Drivers of Epidemic Timing and Size in a Natural Aquatic System

Abstract

Parasite epidemics are embedded in complicated webs of interacting organisms as well as in an abiotic environment, with direct and diffuse connections among these elements impacting epidemic dynamics. In addition, these elements can be dynamic over time and space, and they may be changing due to human impacts. Yet, though myriad factors influence it, infection is fundamentally the result of an interaction between an individual host and parasite and is impacted by the particular defenses and strategies of these antagonists. Therefore, to understand drivers of epidemics, some of which can be catastrophic for host populations, we must incorporate a nuanced understanding of environmental modulators of parasite transmission as well as of the biology of host and parasite populations interacting over space and time. My dissertation elucidates some of the drivers of natural epidemics caused by two environmentally transmitted parasites, the bacterium Pasteuria ramosa and the fungus Metschnikowia bicuspidata, in Daphniid hosts. Chapters 2 and 3 focus on how abiotic factors influence disease outbreaks through effects on parasite transmission stages. In Chapter 2, I evaluated the effects of light variation on parasite infectivity. I found that both parasites were sensitive to light, though the fungus was more sensitive. I related this to natural epidemics and found that epidemics of the less sensitive bacterium started before those of the more sensitive fungus. In addition, looking across lakes that varied in clarity (and therefore light penetration into water), I found that darker lakes had larger epidemics of the bacterial parasite. In Chapter 3, I quantified parasites in the water column of lakes to link lake habitat structure with host and parasite overlap. I found that parasites were unevenly distributed throughout the water column, and that host habitat selection behavior may influence epidemic dynamics. Chapters 4 and 5 use genetic approaches to elucidate how parasites are passed among a community of Daphniid species in lakes and how patterns of infection differ between generalist and specialist parasites. In Chapter 4, I quantified the population structure of the fungal parasite across two common host species and performed a cross infection experiment, finding that this generalist parasite in fact was composed of genotypes that specialized on the divergent hosts and that this specialization might be driven by spore size. In Chapter 5, I genotyped the bacterial parasite across populations, host species, and over time, finding that genetic variation in this parasite was structured by lake and host species, implicating low transmission across these barriers and/or local adaptation to specific host populations. In addition, parasite strain structure changed over time in outbreaks indicating rapid evolution. Overall, my research shows that epidemic dynamics are influenced by environmental conditions through their effects on environmental transmission stages, but also that epidemics depend on the success of specific interactions that occur between hosts and parasite individuals over space and time.