All hosts are not created equal : variation in amphibian responses to an emerging fungal pathogen and why it matters

Graduation date: 2013Emerging infectious diseases are increasing globally and are a threat to human, wildlife, and ecosystem health. The emerging fungal pathogen, Batrachochytrium dendrobatidis (Bd), or amphibian chytrid fungus, is associated with worldwide amphibian population declines and extinctions. Bd has been found on every continent where amphibians exist and has been documented to infect hundreds of species. As with other multi-host pathogens, the outcome of infection with Bd appears to vary among individuals, species, and populations. Variation in host responses to infection can lead to changes in the structure and composition of amphibian assemblages and can affect disease dynamics including pathogen persistence or fadeouts in ecological communities. Understanding the feedback between hosts and pathogens requires disentangling the influence of multiple interacting biotic and abiotic factors, yet fundamentally depends on characterizing intrinsic host responses to infection. This thesis broadly examines variation in species-specific susceptibility to the chytrid fungus among amphibians. I have characterized patterns of mortality, quantitative infection load, feeding behavior, and pathogen avoidance behavior of twenty different amphibian species exposed to the fungus or control conditions (Chapter 2). We found a high degree of variation in pathogen-induced responses to chytrid fungus, ranging from zero mortality to 100% mortality after only six days of pathogen exposure. Variation in infection load was also significant at the species level and there was not always a direct relationship between infection load and mortality. Behavior of pathogen exposed animals was significantly different from that of control animals, and reduced feeding behavior of pathogen-exposed animals is likely to be related to the decline in health of pathogen-exposed animals. In Chapter 3, I examined a special case of amphibian susceptibility to chytrid fungus in the American bullfrog (Lithobates catesbeianus). Bullfrogs are widely reported to be a tolerant host and a carrier of Bd that spreads the pathogen to less tolerant hosts. However, this hypothesis has not been rigorously tested using an experimental approach. In Chapter 3, co-authors and I tested whether bullfrogs raised from eggs to metamorphosis in outdoor mesocosms were susceptible to two different strains of Bd (one strain isolated from a "non susceptible" species, the American bullfrog; the second strain isolated from a highly susceptible species, the Western toad (Anaxyrus boreas)). Bullfrogs were susceptible to strain JEL 274, and this is the first documented case of susceptibility to chytrid fungus in this species. Bullfrogs were not susceptible to strain JEL 630, indicating variation in virulence among chytrid fungus strains and important context dependency when considering the effect of infection non individuals, species, and populations of amphibians. In Chapter 4, I examined fine- scale variation in responses of three key amphibian hosts, and examine the evidence for tolerance (the ability to maintain health as infection severity increases) or resistance (the ability to reduce pathogen infection load) of larval and post-metamorphic animals to Bd. I saw pronounced variation in responses to chytrid fungus among species, between life history stages, over time, and found that responses to the fungal inoculate were not dose dependent in two out of the three species examined. In the final data chapter, Chapter 5, I investigated immunological responses that underlie variation in species-specific responses to Bd. I looked at temporal patterns of functional immune responses during a time-course of early and later stage exposure to Bd, spanning 24 hours after experimental inoculation to 15 days post-inoculation. I uncovered patterns in immune responses that were distinctly associated with susceptibility and temporal patterns in infection load associated with susceptibility and immune response. This thesis provides critical information about variation in host responses to a conservation relevant pathogen. Differences in how host hosts acquire, transmit, and persist with infection have important implications for spatiotemporal disease dynamics. Further, understanding host sensitivity to infection allows for predictive risk management of imperiled species