2009-10-24

A terrible disease that could drive many frogs to extinction appears to kill by interrupting the flow of nutrients through their normally porous skins, which ultimately causes their hearts to shut down, say scientists.

Until now, it wasn’t known how the disease, called chytridiomycosis, does its damage.

“Understanding the pathogenesis is fundamental to understanding this disease,” said Jamie Voyles, a James Cook University biologist and co-author of the paper published Thursday in Science. “Now we can start to develop treatments for frogs in captivity. If it works out, we could potentially treat frogs where outbreaks are happening. We could perhaps help frogs get through the initial catastrophic declines.”

First identified in 1993, chytridriomycosis — chytrid for short — is caused by Batrachochytrium dendrobatidis, a highly virulent fungus that remains in the environment, even after killing its amphibian hosts. It does this with frightening efficiency: In less than two decades, it has killed about 95 percent of all frogs in Colombia and Panama, and driven some 30 species in the Atelopus genus alone to extinction.

Frogs are especially vulnerable, but chytrid affects most amphibian species, and after being spread by the global trade in African clawed toads, is now found in every continent except Antarctica.

Though it’s not the only threat faced by amphibians, who are also squeezed by climate change, habitat destruction and humanity’s harnessing of approximately one-half of Earth’s fresh water, chytrid is perhaps the worst. Unless something can be done about it, many amphibians, whose evolutionary lineage can be directly traced to a time before birds or mammals or even dinosaurs, will almost certainly vanish.

Until now, it wasn’t clear how chytrid kills.

“It’s a fantastic paper,” said Paul Daszak, a disease ecologist with the Wildlife Trust who was not involved in the research. “It finally clinches the cause of death. This isn’t any old disease; it’s emerging on multiple continents, and is probably the most significant disease we’ve ever seen in wildlife. It’s a breakthrough.”

Chytrid has remained a mystery in part because amphibian researchers receive relatively little funding, but also because its victims’ bodies shut down so completely that it’s difficult to know the precise cause of death.

In earlier research on diseased frogs, Voyles’ team had noticed imbalances of electrolytes, the compounds that conduct electrical charges through cells. In frogs as in humans, a healthy electrolyte balance is needed to keep the heart pumping.

Because amphibians absorb electrolytes through their skin, the researchers suspected that the fungus, which resides on their skin, was to blame.

To study the connection, they first measured electrolyte flow across the skin of infected green tree frogs, and found that it dropped by half as the disease progressed. This produced a drop of 20 percent in blood levels of sodium and 50 percent in potassium, two key electrolytes.

Then the researchers implanted miniaturized cardiac activity recorders inside the chests of another group of frogs. The monitors produced frog versions of electrocardiograms, readouts familiar to people who’ve had their hearts monitored. When the frogs were infected, their cardiac systems malfunctioned in tandem with their falling electrolytes.

“Changes in sodium and potassium basically led to a failure of the electrical system. This is exactly what we see in humans as well. It’s a failure of the electrical system, leading to mechanical failure. If you don’t have a normal electrical system pacing the heart, it won’t pump blood,” said Wyatt Voyles, a University of New Mexico cardiologist and co-author of the study.

Exactly how the fungus interferes with electrolyte transport is unknown. The researchers suspect it’s the result of direct cell damage, or the release of a fungal toxin.

“Helping us understand the cause of death is really a step forward,” said Louise Rollins-Smith, a Vanderbilt University microbiologist who studies amphibian immune systems. She was not involved in the study.

Electrolyte supplements slightly prolonged the life of diseased frogs in the study, and Rollins-Smith said the findings may help scientists develop treatments for chytrid. Though probably not practical at the ecosystem level, such treatments could be used on captive frogs bred to restore dwindling wild populations.

The research could also help researchers understand why some species or sub-species are especially resistant to chytrid, and help to guide conservation efforts.

But Jamie Voyles cautioned that stopping chytrid isn’t enough to save amphibians. “This disease is important, but it’s important to recognize that there are many threats. Unfortunately, there’s no silver bullet.”

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