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. vi 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 vii 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 viii 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 ix 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.