This research investigated the impacts of climate and climate change on infectious diseases. We made several significant discoveries. First, we found that vector-borne diseases, which are transmitted by biting mosquitoes, flies, ticks, fleas, and other arthropods, are highly sensitive to temperature and transmission peaks at intermediate temperatures (between 23-29 degrees C or 73-84 degrees F), depending on the disease and vector. These peak temperatures are increasingly occurring in temperate and highland regions where diseases like malaria, West Nile, dengue, and Lyme disease have historically been absent or eliminated. We found that some regions may see shifts in which diseases and mosquito and tick species present the greatest threat. For example, as the temperature warms in sub-Saharan Africa, much of the region that is currently malaria-endemic may see temperatures so warm that malaria (which has peak transmission at 25 degrees C or 77 degrees F) begins to decline, while dengue and its mosquito vector, which thrive at warmer temperatures (peaking at 29 degrees C or 84 degrees F), becomes more common. Mountainous and temperate areas in Africa, Asia, and Latin America are becoming more suitable for malaria, dengue, and other mosquito-borne diseases as the climate warms. Recent warmer winters are driving an expansion of Lyme disease in the northeastern part of the United States, and future warmer winters are likely to further increase the number of Lyme disease cases in the Northeast and Midwest.

We also developed and tested theory that predicts how diseases will spread under warming climates. Across hundreds of different wild animal species, we found support for the "thermal mismatch hypothesis", which predicts that the burden of diseases is highest at temperatures to which a species is poorly adapted. Thermal mismatches are part of the reason that the emerging chytrid fungus disease (Bd) now threatens many cold-adapted frog and salamander species with extinction as the climate warms. We found that how animal metabolisms respond to temperature can predict the effects of climate on mosquito-borne disease transmission, providing an opportunity to understand the effects of climate on newly emerging diseases. Finally, we found that mosquitoes that transmit diseases have potential to evolve to tolerate warmer temperatures as the climate warms, due to their large population sizes, short life cycles, and ecological flexibility in adapting to new habitats. Our work concludes that climate change has the potential to greatly expand the transmission of many devastating diseases, including malaria, dengue, Zika, West Nile, and Lyme disease, into new regions, and to drive unexpected changes in disease that can cause public health crises. In concert with deforestation and other types of land use change, human impacts on the environment are creating major threats to human health.

Finally, we conducted controlled experiments in the laboratory and mosquito trapping in the field to discover how the capacity for mosquitoes to transmit malaria, dengue, chikungunya, and Zika varies with weather and climate in the real world. Comparing sites across both Ecuador and Kenya, we found that measurements of the local weather, combined with a mathematical model, can predict when people get bitten by mosquitoes and get infected with dengue, chikungunya, and Zika viruses, but that the ages of household members, access to piped water, and the construction materials of the home all affected these risks. Based on observations that in regions of Africa where people regularly use bed nets, malaria-transmitting mosquitoes begin to bite earlier in the evening or in the early morning, we tested how these shifts in biting behavior affect mosquito capacity to transmit malaria. We found that while early evening biting increases malaria transmission compared to late night biting, morning biting reduces malaria transmission by exposing mosquitoes to warm temperatures that inhibit parasite development. Together, this research shows that temperature, rainfall, and humidity have powerful influences on mosquito and disease dynamics that are likely to change with climate change, but can be predicted using experiments and mathematical models in order to prevent the worst outcomes of climate change for disease.

This research provided training opportunities for 66 graduate, undergraduate, high school, and postdoctoral students. Our work was covered in major national and international news outlets, in public seminars in Ecuador, Kenya, Australia, and elsewhere, and in classrooms at Stanford University, UCLA, Penn State University, the University of Florida, the University of South Florida, Virginia Tech, and Notre Dame University. We worked with vector control and Ministry of Health officials and community members in Kenya and Ecuador to develop sustainable strategies for reducing mosquito and disease exposure. We presented our work at the White House Office of Science Technology and Policy and participated in analyses of the impacts of climate change on human health for the Intergovernmental Panel on Climate Change Sixth Assessment Report (IPCC AR6) and social costs of carbon estimates to be used in the Biden administration environmental policy.

Last Modified: 09/16/2021
Modified by: Erin Mordecai