Dynamics of the host-pathogen relationship between New Zealand’s threatened frogs (Leiopelma spp.) and the amphibian chytrid fungus, Batrachochytrium dendrobatidis

Abstract

The spread of chytridiomycosis, an emerging infectious disease caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd), is one of many threats facing amphibians worldwide. In New Zealand,Bd has been detected in both threatened native (Leiopelma spp.) and widespread introduced (Litoria spp.) anuran species, and many questions concerning its potential impact remain. In order to aid in the conservation management of New Zealand’s vulnerable amphibians, I tested the susceptibility of native frog species to this emerging pathogen. In addition, I investigated host infectiousness of both introduced and native frog species as a first step in the creation of a New Zealand specific Bd-host interaction model. It has been proposed that novel threats like chytridiomycosis have prompted a shift in the focus of amphibian researchers, resulting in the neglect of wide-reaching threats such as habitat loss. In Chapter 2, I tested the validity of this proposal by reviewing the literature on amphibian declines, categorizing articles by topic and comparing citation rates and journal impact factor among articles on these topics. From 1990-2009, the yearly proportion of papers on habitat loss remained fairly constant. However, papers on chytridiomycosis were more highly cited and published in journals with higher impact factors, which may indicate this topic’s popularity. These results were not consistent with a shift in the research agenda on amphibians, but the perception of such a shift may have been supported by the higher citation rates of papers on chytridiomycosis. In Chapter 3, I assessed the susceptibility of two native species, Le. pakeka and Le. hochstetteri, to chytridiomycosis, utilizing the susceptible introduced species, Li. ewingii, as a positive control.Bd-naïve individuals were exposed to a New Zealand- isolate of Bd, and their infection status monitored using quantitative real-time PCR. Native species demonstrated low susceptibility and all individuals cleared Bd infection (Le. hochstetteri by week 10,Le. pakeka by week 15) before manifesting any clinical signs of chytridiomycosis. However, zoospore load was low and not detected consistently each week. Le. archeyi similarly demonstrated low susceptibility in a previous study, indicating a genus-wide trend. These findings suggest that the risk of Bd for Leiopelmatids in captive populations is low, but care needs to be taken when extrapolating these results to natural populations. Finally, I investigated an aspect of Bd that has yet to be quantified in any study: potential host infectiousness. In Chapter 4, I compared the longitudinal shedding of zoospores in three species with different susceptibilities to Bd:Le. pakeka,Le. hochstetteri and Li. ewingi. Frogs were exposed to Bd and then rinsed with artificial pond water at increasing intervals to measure free zoospores on the skin. Native species demonstrated low infectiousness with an overall mean shedding rate of only 4 zoospore equivalents (ZE)/hr. In the susceptible Li. ewingii, infectiousness was high (peaking at 423 ZE/hr), but mortality rate was low (20%) in comparison to the previous susceptibility trial (100%). These findings indicate that host auto-reinfection may be critical for to reach lethal levels, and rinsing may have removed these zoospores.