HIGHER INFECTION PREVALENCE IN AMPHIBIANS INHABITING HUMAN-MADE COMPARED TO NATURAL WETLANDS

The impacts of water quality on the amphibian chytrid fungal pathogen: A systematic review

The pathogenic fungus Batrachochytrium dendrobatidis has caused declines of amphibians worldwide. Yet our understanding of how water quality influences fungal pathogenicity is limited. Here, we reviewed experimental studies on the effect of water quality on this pathogen to determine which parameters impacted disease dynamics consistently. The strongest evidence for protective effects is salinity which shows strong antifungal properties in hosts at natural levels. Although many fungicides had detrimental effects on the fungal pathogen in vitro, their impact on the host is variable and they can worsen infection outcomes. However, one fungicide, epoxiconazole, reduced disease effects experimentally and likely in the field. While heavy metals are frequently studied, there is weak evidence that they influence infection outcomes. Nitrogen and phosphorous do not appear to impact pathogen growth or infection in the amphibian host. The effects of other chemicals, like pesticides and disinfectants on infection were mostly unclear with mixed results or lacking an in vivo component. Our study shows that water chemistry does impact disease dynamics, but the effects of specific parameters require more investigation. Improving our understanding of how water chemistry influences disease dynamics will help predict the impact of chytridiomycosis, especially in amphibian populations affected by land use changes.

Exposure to a fungal pathogen increases the critical thermal minimum of two frog species

The ability of an organism to tolerate seasonal temperature changes, such as extremely cold temperatures during the winter, can be influenced by their pathogens. We tested how exposure to a virulent fungal pathogen, Batrachochytrium dendrobatidis (Bd), affected the critical thermal minimum (CTmin) of two frog species, Hyla versicolor (gray treefrog) and Lithobates palustris (pickerel frog). The CTmin is the minimum thermal performance point of an organism, which we estimated via righting response trials. For both frog species, we compared the righting response of Bd-exposed and Bd-unexposed individuals in either a constant (15ºC) environment or with decreasing temperatures (-1°C/2.5 min) starting from 15°C. The CTmin for both species was higher for Bd-exposed frogs than unexposed frogs, and the CTmin of H. versicolor was higher than L. palustris. We also found that Bd-exposed frogs of both species righted themselves significantly fewer times in both decreasing and constant temperature trials. Our findings show that pathogen exposure can reduce cold tolerance and limit the thermal performance range of hosts, which may lead to increased overwintering mortality.

Habitat disturbance influences the skin microbiome of a rediscovered neotropical-montane frog

BACKGROUND

The skin microbiome serves as a first line defense against pathogens in vertebrates. In amphibians, it has the potential to protect against the chytrid fungus Batrachochytrium dendrobatis (Bd), a likely agent of amphibian declines. Alteration of the microbiome associated with unfavorable environmental changes produced by anthropogenic activities may make the host more susceptible to pathogens. Some amphibian species that were thought to be "extinct" have been rediscovered years after population declines in the late 1980s probably due to evolved Bd-resistance and are now threatened by anthropogenic land-use changes. Understanding the effects of habitat disturbance on the host skin microbiome is relevant for understanding the health of these species, along with its susceptibility to pathogens such as Bd. Here, we investigate the influence of habitat alteration on the skin bacterial communities as well as specifically the putative Bd-inhibitory bacterial communities of the montane frog Lithobates vibicarius. This species, after years of not being observed, was rediscovered in small populations inhabiting undisturbed and disturbed landscapes, and with continuous presence of Bd.

RESULTS

We found that cutaneous bacterial communities of tadpoles and adults differed between undisturbed and disturbed habitats. The adults from disturbed habitats exhibited greater community dispersion than those from undisturbed habitats. We observed a higher richness of putative Bd-inhibitory bacterial strains in adults from disturbed habitats than in those from undisturbed habitats, as well as a greater number of these potential protective bacteria with a high relative abundance.

CONCLUSIONS

Our findings support the microbial "Anna Karenina principle", in which disturbance is hypothesized to cause greater microbial dispersion in communities, a so-called dysbiosis, which is a response of animal microbiomes to stress factors that decrease the ability of the host or its microbiome to regulate community composition. On the positive side, the high richness and relative abundance of putative Bd-inhibitory bacteria may indicate the development of a defense mechanism that enhances Bd-protection, attributed to a co-occurrence of more than 30-years of host and pathogen in these disturbed habitats. Our results provide important insight into the influence of human-modified landscapes on the skin microbiome and health implications of Bd-survivor species.