Frogs and fungal skin infections
23 October 2009
Researchers from the Discipline of Physiology of the Sydney Medical School have joined forces with scientists at James Cook University to solve the mystery of how a fungal skin infection that has been causing mass extinctions in frog populations causes its hosts' demise.
The team have published a paper about their finding in the latest edition of the prestigious journal Science, published online today here.
Chytridiomycosis or chytrid, is a fungal skin infection affecting amphibians that is unique because it causes death and is so virulent that it leads to the extinction of whole species of frogs. It has been implicated in more than 90 of the 120 amphibian extinctions since 1980 but despite its savage advance through all the continents on which frogs live, it took researchers long time to work out that the infection was what caused such population devastation.
"The greatest mystery of the disease was that the frog appeared normal after death. There was a small patch of fungus on the belly but all the organs were unaffected," said Associate Professor Anuwat Dinudom, who worked on the project and is one of the authors of the paper.
Chytrid was finally identified by Professor Rick Speare'slab at James Cook University about 11 years ago as the cause mass amphibian extinction but how it led to death still remained a mystery. Lee Berger, a researcher who worked on both projects and co-authored the paper, earlier this year told National Geographic, "The impact of chytridiomycosis on frogs is the most spectacular loss of vertebrate biodiversity due to disease in recorded history."
Then a PhD student from Professor Speare's lab, Jamie Voyles, who is the lead author of the paper, noted that the sodium content in the infected frog's plasma dropped in the late stage of the disease. She also found that infected frogs lived for a while longer with sodium supplementation.
Looking to verify whether sodium was a factor in the frogs' death, researchers at James Cook University turned to the expertise of Professor David Cook's Exocrine Physiology and Biophysics lab in the Medical Foundation Building.
"Frogs act as important first indicators of pollution in the environment, especially the waterways, because their skin acts as lungs and kidneys," said Craig Campbell, a PhD student and one of the authors of the paper. "Our lab's skill and technical experience lie in looking at how kidneys, lungs, sweat and salivary glands transport water and electrolytes, especially regulation of epithelial sodium channels."
The researchers at Professor Cook's lab measured sodium transport in the skin of infected frogs and found near-terminal ones had almost completely inhibited sodium absorption. They showed that a lack of sodium and electrolytes was the mechanism leading to death through cardiac arrest.
"For us, it was more important to work out how the fungus causes the inhibition of the sodium channels that leads to the infected frog's demise," Associate Professor Anuwat Dinudom noted. "Working that out may lead to understanding of how to combat illnesses caused by hyperactivity of these sodium channels, such as cystic fibrosis and hypertension, which are what we usually study."
"The exciting thing about this project was being able to apply what I've learnt from studying epithelial sodium channels as part of my doctorate to a real pathophysiological problem and see just how critical sodium channels are," Craig Campbell said. "I wish we could say that we've cured the disease or identified an effective treatment, but understanding the mechanism that causes death is hopefully part of the way to doing that."
Contact: Reema Rattan
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