The so-called "out of Africa" theory held that African clawed frogs which had escaped into the wild had infected vast numbers of native amphibians, in UK and around the world, with a deadly chytrid fungus known as Bd, causing them to die off.
Lead researcher Professor Matt Guille, at the University of Portsmouth's Institute of Biomedical and Biomolecular Science, said: "Amphibians around the world started dying off for a number of reasons but the chytrid fungus is a major contributor to population declines. Many studies suggest that chytrid fungus is responsible for up to 200 frog species extinctions worldwide since the 1970s.
"Finding out what caused this spread of the fungus is really important to minimise the risk to our native species."
The research was carried out with colleagues at the University of Bristol and published in the journal Biological Conservation. It was funded by the Biotechnology and Biological Sciences Research Council (BBSRC), the Wellcome Trust and the Natural Environment Research Council (NERC).
African clawed frogs were, for decades, exported worldwide for use in pregnancy testing and have formed wild colonies across the world.
The so-called 'Frog Test' was the world's first cheap and reliable pregnancy test and became the international standard for many years until, in the late 1950s, technology advanced. The test was done by injecting a sample of the woman's urine into a female African clawed frog. If the frog produced eggs within 24 hours, the test was positive. The frog was not killed and could be re-used.
The African clawed frog is now used as one of the major species for studying human disease – much of what scientists have learnt about cancer and stem cells comes from their study.
DEFRA and the University of Portsmouth designed strict protocols adopted by UK research institutions to prevent the escape of any African clawed frogs, reducing the risk to native amphibians.
Professor Guille said: "There were two colonies of African clawed frog living wild in the UK, in Wales and Lincolnshire. We do not know where they came from, but our study found that both colonies were highly infected with chytrid. This gave us the opportunity to see if the fungus has spread to resident native amphibians living with the infected colonies."
Scientists tested a sample of 174 toads, newts and frogs sharing ponds with the African clawed frogs for signs of infection. Surprisingly, all of the animals tested negative apart from one toad, which was in a control area well away from any of the infected African clawed frog colonies.
"It is a surprise. Most people, including ourselves, would have expected the African clawed frogs with high levels of chytrid to have infected the native amphibians. It certainly doesn't support the 'out of Africa' theory in the conditions found in the UK," Professor Guille said.
"We are unsure what prevented the spread of infection to native amphibians, however it is an important question since what limits the spread in the UK may help in areas where spread is occurring rapidly.
"We still need to understand what controls the spread, but this research tells us that we do not need to be overly concerned that British species could be infected from African clawed frog escape. Sadly, that doesn't mean they won't get it from somewhere else."
Paper: Chytrid fungus infections - risks for UK native amphibians
Professor Matt Guille, University of Portsmouth
Chytrid fungus - Amphibian Ark
African clawed frog - Wikipedia