Feasibility of Stable Isotope Analysis as a Method for Prior Ixodes scapularis Host Identification

Tick-borne diseases pose a prominent threat to humans by causing a wide range of symptoms and occasionally death. I am studying tick-borne disease transmission through an ecological lens, attempting to identify previous tick hosts using stable isotope analysis (SIA). A method for indirect tick host identification would be useful because our current understanding of the roles of various tick hosts might be biased by the ease of capture of some hosts compared to others. SIA of ticks collected in the wild could clarify the relative importance of various hosts or even identify new hosts that have not been previously considered, advancing our understanding of tick-borne disease transmission through communities. To evaluate SIA as a technique for previous tick host identification, I trapped 2 species known to be important hosts for Ixodes scapularis (black-legged tick), Peromyscus leucopus (white-footed mouse, n=26) and Tamias striatus (eastern chipmunk, n=19), and infested them with nymphal I. scapularis. For the 3-4 days that the ticks fed, the hosts consumed a diet enriched with ¹⁵N, an isotope commonly utilized for trophic level identification, to determine if that diet enrichment would be reflected in the molted tick. I chose ¹⁵N because previous research showed that δ¹³C, another useful isotope for studying diets, has a flexible isotopic signature that would likely change according to short-term, opportunistic deviations from a host’s typical diet. By using ¹⁵N, I hoped to find a less flexible isotopic signature to facilitate host identification to species or at least to feeding guild.  The average isotopic signature of ticks feeding on hosts with a ¹⁵N-enriched diet was significantly higher than ticks collected from newly captured animals (which presumably reflect the wild diet; p< 0.0001). This indicates that a short-term diet change was reflected in the molted tick. Further comparisons indicated that the host’s long-term diet is also reflected, revealing a summative quality of the δ¹⁵N signature. Our overall findings suggest that the δ¹⁵N isotopic signature is more flexible than expected and reflects dramatic short-term diet changes, nevertheless host feeding guild identification could still be possible because the long-term diet is also represented.