Tolerance of fungal infection in European water frogs exposed to Batrachochytrium dendrobatidis after experimental reduction of innate immune defenses

Naturally-acquired Bd infections were detected in 10/51 P. lessonae and 4/19 P. esculentus, but no disease outbreaks or population declines have been detected at this site. Thus, this population was immunologically primed, and disease resistant. No mortality occurred during the 64 day experiment. Forty percent of initially uninfected frogs became sub-clinically infected upon experimental exposure to Bd. Reduction of both skin peptide and microbiota immune defenses caused frogs to gain less mass when exposed to Bd than frogs in other treatments. Microbiota-reduced frogs increased peptide production upon Bd infection. Ranavirus was undetectable in all but two frogs that appeared healthy in the field, but died within a week under laboratory conditions. Virus was detectable in both toe-clips and internal organs.Intact skin microbiota reduced immune activation and can minimize subclinical costs of infection. Tolerance of Bd or ranavirus infection may differ with ecological conditions.Experimental studies are needed in disease ecology to determine the relative importance of factors influencing disease outcome. Results can be applied to conservation management of amphibians encountering a variety of infectious diseases emerging with global change [1-5]. Two predominant amphibian pathogens associated with global population declines are ranaviruses and the amphibian chytrid fungus Batrachochytrium dendrobatidis (Bd).Ranaviruses are large icosahedral DNA viruses belonging to the family Iridoviridae. They have been detected in fish, amphibians and reptiles. Ranavirus associated disease in amphibians has been reported in the Americas, Europe, and Asia, and ranaviruses have also been detected in amphibians in Australia [4,6]. To our knowledge, there are no previous reports of ranavirus occurring in wild-caught amphibians in Switzerland.Bd is the causal agent of chytridiomycosis, an amphibian disease capable of producing epizootics and perhaps species extinctions [3,7]. The fungu