Effectiveness of antifungal treatments during chytridiomycosis epizootics in populations of an endangered frog

• Published in: PeerJ (San Francisco, CA) Vol. 10; p. e12712
• Main Authors: Knapp, Roland A, Joseph, Maxwell B, Smith, Thomas C, Hegeman, Ericka E, Vredenburg, Vance T, Erdman, Jr, James E, Boiano, Daniel M, Jani, Andrea J, Briggs, Cheryl J
• Format: Journal Article
• Language: English
• Published: United States PeerJ. Ltd 05-01-2022; PeerJ, Inc; PeerJ Inc
• Subjects: Adaptive immunity; Amphibian chytrid fungus; Amphibians; Animals; Antifungal agents; Antifungal Agents - pharmacology; Antifungal treatment; Anura - microbiology; Bacteria; Batrachochytrium; Batrachochytrium dendrobatidis; Care and treatment; Chytridiomycosis; Chytridiomycota; Conservation Biology; Disease; Ecology; Endangered & extinct species; Endangered populations; Epizootic; Epizootics; Experiments; Extinction; Freshwater Biology; Frogs; Host population decline; Immune response; Infection; Infection control; Infections; Itraconazole; Itraconazole - pharmacology; Microbiomes; Mycology; Mycoses - drug therapy; Pathogens; Population; Population decline; Ranidae; Recruitment; Reptiles & amphibians; Survival; Wildlife disease; Zoology; United States > US; Sierra Nevada Mountain Range; California; Epizootic; Wildlife disease; Host population decline; Amphibian chytrid fungus; Antifungal treatment; Batrachochytrium dendrobatidis
• ISSN: 2167-8359; 2167-8359
• DOI: 10.7717/peerj.12712

The recently-emerged amphibian chytrid fungus
(Bd) has had an unprecedented impact on global amphibian populations, and highlights the urgent need to develop effective mitigation strategies. We conducted
antifungal treatment experiments in wild populations of the endangered mountain yellow-legged frog during or immediately after Bd-caused mass die-off events. The objective of treatments was to reduce Bd infection intensity ("load") and in doing so alter frog-Bd dynamics and increase the probability of frog population persistence despite ongoing Bd infection. Experiments included treatment of early life stages (tadpoles and subadults) with the antifungal drug itraconazole, treatment of adults with itraconazole, and augmentation of the skin microbiome of subadults with
, a commensal bacterium with antifungal properties. All itraconazole treatments caused immediate reductions in Bd load, and produced longer-term effects that differed between life stages. In experiments focused on early life stages, Bd load was reduced in the 2 months immediately following treatment and was associated with increased survival of subadults. However, Bd load and frog survival returned to pre-treatment levels in less than 1 year, and treatment had no effect on population persistence. In adults, treatment reduced Bd load and increased frog survival over the entire 3-year post-treatment period, consistent with frogs having developed an effective adaptive immune response against Bd. Despite this protracted period of reduced impacts of Bd on adults, recruitment into the adult population was limited and the population eventually declined to near-extirpation. In the microbiome augmentation experiment, exposure of subadults to a solution of
increased concentrations of this potentially protective bacterium on frogs. However, concentrations declined to baseline levels within 1 month and did not have a protective effect against Bd infection. Collectively, these results indicate that our mitigation efforts were ineffective in causing long-term changes in frog-Bd dynamics and increasing population persistence, due largely to the inability of early life stages to mount an effective immune response against Bd. This results in repeated recruitment failure and a low probability of population persistence in the face of ongoing Bd infection.