Seasonal variation in the prevalence of a fungal pathogen and unexpected clearance from infection in a susceptible frog species

Abstract

<p>The amphibian chytrid fungus <i>Batrachochytrium dendrobatidis</i> (<i>Bd</i>) causes the disease chytridiomycosis, which is a primary driver for amphibian population declines and extinctions worldwide. For highly susceptible species, such as the green and golden bell frog <i>Litoria aurea</i>, large numbers of <i>Bd</i>-related mortalities are thought to occur during the colder season (winter), when low temperatures favour the growth of the pathogen. However, extant <i>L. aurea</i> populations are persisting with <i>Bd</i>. We measured <i>Bd</i> prevalence and infection levels of wild <i>L. aurea</i> using capture-mark-recapture and radio-tracking methods. Using this information, we sought to determine host and environmental correlates of <i>Bd</i> prevalence and infection load. Mean ± SE infection load was higher in frogs sampled in autumn (431.5 ± 310.4 genomic equivalents; GE) and winter (1147.5 ± 735.8 GE), compared to spring (21.8 ± 19.3 GE) and summer (0.9 ± 0.8 GE). Furthermore, prevalence of <i>Bd</i> infection in <i>L. aurea</i> was highest in winter (43.6%; 95% CI 33.1-54.7%) and lowest in summer (11.2%; 95% CI 6.8-17.9%). Both prevalence and infection load decreased with increasing temperature. Seven frogs cleared their fungal infection during the coolest months when <i>Bd</i> prevalence was highest; however, these clearances were not permanent, as 5 frogs became infected again. Understanding the factors that allow amphibians to clear their <i>Bd</i> infections when temperatures are optimal for <i>Bd</i> growth presents the potential for manipulating such factors and provides an important step in future research.</p>