nile Virus in California ................................................................................10 
West Nile Virus in San Diego ................................................................................12 
Global Climate Change and Public Health ............................................................12 
Global Climate Change and Infectious Diseases ...................................................15 
Global Climate Change and West Nile Virus ........................................................18 
El Niño Southern Oscillation and Public Health ...................................................21 
El Niño Southern Oscillation and Infectious Diseases ..........................................22 
3 METHODS ..................................................................................................................25 
Mosquito Surveillance Data ...................................................................................25 
El Niño Southern Oscillation and Climate Data ....................................................27 
Statistical Analysis .................................................................................................28 
4 RESULTS ....................................................................................................................29 
Mosquito Abundance and West Nile Cases ...........................................................29 
Mosquito Abundance and Climatic Variables .......................................................29 
Enso and Mosquito Abundance .............................................................................33
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5 DISCUSSION ..............................................................................................................36 
6 CONCLUSION ............................................................................................................41 
REFERENCES ........................................................................................................................42 
APPENDIX  
A MOSQUITO SURVEILLANCE TRAPS USED BY THE COUNTY OF SAN
DIEGO VECTOR CONTROL PROGRAM ...............................................................52 
B NATURAL LOGARITH TRANSFORMED DATA FOR MONTHLY,
MINIMUM AND MAXIMUM TEMPERATURE .....................................................54 
C NATURAL LOGARITHM TRANSFORMED DATA FOR ABUNDANCE
OF C. TARSALIS, C. ERYTHROTHORAX AND C. QUINQUEFASCIATUS ............57 
D NATURAL LOGARITHM TRANSFORMED DATA FOR
PRECIPITATION ........................................................................................................60 
E SPEARMAN RHO VALUES BETWEEN MOSQUITO ABUNDANCE AND
CLIMATE VARIABLES ............................................................................................62 
F LINEAR REGRESSION GRAPHS FOR SPRING MOSQUITO
ABUNDANCE AND WINTER MINIMUM TEMPERATURES ..............................64 
G BOXPLOTS GRAPH FOR MOSQUITO ABUNDANCE DURING
NORMAL WEATHER, EL NIÑO AND LA NIÑA ...................................................68 
H BOXPLOTS GRAPHS FOR TEMPERATURES DURING NORMAL
WEATHER, EL NIÑO AND LA NIÑA .....................................................................70 
I BOXPLOTS GRAPH FOR PRECIPITATION DURING NORMAL
WEATHER, EL NIÑO AND LA NIÑA .....................................................................72
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LIST OF TABLES
PAGE
Table 1. Distribution of Positive Mosquito Pools for WNV in San Diego County from
2007-2011 ....................................................................................................................31 
Table 2. Pearson Correlation Values between Mosquito Abundance and Climate
Variables ......................................................................................................................31 
Table 3. Pearson Correlations Values between Spring and Summer Mosquito
Abundance and Winter Climate Variables ..................................................................32 
Table 4. Partial Correlations between Winter Precipitation and Summer Mosquito
Abundance (Adjusted for Summer Temperatures) and between Summer
Temperatures and Summer Mosquito Abundance (Adjusted for Winter
Precipitation) ................................................................................................................32 
Table 5. ANOVA Table for Comparison of Climatic Variables and Mosquito
Abundance during Normal Weather, El Niño and La Niña Episodes .........................34 
Table 6. Series of T-Tests Conducted between Normal Weather, El Niño and La Niña
to Evaluate Differences among Climate Variables and Mosquito Abundance ............35 
Table 7. Spearman Rho Correlation Values between Mosquito Abundance and
Climate Variables .........................................................................................................63
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LIST OF FIGURES
PAGE
Figure 1. The complex cycle of West Nile virus .......................................................................6 
Figure 2. The enzootic spread of West Nile virus across the continental United States
since its first introduction in the Western Hemisphere in 1999. ....................................9 
Figure 3. Mosquito abundance and West Nile virus cases in San Diego County from
1994-2011. ...................................................................................................................30 
Figure 4. Carbon dioxide baited trap used by the County of San Diego vector control
program. .......................................................................................................................53 
Figure 5. Gravid trap used by the County of San Diego vector control program. ...................53 
Figure 6. Natural logarithm transformation for monthly temperatures. ..................................55 
Figure 7. Natural logarithm transformation for minimum temperatures. ................................55 
Figure 8. Natural logarithm transformation for maximum temperatures. ...............................56 
Figure 9. Natural logarithm transformation for abundance of C. tarsalis. ..............................58 
Figure 10. Natural logarithm transformation for abundance of C. erythrothorax. ..................58 
Figure 11. Natural logarithm transformation for abundance of C. quinquefasciatus. .............59 
Figure 12. Natural logarithm transformation for monthly precipitation. .................................61 
Figure 13. Linear regression lines between winter minimum average temperatures and
spring mosquito abundance for C. tarsalis. .................................................................65 
Figure 14. Linear regression lines between winter minimum average temperatures and
spring mosquito abundance for C. erythrothorax. .......................................................66 
Figure 15. Linear regression lines between winter minimum average temperatures and
spring mosquito abundance for C. quinquefasciatus. ..................................................67 
Figure 16. Boxplot graph depicting mosquito abundance during normal weather, El
Niño and La Niña. ........................................................................................................69 
Figure 17. Boxplot graph depicting monthly, minimum and maximum temperatures
during normal weather, El Niño and La Niña. .............................................................71 
Figure 18. Boxplot graph depicting monthly precipitation during normal weather, El
Niño and La Niña. ........................................................................................................73
x
ACKNOWLEDGEMENTS
I would like to express my sincere appreciation to the County of San Diego Vector
Control Program staff for providing me with the data for my thesis and for giving me
guidance and support throughout my whole graduate studies.
I would also like to give my sincere thanks to all the professors of San Diego State
University Graduate School of Public Health because I have learned so much from them and
because they were always available to help me with my studies.
Finally, I would like to thank my thesis committee, Dr. Gersberg, Dr. Hoh and
Dr. Hope for helping me throughout the entire thesis process and for helping me to analyze
data, interpret results and for giving me guidance.