ECOLOGICAL FACTORS UNDERPINNING EXPANSION OF <i>BORRELIA</i>-INFECTED TICK POPULATIONS IN THE SOUTHEASTERN UNITED STATES

In the eastern United States, Lyme disease is caused by the bacterium Borrelia burgdorferi sensu stricto (Bb) and vectored by the blacklegged tick Ixodes scapularis. This tick is found in all eastern states, yet Lyme disease has been considered endemic only in the North. Bb infection was found only rarely in I. scapularis in southern states in past decades, but infected tick populations have recently expanded into southwestern Virginia. There is concern that further southwards spread of infected tick populations into Tennessee could occur either from Virginia, to the east of the Appalachians, or from Kentucky, to the west. There are, however, two hypotheses as to why there might be an ecological barrier to such spread: i) I. scapularis densities in eastern Tennessee may be too low to support Bb transmission cycles; or ii) immature ticks in eastern Tennessee may feed primarily on non-reservoir competent lizards. This study set out to assess whether or not Bb-infected tick populations have become established in eastern Tennessee and to investigate these hypotheses. Winter surveys for adult I. scapularis at 130 sites in Tennessee, Kentucky, and Virginia in 2017-2018 and 2018-2019 determined that most eastern Tennessee counties have established blacklegged tick populations. Bb-infected ticks were found in four of these counties. Comparisons of Borrelia strain types between states suggested that southwards movement of infected ticks into eastern Tennessee has been primarily from Virginia rather than Kentucky. Live-trapping at selected sites indicated that while some immature I. scapularis do fed on reservoir incompetent lizards, mice are abundant and are also serve as hosts. Stable Isotope Analysis supported predictions that Bb-positive ticks were more likely to have fed on herbivores (e.g., rodents) than on insectivores (e.g., lizards) and that nymphal I. scapularis were more likely to have fed on rodents than lizards in eastern Tennessee. I conclude, therefore, that neither low tick abundance nor immature tick host selection represent a significant barrier to Bb emergence in eastern Tennessee. Health practitioners and the public in this area should be alert to the potential for increasing Lyme disease risk in coming years.