Online

Ash, Lauren V.

Biology

2022

Ph. D.

Emerging infectious diseases threaten amphibian species around the globe. Ranavirus (Family: Iridoviridae) is associated with the majority of amphibian deaths in North America, with some species exhibiting mass mortality events in short periods of time. In contrast, some amphibian species show the capacity to resist or tolerate infection. Resistant individuals exhibit reduced infection intensity, while tolerant individuals display reduced fitness effects given the infection intensity. My research focused on how amphibian host dynamics at community, population, and individual-level scales relate to the wide variation in Ranavirus prevalence and severity. Through my work, I documented the first reports of Ranavirus in the state of Vermont and recorded the species Frog-virus 3 (FV3) at relatively low prevalence in many amphibian species and life stages across wetlands. Overall, my results contribute to our understanding of host-pathogen dynamics, particularly for this deadly amphibian disease.I investigated the relationship between host biodiversity and Ranavirus in my first chapter. Previous studies in other systems have documented a negative relationship, with host diversity 'diluting' disease risk, while other systems have observed the opposite, with diversity amplifying disease risk. Although I hypothesized a dilution effect, I found the presence and abundance of certain amphibian species (e.g., Pseudacris crucifer) positively related to and amplified Ranavirus prevalence. However, other species (e.g., Notophthalmus viridescens) were associated with reduced population or community prevalence, and many individuals harbored asymptomatic infections. Little is known about the capability for these more resistant and tolerant species to serve as reservoirs, or sources of infection, for other more susceptible species. The American bullfrog (Lithobates catesbeianus) is a species that has shown experimental resistance and tolerance to FV3. For my second chapter, I provided the first assessment of concomitant infection of Ranavirus and helminth macroparasites in invasive populations of bullfrogs in South America. In Brazil, bullfrogs are farmed for their meat and have invaded the Atlantic Forest, which houses many of Brazil's endemic anuran species. Asymptomatic Ranavirus infection was common in invasive bullfrogs, suggesting they can serve as sources of infection to native species. Co-infection by helminth macroparasite taxa was also common. I found a significant negative relationship between Ranavirus viral load and nematode abundance, and Ranavirus-infected bullfrogs had lower total macroparasite abundances. This suggests a trade-off in immune response may be occurring: individuals who may be effective at controlling macroparasite abundances could be more susceptible to pathogen infection and vice-versa. Individual host immune response most likely contributes to the variation in Ranavirus susceptibility and mortality observed even within populations. The Major Histocompatibility Complex (MHC) generates and modulates acquired immune response in all jawed vertebrates. For my final chapter, I used an experimental approach to determine whether MHC haplotypes were associated with the mortality and viral loads of experimentally infected wood frog (L. sylvaticus) tadpoles. I detected 24 unique haplotypes in 33 of the most susceptible and resistant individuals, which formed 4 haplotype supergroups. One supergroup (VT-02) had more resistant and fitter individuals, while another had higher mortality (VT-03). I detected diversifying selection on multiple amino acid sites across the MHC gene in all supertypes, indicating positive selection on the MHC gene may reflect pressure from other sources.