By Richard Black

Environment Correspondent, BBC News website, Washington DC



The summit aims to produce an action plan (Images: Conservation International/Don Church)

The theory is being debated at a summit in Washington DC where scientists hope to produce an action plan to conserve frogs, toads and salamanders.

In the 1930s, African frogs were exported for use in human pregnancy tests and it is suggested they may have carried a fungal disease with them.

The spread of chytridiomycosis is now a major cause of amphibian decline.

According to the most recent global assessment, almost a third of this animal grouping - a category that includes frogs, toads, salamanders and caecilians (legless amphibians) - is judged to be at risk of extinction.

Out of Africa?

Chytridiomycosis is caused by the fungus Batrachochytrium dendrobatidis, but where the fungus originated and how it spread have not been established.

WHAT ARE AMPHIBIANS? Group includes frogs, toads, salamanders and caecilians First true amphibians evolved about 250m years ago Adapted to many different aquatic and terrestrial habitats Present today on every continent except Antarctica Undergo metamorphosis, from larvae to adults

They examined specimens of a South African clawed frog, Xenopus laevis, in museums in southern Africa. They found evidence of Batrachochytrium in specimens dating back to 1938.

They also showed that the incidence of fungal disease in Xenopus in southern Africa had not changed since 1938.

This suggests that frog and fungus had co-existed for a long period, with Xenopus developing the ability to resist infection.

"The idea makes sense," Dr Peter Daszak, co-chair of the working group on disease at the Washington meeting, told the BBC News website.

"This is the oldest record of the fungus anywhere, so it could be the origin."

Frog tests

In the 1930s and 40s, live female Xenopus frogs were used widely in Europe, Australasia and north America in pregnancy testing.

More than 1,800 amphibian species are now judged to be at risk of extinction

Alternative tests involved male frogs and toads, which produced sperm in response to the human hormone gonadotrophin.

Thousands of Xenopus were exported from Africa each year, potentially carrying Batrachochytrium with them, and - perhaps through occasional escapes - delivering it to the habitats of other continents, where it could inflict major damage on amphibian species that were more vulnerable.

Urgent research

The origin and transmission of Batrachochytrium are key topics at the summit here, which will conclude on Monday with the release of an action plan aimed at stemming the global decline in amphibians, which sees more than 1,800 species facing extinction.

The causes include habitat loss, deforestation, climate change and pollution; but fungal disease is clearly a major and perplexing threat.

"We still don't know how it causes death, and we need to know that," said Dr Daszak, director of the Consortium for Conservation Medicine in the US.

"It may hamper oxygen uptake through their skin, though that's not so certain now.

"Alternatively it may block osmo-regulation - the flow of salts through the skin, which then upsets the salt balance - or it may release a toxin, and there's evidence for that, too."

Understanding how some amphibians become immune to fungal infection could be a starting-point for the development of treatments or even vaccines.

But so urgent is the plight of some species that the action plan issued here is likely to recommend captive breeding as the only short-term option.