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The sly maneuvers of the fungus fatal to frogs

A cross-banded treefrog (Smilisca sila) of Panama. This species has been affected by the pathogenic chytrid fungus, Batrachochytrium dendrobatidis.
[Photo courtesy of Louise Rollins-Smith]

Like subsurface ninjas, the cells of a newly discovered fungus are slipping into the skins of frogs worldwide, killing them, and now a new study in the journal Science study hints at how.

In 1998, a new species of chytrid fungus called Batrachochytrium dendrobatidis was identified. In recent decades, it has contributed to rendering dozens of frog species extinct, researchers think.

They know the fungus inserts itself into the skin of frogs, drying out a layer the animals require to be wet in order to breathe, but just how the fungus renders its final fatal jab hasn't been clear.

Vanderbilt University's J. Scott Fites, together with his colleagues, suggested that B. dendrobatidis followed in the footsteps of other harmful fungi, blocking its host's immune system. To figure out which of the fungus's molecules could perform this trick, the researchers mixed fungal cells with cells from the amphibian immune system.

They used two kinds of immune cells -- those involved in amphibians' first and second lines of defense, or their "innate" and "adaptive" immune responses, respectively. Both types of immune response are needed to eliminate fungal infection.

The fungus had no effect on cells involved in the innate immune response, the researchers found, but it greatly affected the adaptive immune response, which is carried out by cells called lymphocytes. Indeed, fungus prevented lymphocytes from growing and multiplying.

The scientists conducted more experiments, this time in lymphocytes of mammals (specifically, humans and mice), finding the same result; only the second type of immune response was negatively affected.

Because lymphocyte growth was only blocked when the researchers used mature fungal cells, and not young zoospores (which lack a cell wall), Fites and colleagues propose that whatever fungal molecule is causing deadly damage to frogs is in the fungus cell wall. Further studies will be needed to pinpoint it.

This study, which shows how B. dendrobatidisescapes the amphibian immune system, explains why it has been so devastating to amphibian populations; even frogs with a strong innate immune response can't escape it.