When Defenses Fail: Atelopus zeteki Skin Secretions Increase Growth of the Pathogen Batrachochytrium dendrobatidis

Abstract

Synopsis To combat the threat of emerging infectious diseases in wildlife, ecoimmunologists seek to understand the complex interactions among pathogens, their hosts, and their shared environments. The cutaneous fungal pathogen Batrachochytrium dendrobatidis (Bd), has led to the decline of innumerable amphibian species, including the Panamanian golden frog (Atelopus zeteki). Given that Bd can evade or dampen the acquired immune responses of some amphibians, nonspecific immune defenses are thought to be especially important for amphibian defenses against Bd. In particular, skin secretions constitute a vital component of amphibian innate immunity against skin infections, but their role in protecting A. zeteki from Bd is unknown. We investigated the importance of this innate immune component by reducing the skin secretions from A. zeteki and evaluating their effectiveness against Bd in vitro and in vivo. Following exposure to Bd in a controlled inoculation experiment, we compared key disease characteristics (e.g., changes in body condition, prevalence, pathogen loads, and survival) among groups of frogs that had their skin secretions reduced and control frogs that maintained their skin secretions. Surprisingly, we found that the skin secretions collected from A. zeteki increased Bd growth in vitro. This finding was further supported by infection and survival patterns in the in vivo experiment where frogs with reduced skin secretions tended to have lower pathogen loads and survive longer compared to frogs that maintained their secretions. These results suggest that the skin secretions of A. zeteki are not only ineffective at inhibiting Bd but may enhance Bd growth, possibly leading to greater severity of disease and higher mortality in this highly vulnerable species. These results differ from those of previous studies in other amphibian host species that suggest that skin secretions are a key defense in protecting amphibians from developing severe chytridiomycosis. Therefore, we suggest that the importance of immune components cannot be generalized across all amphibian species or over time. Moreover, the finding that skin secretions may be enhancing Bd growth emphasizes the importance of investigating these immune components in detail, especially for species that are a conservation priority.

Evolutionary ecology of host competence after a chytrid outbreak in a naive amphibian community

“…Immune resistance in frogs is achieved by mounting T-cell mediated responses to infection [48][49][50], by innate factors such as antimicrobial peptides [51] or combinations of these strategies. However, the efficacy of resistance to chytridiomycosis varies among species [52], depending on their ability to escape the immunosuppressing activity of the fungus [48]. The evolution of resistance effectively decreases pathogen fitness, potentially leading to antagonistic coevolution [53,54].…”

Towards the generation of gnotobiotic larvae as a tool to investigate the influence of the microbiome on the development of the amphibian immune system

“…On week five of tadpole development, we humanely euthanized N = 6 (Experiment 1) and N = 10 (Experiment 2) tadpoles per treatment group for 16S rRNA targeted amplicon microbiome sequencing analysis. We randomly selected tadpoles from each tank and weighed tadpoles to the nearest 0.1 g and measured snout-to-vent length (hereafter, SVL) to the nearest 0.1 mm to calculate body condition (mass/SVL; [ 41 ]). We euthanized the tadpoles using sterile MS-222 that was neutralized by adding sodium bicarbonate to pH 7.0 [ 42 ].…”

“…Xenopus laevis is regarded as resistant to chytridiomycosis [ 43 , 44 ], which made it an ideal species to determine if development in germ-free conditions altered susceptibility to infection, disease development, and mortality. As an additional advantage, we opportunistically conducted this exposure experiment at the same time as one with an additional host species, Atelopus zeteki , which is known to be highly susceptible to lethal chytridiomycosis [ 41 , 45 ]. Since we used the same Bd isolate, the groups of A. zeteki frogs could serve as a positive control for Bd pathogenicity and disease development (see electronic supplementary material).…”

“…We cleaned tanks, fed frogs aquatic frog pellets three times per week, and added fresh water twice per week. At the start of the inoculation experiment, we weighed all frogs to the nearest 0.1 g and measured SVL to the nearest 0.1 mm, which we used to calculate body condition (mass/SVL; [ 41 ]). We collected skin swab samples to test for Bd presence and infection intensity using standardized swabbing techniques [ 46 ].…”

Release trial of captive-bred variable harlequin frogs Atelopus varius shows that frogs disperse rapidly, are difficult to recapture and do not readily regain skin toxicity

“…In addition, some persisting populations have evolved anti-Bd skin secretions that could explain their persistence (Voyles et al, 2018), and recent migration and gene flow are evident in these populations, raising the potential of genetic rescue through outbreeding (Byrne et al, 2021). However, it was recently demonstrated that skin secretions of the Panamanian golden frog Atelopus zeteki could increase susceptibility to chytridiomycosis (Gass & Voyles, 2022), and thus understanding specific components of skin defence is required (reviewed in Woodhams et al, 2023).…”