PS 6-79 - Modeling the Historical and Future spread of Lyme Disease in Response to Climate and Land Cover in the Northeastern United States

Abstract: The prevalence of Lyme disease is rapidly increasing in the United States. According to the Centers for Disease Control and Prevention, Lyme disease cases nearly doubled between 2004 and 2016, and between 1993 and 2012 the number of counties classified as high-incidence for Lyme disease more than quadrupled. Estimated cases are nearing 500,000 annually. Despite the clear rise in cases, the causes of this rise in Lyme disease are unclear. Modeling the primary tick vector in the eastern US, Ixodes scapularis, and Lyme disease transmission presents an opportunity to disentangle drivers of Lyme disease expansion, including climate impact, altered land cover, changing host abundance, and encroaching human settlement in forested areas. We developed a compartmental model of ordinary differential equations to simulate tick development and Lyme disease transmission in the Northeast. The model explicitly represents I. scapularis population dynamics at each life stage, climate effects on tick growth and death, land cover impacts on hosts, and the likelihood of transmission based on tick and human interactions. Following model calibration to sites in the Northeast, we have expanded the scope of the model to cover the entirety of the Northeast as well as New York. We are collaborating with public health agencies and academic institutions across this region to utilize tick abundance and pathogen prevalence data from Connecticut, Maine, New Hampshire, New York, and Vermont. Initial results show that tick abundance is sensitive to land cover, with favorable land cover enhancing host populations and increasing the success of questing ticks at all stages. Lyme disease incidence is sensitive to land cover through host and tick abundance, as well as to host type, since some hosts are competent (can transmit Lyme disease) while others are not. Assumptions surrounding the probability of ticks locating human hosts and transmitting disease also strongly influences disease incidence.