Establishment of Batrachochytrium dendrobatidis in anuran epidermis and experimental transmission from bullfrogs to wood frogs

Amphibians are a class of animals that includes frogs, toads, salamanders, newts, and caecilians.  With approximately 6,000 known species worldwide, amphibians are a lesson in adaptability to millennia of environmental change.  Scientists have observed an alarming shift, however, since the late 1980s.  Across the planet, amphibian populations are steadily declining.  The causes of these declines include pollution, habitat loss, climate change, and infectious diseases.  The spread of chytridiomycosis, a recently described and highly infectious disease of amphibians caused by the microscopic fungus Batrachochytrium dendrobatidis (Bd for short), is a major driver of global amphibian decline.  Effects of chytridiomycosis on frog species are variable. Some species typically develop minor infections and may function as carriers; others typically develop lethal infections that can lead to population declines. Mild infections in the bullfrog (Lithobates catesbeianus) are well-documented.  In contrast, recently metamorphosed wood frogs (L. sylvaticus) can die from chytridiomycosis.

One component of my research involved a fine-scale investigation of Bd inside frog tissue to find out how it invades amphibian skin cells.  I humanely infected bullfrogs and wood frogs with Bd by exposing them to water containing Bd spores; it is probable that this method of experimental infection mimics how a frog would become infected in the wild.  I then examined the infected tissue with a transmission electron microscope, a tool that is necessary to document biological processes at the sub-cellular level.  Included among my research findings was photographic documentation of a never before seen morphological feature of Bd that is used for invading host cells.  These findings fill a critical gap in knowledge of Bd development and morphology.  I also showed, through a laboratory experiment, that Bd-infected bullfrogs transmitted the fungus to wood frog tadpoles when the two species shared a body of water and therefore could serve as disease vectors in wetland systems.  Given the low infection loads bullfrogs tend to carry, I highlight bullfrog disease screening as a management challenge, especially in light of exotic bullfrog colonies on multiple continents and large-scale global trade in this species.