Estimated Effects of Projected Climate Change on the Basic Reproductive Number of the Lyme Disease Vector Ixodes scapularis

Background: The extent to which climate change may affect human health by increasing risk from vector-borne diseases has been under considerable debate. Objectives: We quantified potential effects of future climate change on the basic reproduction number (R-0) of the tick vector of Lyme disease, Ixodes scapularis, and explored their importance for Lyme disease risk, and for vector-borne diseases in general. Methods: We applied observed temperature data for North America and projected temperatures using regional climate models to drive an I. scapularis population model to hindcast recent, and project future, effects of climate warming on R-0. Modeled R-0 increases were compared with R-0 ranges for pathogens and parasites associated with variations in key ecological and epidemiological factors (obtained by literature review) to assess their epidemiological importance. Results: R-0 for I. scapularis in North America increased during the years 1971-2010 in spatiotemporal patterns consistent with observations. Increased temperatures due to projected climate change increased R-0 by factors (2-5 times in Canada and 1.5-2 times in the United States), comparable to observed ranges of R-0 for pathogens and parasites due to variations in strains, geographic locations, epidemics, host and vector densities, and control efforts. Conclusions: Climate warming may have co-driven the emergence of Lyme disease in northeastern North America, and in the future may drive substantial disease spread into new geographic regions and increase tick-borne disease risk where climate is currently suitable. Our findings highlight the potential for climate change to have profound effects on vectors and vector-borne diseases, and the need to refocus efforts to understand these effects.