Lyme Disease is a zoonotic disease of increasing impact on human health. Multiple mammalian hosts are responsible for tick success and disease prevalence. Quantifying and predicting the outcomes of changing mammalian communities on human disease risk has proven difficult as hosts may interact directly and indirectly, and some may promote tick abundance while reducing pathogen prevalence. Controlled studies of mammalian host communities are extremely rare because of the difficulty of manipulating the abundance of species at spatial scales large enough to capture relevant pathogen dynamics. To address this gap, this project will initiate studies related to a large-scale experimental manipulation of a key tick host, the white-tailed deer, to evaluate how deer alter the risk of transmission of tick-borne pathogens. The State of Wisconsin will erect 16 large enclosures at 4 locations in hardwood forests of northern Wisconsin and will stock these pens with deer at different densities. In this pre-intervention study, Ixodes scapularis ticks will be collected from all 4 sites to assess abundance and samples will be archived for comparisons of pathogens following the enclosure construction.
This project will have broader impacts through examining the threshold deer density that supports high tick populations, thus contributing to recommendations for wildlife management and disease control. Importantly, this work also will have significant impacts through the training of graduate students, as well as by providing many opportunities for undergraduates to participate in interdisciplinary research.
The long term goals of the study are to examine how deer activity and density affect the transmission of tick-borne pathogens. For this pilot study, we used motion activated cameras to capture deer activity patterns within a 500 acre site. Each of the five sites were placed in hardwood forests distributed across northern Wisconsin. These sites were dominated by sugar maple and four of the five sites were matched for proximity to water and the presence of winter cover (hemlock and white cedar) for white-tailed deer. Collaborators collected extensive data on the plant communities at each of the 88 camera subsites (16-22/forest). The density of black-legged ticks was determined through tick dragging and sampling of small mammals. The type of small mammals present at each site as well as the size of the populations of abundant species was assessed. Finally, we measured the prevalence of Borrelia burgdorferi (BB), the agent of Lyme Disease, and Anaplasma phagocytophilum (AP), the agent of human anaplasmosis, in both ticks and the most abundant species of small mammals. All data were collected in 2012 and 2013. There were several key outcomes. First, the five forests differed dramatically in the abundance of black-legged ticks and this was consistent between the two years. Overall, the density of nymphal ticks was low to moderately high (2-72 nymphs/1000 m2) in these forests. Deer mice were abundant tick hosts but the difference between forests in tick density was not due to differences in the abundance of deer mice or white footed mice. Deer density varied across the five forests (range of 7-52 deer/km2) and the forest with the lowest nymphal tick density was also the forest with the lowest deer density. This may indicate an important deer-density threshold but additional low density sites are needed to confirm this hypothesis. Second, a role for deer at fine scales was suggested by the correlation between deer activity in the fall of 2012 and larval tick abundance in 2013. Features that influenced the risk of contacting a nymphal tick at the fine scale included the density of larvae in the prior year, soil compaction, distance to major water features, woody debris, and the number of mice trapped at individual plots. Third, infection with BB ranged from 0-36% of nymphs while AP prevalence in nymphs ranged from 0-6.5%. Sites with low tick density exhibited low infection rates in both ticks and small mammals. Fourth, analysis of the plant community suggested that the abundance of an invasive sedge, Carex pennsylvanicus, is correlated with the abundance of ticks. The abundance of other sedges and grasses was not correlated with tick density, indicating that the relationship is not due to more efficient drag sampling in grass or sedge-dominated habitats. Taken together, these outcomes suggest that these five forest locations provide an excellent foundation for studies that investigate the stability and ecology of disease coldspots and hotspots at large and small scales. Finally, the project also had broader impacts on the development of human resources. During the two years, eight undergraduates were involved in the field collections. Seven of these students continued working on independent projects related to tick-borne disease and vector biology. An additional ten undergraduates were involved in laboratory diagnostic work to identify the prevalence of pathogens in ticks and small mammals. Many of these students are interested in pursuing careers in public health, entomology, or microbiology and these laboratory experiences will provide strong background and experience as they apply for those positions.
