Scientists have unravelled¹ the mechanism² by which the fungal disease chytridiomycosis kills its victims.

科学家已经解开真菌病如何伤害油彩蛙的机理。

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A world of amphibians

The fungus⁵（真菌，霉菌） is steadily⁶ spreading through populations of frogs and other amphibians（两栖类） worldwide, and has sent some species extinct in just a few years.

Researchers now report in the journal Science that the fungus kills by changing the animals' electrolyte balance（电解质平衡）, resulting in cardiac arrest（心搏停止）.

The finding is described as a "key step" in understanding the epidemic⁷（传染病，流行病）.

Karen Lips, one of the world authorities on the spread of chytridiomycosis（油彩蛙）, said the research was "compelling（令人注目的，强制的）".

"They've done an incredible amount of work, been very thorough, and I don't think anybody will have problems with this.

"We suspected something like this all along, but it's great to know this is in fact what is happening," the University of Maryland professor told BBC News.

Skin deep

Amphibian³ skin plays several roles in the animals' life.

Most species can breathe through it, and it is also used as a membrane⁸（薄膜，羊皮纸） through which electrolytes such as sodium⁹（钠） and potassium（钾） are exchanged with the outside world.

The mainly Australian research group took skin samples from healthy and diseased green tree frogs, and found that these compounds passed through the skin much less readily when chytrid（壶菌） was present.

Samples of blood and urine from infected frogs showed much lower sodium and potassium concentrations than in healthy animals - potassium was down by half.

In other animals including humans, this kind of disturbance¹⁰ is known to be capable of causing cardiac arrest.

The scientists also took electrocardiogram recordings¹¹ of the frogs' hearts in the hours before death; and found changes to the rhythm culminating（终极的，最后的） in arrest.

Drugs that restore electrolyte balance brought the animals a few hours or days of better health, some showing enough vigour¹²（精力） to climb out of their bowls of water; but all died in the end.

Grail quest

Lead scientist Jamie Voyles, from James Cook University in Townsville, said the next step was to look for the same phenomenon in other species.

"This is lethal¹³ across a broad range of hosts, whether terrestrial（地球的，地上的） or aquatic¹⁴（水生的，水中的）, so it's really important to look at what's happening in other susceptible¹⁵（易受影响的） amphibians," she said.

Another step will be to examine how the chytrid fungus (Batrachochytrium dendrobatidis - Bd) impairs¹⁶ electrolyte transfer.

"What this work doesn't tell us is the mechanism by which chytrid causes this problem with sodium," said Matthew Fisher from Imperial College London.

"It could be that Bd is excreting a toxin¹⁷, or it could be causing cell damage. This causative action is actually the 'holy grail' - so that's another obvious next step."

The finding is unlikely to plot an immediate¹⁸ route to ways of preventing or treating or curing the disease in the wild.

Curing infected amphibians in captivity¹⁹ is straightforward²⁰ using antifungal（抗真菌的） chemicals; but currently there is no way to tackle it outside.

Various research teams are exploring the potential of bacteria that occur naturally on the skin of some amphibians, and may play a protective role.

Understanding the genetics of how Bd disrupts electrolyte balance might lead to more precise identification of protective bacteria, suggested Professor Lips, and so eventually play a role in curbing²¹ the epidemic.