The Sensitivity of Lyme Disease to Climate and Land Cover in the Northeastern United States

The prevalence of Lyme disease in the United States is rapidly increasing. According to the Centers for Disease Control and Prevention, between 2004 and 2016 Lyme disease cases almost doubled, and between 1993 and 2012 the number of counties classified as high-incidence for Lyme disease more than quadrupled. While the rapid rise in Lyme disease is clear, the causes of it are not. Modeling Ixodes scapularis, the primary Lyme disease tick vector in the eastern US, and Lyme disease transmission presents an opportunity to disentangle the drivers increasing Lyme disease, including climate impacts on ticks, altered land cover, changing host abundance, and the expansion of human settlement into forested areas. However, to date this modeling is limited and typically simplistic.

We developed a compartment model of ordinary differential equations to simulate tick development and Lyme disease transmission in the Northeast. The model includes explicit representations of Ixodes scapularis population dynamics at each life stage, as well as climate effects on tick growth and death, land cover impacts on hosts, and the likelihood of transmission based on tick and human interactions. After calibrating the model to reasonably simulate the observed seasonal timing and relative abundance of questing ticks for each growth stage (larva, nymph, adult), we analyzed the sensitivity of the model across climate and land cover. Specifically, we ran an ensemble of eight simulations with climate data from Hanover, New Hampshire and Storrs, Connecticut, and land cover data from Hanover and Cardigan, New Hampshire, and Windsor and Danielson, Connecticut. Initial results show that tick abundance is more sensitive to land cover, where favorable land cover enhances host populations and increases the success of questing ticks at all stages. Lyme disease incidence is sensitive to land cover through host and tick abundance, as well as host type, since some hosts are competent (can transit Lyme disease) while others are not. Lyme disease incidence is also strongly influenced by assumptions about the probability ticks finding a human host and the probability of human infection.