By Paul Rincon
BBC News science reporter
A fungus that is deadly to many species of amphibians has been found in wild animals in the UK for the first time.
The bullfrogs may have been introduced as pets (Image: Stephen Price)
Chytrid fungus is a major contributor to the decline of amphibian populations around the world and may have already made one species extinct.
Its presence was detected in American bullfrogs that had set up home in two small lakes in South-East England.
Scientists are now trying to establish whether the fungal disease has spread to native amphibian species.
The fungus, Batrachochytrium dendrobatidis, has been found in captive animals in this country before, but never in the wild.
It was identified by Dr Andrew Cunningham of the Zoological Society of London (ZSL) and colleagues, and reported in the Veterinary Record.
The American bullfrog (Rana catesbeiana) is native to the central and eastern US and parts of Canada. The UK colony was probably derived from animals kept as pets that escaped or were released.
"If it does get into British species, it's going to be very difficult to get rid of," Dr Cunningham told the BBC News website.
"This disease is a major cause of amphibian population declines and extinctions worldwide. So it's pretty bad news that it has been found in the wild in this country."
English Nature has been eradicating the invasive frogs, which numbered more than 11,000, and had sent specimens to ZSL for testing.
The fungus was identified six years ago and is firmly established in parts of the Americas, Australia and Europe. The disease it causes, chytridiomycosis, appears to kill amphibians by damaging their sensitive skins, blocking the passage of air and moisture.
"I think there is great concern," said Dr Richard Griffiths, an amphibian specialist at the University of Kent in Canterbury.
"It will take more work to see if infected animals can be taken out of the wild, cleaned up and released. At the moment, people are concentrating on keeping [the disease] out."
American bullfrogs carry the fungus but do not develop the disease. It is not known how native amphibian species respond to it.
"We know that in Spain, the common toad - Bufo bufo - does get killed by this. The species also lives in the UK, so it would potentially kill the toad in this country," said Dr Cunningham.
Dr Cunningham said vigilance for the disease should be maintained throughout the country.
"The fungus has turned up in many captive collections of exotic amphibians. And it can spread by motile zoospores in water," he explained.
"So someone washing out their vivarium and pouring the water into the garden could inadvertently bring native species into contact with it."
The bullfrog colony, at an undisclosed site on the border of Kent and East Sussex, has been known about for some time.
There was a significant trade in the animals during the 1980s and 1990s, when they were sought after as pets. The European Commission banned their import in 1997 due to fears that the frogs, which have voracious appetites and are not choosy eaters, would destroy native wildlife.