ABSTRACTINVESTIGATING THE IMPORTANCE OF VERTEBRATE HOSTS FOR LYME DISEASE ECOLOGY: A NATURAL EXPERIMENT PRESENTED BY LAKE MICHIGAN ISLANDS AT SLEEPING BEAR DUNES NATIONAL LAKESHORE ByJennifer Lois Sidge In eastern North America, Lyme disease is caused by the bacterium, Borrelia burgdorferi, and is transmitted by the blacklegged tick, Ixodes scapularis. In Michigan’s Lower Peninsula, the blacklegged tick and the bacterium are invading from the southwest corner, northward along the Lake Michigan coast with the presumed leading edge at Sleeping Bear Dunes National Lakeshore (SLBE). How the tick spreads and becomes established is of great public health importance. White-tailed deer (Odocoileus virginianus) are believed to be the most important hosts for adult I. scapularis and critical for its spread and maintenance, but few opportunities exist to investigate tick and pathogen dynamics in their absence. Two Lake Michigan islands, at SLBE, one with deer and one without, presented this opportunity. The overall objective of this dissertation was to establish a baseline of abundance for the tick and pathogen on both islands and compare it to the ecologically diverse mainland, and assess the role of other mammals as alternative hosts for the adult stage of the tick in areas absent of deer. My hypothesis was that in locations devoid of deer, the blacklegged tick would not be established and/or would exist at much lower densities in comparison to areas with resident deer populations. Also, other medium-sized mammals would serve as hosts for the adult ticks that theoretically could support a tick population. In Chapter 1, I continued to track the invasion of the Lyme disease pathogen and vector at SLBE over a nine-year period. I found that there was a four year delay between the first detection of blacklegged ticks and the presence of B. burgdorferi and there was a trend illustrating an increase in I. scapularis and B. burgdorferi over time. At an additional site on SLBE’s mainland, the tick and the pathogen were detected at the same time supporting the “dual-invasion” scenario of invasion, yet, this may have been a consequence of when sampling began. At recently-invaded Lyme disease areas, such as these two SLBE mainland locations, I found eastern chipmunks (Tamias striatus) to be an earlier indicator of the pathogen’s presence in comparison to white-footed mice (Peromyscus leucopus). SLBE’s two offshore islands, one with white-tailed deer and the other devoid, were the focus of Chapter 2 as a means to evaluate the success of mammalian hosts for maintaining I. scapularis in the absence of deer. I found that although the island with the deer had a greater density of ticks and greater B. burgdorferi infection prevalence, the island that was deer-free had all three life stages of the blacklegged tick and B. burgdorferi was present. Thus, alternative hosts for the adult tick, including snowshoe hares (Lepus americanus), passerine birds, and coyotes (Canis latrans), were captured and it was determined that coyotes were maintaining the established tick population on the island. Eastern chipmunks played a crucial role with maintaining the juvenile stages of the tick on the islands. Chapter 3 then compared SLBE’s host-diverse mainland to the host-limited islands, testing the dilution and multiple niche polymorphism hypotheses. In order to test the dilution effect, the larval I. scapularis prevalence on white-footed mice between the two locations was compared. The proportion of mice infested with at least one I. scapularis larvae nor the larval burden on the mice supported the dilution hypothesis. However, on the islands and the mainland, more eastern chipmunks were captured than anticipated and this consequently reduced the larval burden on the mice in each community, thus, supporting the dilution effect at the location level. Host-seeking I. scapularis adult/nymphal infection prevalence and adult/nymphal density of infected ticks were greater on the host-limited islands, supporting the dilution hypothesis. However, B. burgdorferi IGS strain diversity was greater on the islands in comparison to the host-diverse mainland, which was unlike what was predicted by the multiple niche polymorphism hypothesis. Future studies to better estimate the island mammalian population sizes and diversity, in addition to comparing the B. burgdorferi island diversity to an area with a known long-established population of ticks would be advantageous to further our Lake Michigan island Lyme disease ecology knowledge. Also, given that SLBE is now most likely not at the leading edge of the Lyme disease invasion, future work should evaluate if the tick and bacterium have spread into neighboring counties.
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