Host species determines whether infection load increases beyond disease‐causing thresholds following exposure to the amphibian chytrid fungus

Abstract

Introduced pathogens are increasingly being implicated in population declines and their effects are difficult to manage. In the absence of methods to eradicate pathogens acting as threatening processes, intervention before population decline is necessary. Such an intervention requires an ability to predict when population declines will occur, and therefore, an understanding of when exposure will lead to infection, disease, death and population decline. This study investigates when pathogen exposure leads to disease for the amphibian chytrid fungus Batrachochytrium dendrobatidis, which has been implicated as a causal agent in the global amphibian decline. Susceptibility studies were conducted on two anuran species, the green and golden bell frog Litoria aurea and the striped marsh frog Limnodynastes peronii, when exposed to the fungus as either tadpoles or juveniles. Host species was found to significantly affect the outcome of exposure, with infection loads in L. aurea increasing over time and resulting in significantly lower survival rates than unexposed. By comparison, infection loads in L. peronii remained the same or decreased over time following the initial infection, and survival rates were no different whether exposed to B. dendrobatidis or not. These outcomes were independent of the life stage at exposure. Individuals with higher infection loads were not found to have lower survival rates; rather, an infection load threshold was identified where individuals with infection loads that crossed this threshold had high likelihoods of showing terminal signs of chytridiomycosis. Therefore, host species determined whether infection load crossed this threshold and the crossing of the threshold determined the incidence of disease and survival. The quantification of infection load thresholds for survival, along with the time it takes to reach them, will enable infection loads in wild populations to be related to the likelihood of disease and is the first step in the understanding and prediction of when exposure will result in population decline.

Assessing host extinction risk following exposure to Batrachochytrium dendrobatidis

“…For example, the yearly host survivorship is halved at a certain DOI tolerance level, T s and the adult breeding rate is halved at a certain DOI, T br . Individuals die immediately if their DOI reaches a lethal threshold [28,32,34]. We also explicitly considered the case where tadpoles completely tolerate their infections, e.g.…”

Chytridiomycosis in Amphibians

“…\ small doses of the pathogen have been shown to cause fatal chytridiomycosis in infected frog metamorphs. 18,19 Three of 3 frogs, each exposed to an estimated 1000 zoospores, died or became terminally ill between 23 and 38 days post exposure, 3 of 3 frogs exposed to approximately 100 zoospores died between 35 and 47 days post exposure, however 3 frogs exposed to approximately 10 zoospores did not succumb to chytridiomycosis. 18 The mortality in naïve amphibians, in both natural and captive populations, is normally high and can reach up to 100% of all infected animals.…”

Clinical trials with itraconazole as a treatment for chytrid fungal infections in amphibians

“…Clinical signs of chytridiomycosis are non-specific and include irregular skin sloughing, lethargy, weight loss, ventral and hind-limb redness and loss of righting reflex. The motile zoospores of Bd infect the keratinized epithelium in the outer epidermal layers of post-metamorphic amphibians resulting in thickening of the epidermis (Stockwell et al 2010). Infection seems to cause osmoregulatory dysfunction (Voyles et al 2009a(Voyles et al , 2012, though it is not clear whether this effect is caused directly by the fungus or perhaps by a toxin it produces (Voyles et al 2007).…”

“…If an animal was unable to right itself within 8 s after being placed on its back, we assumed that it would have died within 24 h and the animal was euthanized (Stockwell et al 2010) with 0.2% MS-222 (Berger et al 2004). All animals were euthanized upon completion of the experiment.…”