Interaction between temperature and sublethal infection with the amphibian chytrid fungus impacts a susceptible frog species

Abstract

The amphibian chytrid fungus Batrachochytrium dendrobatidis is an emerging infectious pathogen present on every continent except Antarctica. It causes the disease chytridiomycosis in a subset of species but does not always result in disease or death for every host. Ambient temperature influences both amphibian metabolism and chytrid pathogenicity, however the interactive effects on host physiology is not well understood. We investigated the sublethal effect of B. dendrobatidis infection on a susceptible host, Litoria aurea to test (1) whether the infection load, metabolic activity, body fat and gonad size differed in L. aurea at either 24 °C or 12 °C ambient temperatures and (2) whether previous Bd infection caused long-term changes to body fat and gonad size. Litoria aurea in 12 °C treatments had higher infection loads of B. dendrobatidis and lower survivorship. Metabolic rate was higher and fat mass was lower in infected individuals and in animals in 24 °C treatments. Male L. aurea previously infected with B. dendrobatidis had smaller testes 5 months-post clearance of infection, an effect likely to translate to fitness costs in wild populations. These experiments demonstrate a physiological cost to sublethal B. dendrobatidis infection, which suggests a reduction in host fitness mediated by temperature in the host's environment regardless of whether infection leads to mortality.

Figure 1

Effect of temperature and Bd infection in L. aurea for experiment 1. (a) Effect of temperature (12 °C or 24 °C) on mean infection load in Bd infected frogs. Values are least squares means ± 1CI (95%); (b) Effect of temperature (12 °C or 24 °C) and Bd infection (infected or uninfected) on survival over 48 days; (c) Mean SMR in infected and uninfected frogs at 12 °C or 24 °C. Values are least squares means ± 1CI (95%); (d) Mean fat mass in infected and uninfected frogs at 12 °C or 24 °C. Values are log transformed least squares means ± 1CI (95%).

Figure 2

Regression of L. aurea testes mass data from experiment 2 against body weight. Broken lines = 95% confidence of fit.