Climate-associated changes in a mosquito's ability to transmit tropical disease

by Eileen Jeffrey Gutierrez, 2017

My dissertation focuses on testing surveillance techniques that can be used to track changes in a mosquito population’s ability to transmit disease. I focus on Aedes aegypti mosquitoes throughout my chapters due to the public health risk that they pose to populations in the Sonoran Southwest, including Pima County. Ae. aegypti is highly anthropophilic and is the primary vector (organism that transmits a pathogen) for many diseases including dengue fever, yellow fever, chikungunya and zika. This mosquito is also very adept at exploiting human travel and trade to colonize new locations. The man-power and resources required to reduce or eliminate mosquito abundance in an area is significant, therefore techniques that allow us to evaluate where and when transmission risk is highest are necessary in order to have more efficient and sustainable mosquito control programs. For example, it is not feasible to spray insecticide indiscriminately throughout a city for both economic, environmental, and public health reasons. Methods for pinpointing transmission risk will allow for minimal use of chemical interventions.


For the first chapter of my dissertation, I used Ae. aegypti derived from egg collections in Tucson, Arizona to see if measurements of body size could be used to track variation in lifespan. Longer lifespans in adult mosquitoes give infected females greater opportunity to transmit their infection to a human. In a laboratory experiment, I reared larvae under a range of temperature and crowding conditions to produce adult mosquitoes with a range of body sizes. I then maintained the adults under a range of humidity conditions and kept track of when each individual died and their body size. I found that there is a positive relationship between body size and age at death for local, female mosquitoes. Using a statistical technique for pathway analysis (known structural equation modeling), I found that, of the variables I measured, the most important for predicting variation in adult longevity were temperature during larval development, body size, and relative humidity during adulthood. These findings confirmed my hypothesis that body size measurements can help strengthen prevention and control activities.


These findings are particularly interesting because Tucson has a large seasonal population of Ae. aegypti but has yet to have local transmission of the dengue, chikungunya, or zika viruses. This is despite having close economic and social ties to another city in the Sonoran Desert which sees regular dengue transmission every year, Hermosillo, Sonora, Mexico. There is also the potential that rising temperatures due to increasing urbanization (via the “heat-island effect”) and global warming will impact the distribution of mosquito-borne disease. For the second chapter of my dissertation, I am using several years of mosquito body size, temperature, and humidity field data from Tucson, Arizona, and Hermosillo and Nogales, Sonora Mexico to see if the positive association between body size and age produced in the laboratory experiment can be observed in local field populations. By identifying associations and trends in transmission risk, the scientific community can make progress towards the goal of predicting disease transmission using meteorological and entomological indices.