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Secrets of frog killer laid bare

人氣2413

By Richard Black
Environment correspondent, BBC News website

Scientists have unravelled the mechanism by which the fungal disease chytridiomycosis kills its victims.

The fungus is steadily spreading through populations of frogs and other amphibians 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, resulting in cardiac arrest.

The finding is described as a "key step" in understanding the epidemic.

Karen Lips, one of the world authorities on the spread of chytridiomycosis, said the research was "compelling".

"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.

 Signs of infection. Researchers found that chytrid infection on the frogs' undersides became fatal above a certain "load" of spores - backing findings from other research groups.

Skin deep

Amphibian skin plays several roles in the animals' life.

Most species can breathe through it, and it is also used as a membrane through which electrolytes such as sodium and potassium 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 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 disturbance is known to be capable of causing cardiac arrest.

The scientists also took electrocardiogram recordings of the frogs' hearts in the hours before death; and found changes to the rhythm culminating in arrest.

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

Hundreds of amphibian species will become extinct unless a global action plan is put into practice very soon, conservationists warn. (Image: Glass tree frog - R.D.Holt)
Campaigners are forming an Amphibian Survival Alliance to carry through a rescue strategy. (Image: Centrolene tayrona - Fundacion Pro aves)
There are about 6,000 known amphibians, a category that includes frogs, toads, salamanders and caecilians (legless amphibians). (Image: Albericus siegfriedi - S.Richards)
More than a third of all amphibian species are said to be in peril. (Image: Golden mantella - R.A.Mittermeier)
The two main threats are habitat loss and the fungal disease chytridiomycosis. Climate change, hunting and pollution are other issues of concern. (Image: Sphenophryne cornuta - S.Richards)
In particular, the fungus Batrachochytrium dendrobatidis, first identified 1998, is doing widespread damage in parts of the Americas, Australia and Europe. (Image: Nimbaphrynoides occidentalis - P.Naskrecki)
The new Amphibian Survival Alliance will co-ordinate conservation efforts with a particular interest in field treatments for the chytrid fungus. (Image: Dendrobates azureus - R.A.Mittermeier)
"We simply cannot afford to let this current amphibian extinction crisis go unchecked," said Simon Stuart, chair of IUCN's Species Survival Commission. (Image: Centrolene tayrona - Fundacion Pro aves)

 

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 lethal across a broad range of hosts, whether terrestrial or aquatic, so it's really important to look at what's happening in other susceptible amphibians," she said.

Another step will be to examine how the chytrid fungus (Batrachochytrium dendrobatidis - Bd) impairs 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 toxin, 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 immediate route to ways of preventing or treating or curing the disease in the wild.

Curing infected amphibians in captivity is straightforward using antifungal 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 curbing the epidemic.

 Ways to stop chytrid in the wild are the "holy grail" for researchers

Richard.Black-INTERNET@bbc.co.uk

http://news.bbc.co.uk/2/hi/science/nature/8319467.stm

 

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