Page last updated at 04:55 GMT, Monday, 1 September 2008 05:55 UK

Q&A: The frog-killer fungus

Dead rainforest frogs
Amphibians have been dying out around the globe

Amphibians around the world have suffered a dramatic decline in numbers over the last few decades.

Scientists now estimate that nearly one-third of all species are threatened by extinction and populations are falling rapidly.

A deadly fungus has been held up as one of the prime suspects for the crash in numbers - but where did it come from, how did it take hold and what can be done about it?


Batrachochytrium dendrobatidis was only discovered in 1998.

Harlequin toad
The harlequin toad has been badly hit by the chytrid fungus

Two teams of researchers investigating the huge number of frog deaths in Central America and Australia isolated the fungus and discovered that it could have a deadly effect if frogs became infected with it.

It belongs to a family of fungi known as the chytrids (Batrachochytrium dendrobatidis is also commonly referred to as the chytrid fungus).

Chytrid species are extremely widespread - they are found in the soil and play a key role in the ecosystem, breaking down matter so its nutrients can be released back into the earth.

Yellow-eyed leaf frog

Batrachochytrium dendrobatidis is the first chytrid that has been shown to be deadly to vertebrates.

In many amphibians, the fungus can cause a disease called chytridiomycosis.

It infects the animals' skin, causing skin lesions, effectively suffocating them.

It takes between one to two weeks for the frogs to die.

Not all amphibians develop the disease.

Some species can carry and spread the fungus without developing any symptoms.


Scientists are not sure where the fungus came from, and are currently investigating two possibilities.

Jon Bielby, an amphibian expert from the Zoological Society of London, explains: "One hypothesis is that is was that was already all over the place - and it could be that something suddenly changed, like the weather, causing amphibians to become more vulnerable to the fungus or the fungus to become more pathogenic."

This is called the "endemic pathogen hypothesis".

The other is described as the "na´ve pathogen hypothesis".

Dr Bielbly said: "This is the idea that it came from one area and then spread all around the world - and because it is being introduced to populations that have never seen the fungus before - na´ve populations - it has a serious effect.

"The evidence at the moment seems to favour this slightly - the first records of the fungus go back to Southern Africa in the 1920s."

Around this time, he said, an aquatic amphibian called the xenopus toad, that was native to the area, was being exported all over the world for use in pregnancy tests - certain hormones only found in pregnant females make the amphibian spawn.

This toad is able to carry - and spread - the fungus without suffering any ill effects.

Dr Bielby added: "This was a possible way that it spread from South Africa."


The fungus is now found all over the world.

The Batrachochytrium dendrobatidis Mapping Project says that it has been so far been detected in 38 countries and in more than 250 amphibian species


Batrachochytrium dendrobatidis can be killed off very easily using fungicides.

American bullfrog
The American bullfrog seems to be able to spread the fungus

While this is good news for captive amphibians, treating populations with this method in the wild would be impossible, so scientists are now looking at ways to stem the spread.

There are calls for more stringent rules on importing and exporting amphibians, which may soon be put into place.

Some countries, such as the UK, are also trying to eradicate invasive amphibians such as the North American bullfrog, which is able to carry the fungus and spread it to other more susceptible animals.

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