Boreal Toad Survival Varies by Degree of Attenuation and Adaptation of a Fungal Pathogen

Batrachochytrium dendrobatidis (Bd) is a fungal pathogen responsible for dramatic declines of amphibian populations around the world. Experimental exposure studies have documented differences in host susceptibility to Bd, but variation in study designs may limit our ability to compare inferences across studies. Using laboratory-maintained pathogen cultures that may lose virulence over time (i.e., pathogen attenuation) or exposing hosts to foreign or local strains are choices that could affect hosts differently. To test how these study design choices affect resulting inference, we exposed a vulnerable anuran species, the boreal toad (Anaxyrus boreas boreas), to three strains of Bd that varied in passage history and geographic origin and measured differences on host survival. We also examined strain-specific information reported in 58 publications of Bd exposure experiments to investigate the potential for reproducibility across studies. We found that boreal toads exposed to strains with many passes had higher weekly survival probabilities than those exposed to the strain with few passes, indicating likely pathogen attenuation in strains with higher passage history. We also found evidence for local adaptation of Bd to its host. Eighty-eight percent of publications summarized did not report the number of Bd passages. Our findings suggest that strains with fewer passes in culture are better suited for exposure studies seeking to understand differences in host susceptibility and that the amount of passes can dramatically affect inference gained across studies.