The frog-killing fungus Batrachochytrium dendrobatidis (Bd), which causes the disease chytridiomycosis, has been blamed for about 100 amphibian extinctions around the globe since it was first observed in 1998, but clear information on exactly how it spreads has remained a mystery.

Now a team of scientists working in Belgium have come up with one potential clue: the chytrid fungus may sometimes be carried to new habitats on the toes of waterfowl such as geese.

According to research published April 13 in the journal PLoS One, geese are "potential environmental reservoirs" for the Bd fungus, because waterfowl and amphibians often co-occur in the same habitats. The team studied 497 wild geese—which had been rounded up from six wildlife areas in East Flanders as part of an invasive species eradication program—and found that the keratinous toe scales of 76 of the birds tested positive for Bd. The fungus was present on both species of geese that they tested: invasive Canada geese (Branta Canadensis) and domesticated geese (Anser anser domesticus) that had been living in the wild.

The researchers then took some toe scales, heated them in an autoclave, and exposed them to Bd zoospores, which over the course of the in vitro experiment both adhered to and proliferated on the toe cells. After four days, the fungus developed discharge tubes and released new zoospores, showing that the geese's toe scales provided not just a site for the fungus to reside but also to reproduce.

The team also incubated Bd zoospores on the surface of toe tissue from a dead whooper swan (Cygnus cygnus) and two dead Moscovy ducks (Cairina moschata) that had been brought to their facility for postmortem examination, with similar results.

Whereas Bd thrives in moist environments, the paper points out that it can survive a drying period of up to 30 minutes, during which time a goose could fly up to 30 kilometers—more than enough time for a bird to move from one pond to another.

The researchers did add two caveats: first, because the in vitro experiment used scales from destroyed birds that had been heated in an autoclave, the tissue may have been altered in the process, although they saw no specific macroscopic evidence of such. Second, they note that direct contact between amphibians and geese "might be rather limited."

Although this research does not prove avian-to-amphibian fungal transmission, it appears to be an important step into understanding the possible transmission paths of this deadly fungus.

Read my previous stories on the chytrid fungus here.

Photo 1: An invasive Canada goose in Belgium by Peter Van den Bossche. Photo 2: A dead frog infected with Bd by Brian Gratwicke. Used under Creative Commons license