A multi-level study of disease dynamics in desert tortoise (Gopherus agassizii) with implications for translocation risk assessments

Open Access
Aiello, Christina Marie
Graduate Program:
Doctor of Philosophy
Document Type:
Date of Defense:
February 28, 2018
Committee Members:
  • Peter John Hudson, Dissertation Advisor
  • Tracy Lee Langkilde, Committee Chair
  • Peter John Hudson, Committee Member
  • David Andrew Miller, Committee Member
  • Ephraim Mont Hanks, Outside Member
  • wildlife epidemiology
  • wildlife translocation
  • behavioral ecology
  • contact networks
  • applied ecology
  • wildlife management
In an age of increasingly human-modified ecosystems, wildlife must interact with numerous threats and sources of disturbance. Attempts to mitigate the damage caused by human activity, such as wildlife translocations, may also be viewed as a type of disturbance despite the intended positive goals of many animal relocations. My research examines the risks wildlife translocations present relative to introduced or endemic infectious diseases and is driven by this over-arching question: what is the impact of augmenting populations with new hosts relative to the outbreak potential of pathogenic infections? For directly transmitted diseases, pathogens rely on host interaction and infectiousness to spread. Therefore, to truly assess the impact translocation events may have on local disease dynamics, we need to examine processes occurring across the landscape as well as between hosts and within hosts. My research focuses on the intersection between animal management, behavior, and epidemiology using the desert tortoise (Gopherus agassizii) as a case study. I first approach the translocation question at the population level (Chapter 1) and use tortoise movement data from a controlled, replicated translocation study to estimate changes in population connectivity using proximity networks. This study shows that translocation creates many new opportunities for contact and transmission that could result in higher prevalence of local pathogens. I next examine how pathogen characteristics like transmission efficiency per contact might affect these conclusions. In Chapter 2, I focus on between-host transmission of the pathogen Mycoplasma agassizii - the most significant pathogen identified in tortoise populations. By initiating controlled contact events between infected and uninfected tortoises, I was able to estimate how much interaction would be necessary between hosts to establish infection, relative to the infectiousness of the transmitting host. By comparing these transmission estimates to contact data collected from wild tortoises, Chapter 2 shows that transmission-efficient contact events will likely be distributed in a highly skewed fashion compared to the assumptions made in Chapter 1. In Chapter 3, I then explore what underlying factors may drive heterogeneity in tortoise contacts and thus aid in identifying circumstances that facilitate M. agassizii transmission events. Considering tortoise behavioral and spatial ecology, I identified metrics that improve prediction of contacts between tortoises and show that mating behavior largely underlies skewed contact rates. Finally, in Chapter 4, I return to tortoises infected with M. agassizii during the Chapter 2 study to determine how host response to infection and patterns of disease progression might affect host infectiousness and the length of the infectious period. I found varying levels of pathogen resistance and tolerance among infected hosts, which had a large impact on the quantity of pathogen shed and the duration of shedding. Collectively, these studies lay the groundwork for more comprehensive studies of M. agassizii epidemiology in both disturbed and undisturbed populations. I conclude with a discussion of future work incorporating these data into experimental translocations conducted as part of a larger project.