EEB Student Dissertation Defense: Causes and consequences of disease outbreaks in aquatic environments

Libby Davenport, EEB PhD student, presents their dissertation defense.

Abstract: Disease outbreaks of environmentally transmitted parasites require that susceptible hosts encounter infective transmission stages (spores) distributed within environmental reservoirs and become infected. However, we have limited empirical data about how changes in parasite environmental spore abundance influence disease dynamics. This is concerning because many environmentally transmitted parasites (e.g., chronic wasting disease, white-nose syndrome, anthrax, etc.) pose major threats to human, livestock, and wildlife health. Moreover, factors such as resource availability may affect the infection probability of a parasite during an encounter with a host as well as impacts of that parasite on exposed or infected hosts. Chronic sublethal impacts of parasites on host traits related to gut function may have consequences that scale up to affect ecosystem processes, such as primary productivity and biogeochemical cycling, although these potential impacts of parasitism remain relatively unexplored. My dissertation combines a multi-faceted six-month field study with a series of laboratory experiments to address the following questions: How does parasite spore abundance in the environment change over time and space? What is the relationship between disease outbreaks and spore abundance in the environment? How does resource availability affect infection success and impacts of a microsporidian parasite on its dominant grazer host?
In my dissertation research, I used the freshwater invertebrate host Daphnia dentifera and five of its naturally occurring environmentally transmitted parasites to answer these questions. During my six-month field study, I quantified spore abundance of the five parasites at every meter of the water column in the deep basin of six lakes in southeastern Michigan every two weeks using a digital PCR assay that I designed. Field-collected spore abundance data revealed that environmental spore abundance changed by several orders of magnitude over time and space and that lake basin shape and the strength of seasonal thermal stratification affected the evenness of spore distributions, altering host-parasite encounter rates. When examining the relationship between environmental spore concentration and disease outbreaks, I found significant time lags between the peaks in environmental spore concentration and infection prevalence, which may have been influenced by parasite traits.
In my laboratory experiments, I found that a microsporidian parasite was more likely to infect hosts when resources were abundant and this parasite significantly reduced grazing rates of infected hosts (although this impact was present at different times depending on resource availability). Thus, the parasite-induced reduction in grazing rate could have broad impacts on lake food webs since the host is a dominant grazer; these results set the stage for further investigation of how and when effects of parasitism scale up to influence higher-order ecological phenomena.
Overall, my dissertation contains one of the most comprehensive field studies examining the dynamic relationship between disease outbreaks and spore abundance in the environment and identifies resource-dependent impacts of a parasite on host density and traits that may have broader ecological consequences.

This is a hybrid event.
To join via Zoom: https://umich.zoom.us/j/94783248528
Passcode: daphnia