Master of Science (M.Sc.)
Harmony J Dalgleish
John P Swaddle
The incidence of tick-borne diseases is on the rise in the US and around the world, due in part to emerging pathogens. However, the environmental drivers affecting these pathogens remain unclear. Most research on the topic in the US has focused on Borrelia burgdorferi, which causes Lyme, but it is unknown if the same conditions that affect B. burgdorferi also affect other pathogens, which may be carried by other ticks or reservoirs. The answer will help determine generalizable principles in tick-borne pathogen ecology, if they exist, as well as better manage for tick-borne pathogen risk in areas at risk from new and often unfamiliar pathogens. One such pathogen in the eastern US is Ehrlichia chaffeensis, which is transmitted by the lone star tick (Amblyomma americanum) and is the causative agent of monocytic ehrlichiosis, a potentially fatal illness. Here, I examine spatial and temporal variation in E. chaffeensis prevalence in southeastern Virginia and how this is influenced by the environment. in Chapter 1, I used four years of data to ask how E. chaffeensis prevalence changed between years and how this was affected by seasonal weather patterns. Using mixed-effect models, I related E. chaffeensis occurrence to temperature, humidity, vapor-pressure deficit, and precipitation up to 21 months prior to sampling. Annual prevalence varied significantly from 0.9% - 3.7%, and was positively affected by temperatures during the previous winter (i.e. before the current cohort of nymphs hatched). I hypothesize this is because winter temperature affects reservoir host mortality or natality, which would in turn affect the availability of naïve reservoir hosts in the spring. Regardless of mechanism, my findings have implications for the future because winters in this region are predicted to grow warmer, which could increase E. chaffeensis prevalence. in Chapter 2, I used five years of field data to ask how landscape context affects spatial variation in the prevalence of E. chaffeensis and interannual occupancy dynamics of its vector, A. americanum. Under a Bayesian framework, I created a metric- and scale-optimized model to relate E. chaffeensis prevalence and A. americanum turnover to the availability, quality, and fragmentation of habitat. Prevalence was highest and turnover was lowest in areas of low forest cover and low edge density, dominated by deciduous trees. Thus, highest disease risk is predicted in areas of forested areas that are either isolated or abutted against impermeable boundaries, both characteristic of many parks. Many of my results highlight the complexity of tick-borne disease dynamics and the challenges inherent to the subject; some results ran counter to my predictions and E. chaffeensis prevalence remains rare, which makes it challenging to model. That said, my work also represents important progress in an often-neglected area of tick-borne disease ecology. to my knowledge, this is the first study to address temporal variation in E. chaffeensis prevalence, and is one of few studies to relate E. chaffeensis prevalence to landscape context at a scale relevant to the pathogen's hosts and to disease-risk management.
© The Author
Simpson, Dylan, "Investigating the Spatiotemporal Distribution of a Tick-Borne Pathogen, Ehrlichia Chaffeensis" (2018). Dissertations, Theses, and Masters Projects. Paper 1550153994.