Amphibian larvae benefit from a warm environment under simultaneous threat from chytridiomycosis and ranavirosis

Rising temperatures can facilitate epizootic outbreaks, but disease outbreaks may be suppressed if temperatures increase beyond the optimum of the pathogens while still within the temperature range that allows for effective immune function in hosts. The two most devastating pathogens of wild amphibians, <em>Batrachochytrium dendrobatidis</em> (<em>Bd</em>) and ranaviruses (<em>Rv</em>), co-occur in large areas, yet little is known about the consequences of their co-infection and how these consequences depend on temperature. Here we tested how exposure to <em>Bd</em> and subsequent exposure to <em>Rv</em>, followed by treatment at elevated temperatures (28 and 30°C vs. 22°C) affected <em>Bd</em> and <em>Rv</em> prevalence, infection intensities, and resulting mortalities in larval agile frogs and common toads. We found multiple pieces of evidence that the presence of one pathogen influenced the prevalence and/or infection intensity of the other pathogen in both species, depending on temperature and initial <em>Rv</em> concentration. Generally, the 30°C treatment lowered the prevalence and infection intensity of both pathogens, and, in agile frogs, this was mirrored by higher survival. These results suggest that if temperatures naturally increase or are artificially elevated beyond what is ideal for both <em>Bd</em> and <em>Rv</em>, amphibians may be able to control infections and survive even the simultaneous presence of their most dangerous pathogenic enemies.