The population dynamics of Ixodes scapularis is complex, with multiple ecological relationships between tick vectors and wildlife hosts. Ixodes scapularis has a two-year lifecycle with the occurrence of its three life stages coinciding with seasonal patterns and the presence of wildlife hosts. Previous models are limited in their ability to identify the mechanisms due to over-simplified assumptions of aggregation that ignores the mechanisms by which individual host species can influence tick populations. This tick requires blood meals from vertebrate hosts to complete its lifecycle, and hosts, in turn, vary in their permissiveness (i.e. the ability for ticks to successfully feed). We, therefore, developed a fine-scaled individual based model in NetLogo to simulate the relationships between I. scapularis, and two hosts Peromyscus leucopus (white-footed mouse) and Odocoileus virginianus(white-tailed deer). As tick development rates and activity levels are temperature dependent, we used Quality Controlled Local Climate Data from Connecticut to simulate patterns of tick phenology. We identified unknown parameters for our model using categorical calibration to ensure tick infestation rates mimic those observed in nature. We also performed sensitivity analyses to evaluate the robustness of the model to parameter uncertainty.
Results/Conclusions
Based on the model output produced from 40,000 iterations, our model accurately simulates the phenology of I. scapularis, with nymphs beginning to appear in late April and peaking in abundance in mid-May. The model also accurately has tick larvae emerging in early July, peaking in August, and subsiding in September. To evaluate species-specific host relationships to I. scapularis, we calibrated larval, nymph, and adult attachment rates to obtain white-footed mice infestation within the range of 0-43 ticks per mouse. Sensitivity analyses, or varying these parameters by 10% above and below the estimated value, demonstrate that our model output is robust to parameter uncertainty. Small changes to nymph molting rates and deer grooming also do not result in drastic changes in model output. This model is the first step to creating a multi-host model to explore how community composition influences the maintenance of tick populations and pathogen prevalence. While it may be impractical (and potentially unethical) to experiment on disease systems in the wild, an individual-based model approach that allows for testing of mechanisms provides a user-friendly tool for epidemiologists and ecologists to quantify and manage tick-borne risk in specific areas of concern.