Richard Black, BBC News 22 Oct 09;
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.
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.
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.
'Gatorade for frogs' could stymie fungal killer
Bob Holmes, New Scientist 22 Oct 09;
The fungus now decimating frog populations around the world does its damage by impairing the animals' ability to absorb electrolytes through their skin. This discovery may eventually lead to treatments that make the disease less lethal.
Biologists now generally agree that the fungal disease known as chytridiomycosis is responsible for the worldwide die-off of frogs that has caused a conservation crisis in recent years. However, the fungus affects only the outer layers of the skin, leaving few clues to why it is so lethal.
But now Jamie Voyles of James Cook University in Townsville, Australia, and colleagues have an answer. In diseased frogs, the skin's ability to take up sodium and potassium ions from the water decreases by more than 50 per cent, they found. As a result, the concentration of these two ions in the frogs' blood fell by 20 and 50 per cent, respectively. This ion loss – similar to the hyponatraemia that a human athlete might experience from drinking too much water too fast – eventually leads to cardiac arrest and death.
The researchers found they could delay death by giving diseased frogs an oral electrolyte-replacement solution – a sort of froggy Gatorade. Fungal damage to the skin was too extensive for this to prevent death altogether, the study represents a first step toward finding an effective treatment for the disease, Voyles says.
Journal reference: Science, DOI: 10.1126/science.1176765
