Deadly toad fungus blamed on captive breeding programme

29 September 2008

Careful detective work by a group of researchers has shown how a programme to re-introduce a rare species of toad to Mallorca may have accidentally introduced a deadly fungus to the island.

Mallorcan midwife toad

Mallorcan midwife toad

In the 1990s, conservationists instigated a captive breeding programme for Mallorcan midwife toads on Spain's largest island. The unusual toads - the males look after the eggs - were thought to have gone extinct thousands of years ago, existing only in fossil records. In 1978, a few small populations were discovered in pools at the bottom of limestone gorges in the arid landscape in the north of the island.

But in 2005, after conservationists had successfully re-introduced the midwife toads, a group of NERC-funded scientists started investigating toad deaths on the island. They screened the whole island, catching tadpoles and taking mouth swabs.

This screening programme showed that fungal infection was clustered around two adjacent gorges on the north of the island. When scientists analysed the DNA of the fungus they found all samples were identical, suggesting a single source of the infection.

Dr Mat Fisher from Imperial College London and international colleagues checked the captive breeding records and found an unusual spike in toad deaths around 1991. When they re-examined five toad carcasses preserved by the conservationists from that time, they found fungal DNA in three toads.

Sampling the toad

Sampling a Mallorcan midwife toad for the fungus

Going back to the breeding records again, they found that the red-listed toads had been housed in the same room as an endangered species of frog from South Africa. The South African species almost always carries the fungus, which travels through water and burrows under skin.

Further DNA analysis confirmed this toad carried the fungus Batrachochytrium dendrobatidis (Bd).

Lethal fungus spreads

According to the paper, published in Current Biology, the Bd fungus, first discovered in 1998, is now recognised as one of the principal drivers of amphibian extinction. Health screenings regularly check for the fungus, but in the early 1990s, the deadly nature of the fungus was unknown.

The researchers were surprised to find that the Mallorcan toads appear to be doing well in three out of the four infected populations. This may mean there are unidentified factors protecting these populations from extinction.

Fisher said, "There are around 19 populations on the island. This translates to around 3000 adults." Over half of these come from reintroductions.

"We are working on the long term prognosis of the infected populations," he added.

Worldwide, amphibians are facing 'a mass extinction crisis' according to the researcher and international colleagues in the paper. The disease has spread to over 87 countries and has driven rapid amphibian declines in areas including Australia and Central America, pushing some species to extinction.

"We think that Mallorca is not going to see the sorts of declines seen in other parts of the world. We are working on why this is, and the answer is likely to be very interesting," said Fisher.

The paper is part of a larger NERC-funded project to work out the environmental, ecological and genetic drivers to explain why this fungus is emerging in amphibians.

Fisher says, "We hope to publish a large proportion of our data on the emergence of this pathogen across Europe in the coming months."

"We are also developing the Global Bd Mapping project with our colleagues in the United States"

The fungus is rare in the UK and has been detected in just three locations.