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Frog trade link to killer fungus revealed

9 November 2011

The global trade in frogs, toads and other amphibians may have accidentally helped create and spread the deadly fungal disease chytridiomycosis, which has devastated amphibian populations worldwide.

What's more, researchers say that unless the trade is regulated, even deadlier strains of the disease may soon emerge.

An international team of scientists, led by Dr Matthew Fisher from Imperial College London, found that the trade may have let non-lethal strains of the chytrid fungus from different parts of the world come into contact with each other.

This means they've exchanged genes in a process called recombination, creating a new and lethal strain which has decimated frog populations around the world in recent years.

"It's likely that the amphibian trade has allowed different populations of the fungus to come into contact with each other, allowing recombination to occur," said Rhys Farrer from Imperial College London and ZSL's Institute of Zoology, lead author of the study published in Proceedings of the National Academy of Sciences.

"This has created a hypervirulent strain leading to losses in amphibian biodiversity."

The chytrid fungus, or Batrachochytrium dendrobatidis (Bd) as it is sometimes called, infects the skins of amphibians like frogs, toads, salamanders and newts.

Declines in many amphibian populations around the world are due to the disease and over 200 species are suspected to have become extinct as a result. In Central America alone, chytridiomycosis has led to the loss of up to 40 per cent of wild amphibians, including the Panamanian Golden Frog.

Despite much research on the disease, scientists have struggled to figure out where it came from or explain how it spread. The problem is even more puzzling because some amphibians coexist alongside Bd with no sign of disease.

"This strongly suggested there may be more than one type of strain of chytrid fungus," said Farrer.

Farrer and his colleagues decided to sequence and compare Bd genomes from 20 disease samples isolated from 11 amphibian species worldwide to find out more about the fungus' ancestry.

They found three different strains. One of these, the Global Panzootic Lineage (GPL), has made its way to at least five continents and has caused infections in North America, Central America, the Caribbean, Australia and Europe.

The researchers found evidence of gene exchange in this lineage, which turned out to be the deadliest of the three strains.

In one example, a captive breeding and reintroduction programme to boost numbers of the endangered Mallorcan midwife toad may have helped Bd spread from captive African clawed frogs to the toads.

The fact that they found three strains in just 20 samples also suggests Bd is much more diverse than previously thought.

"What's interesting is that they're not all causing disease," said Fisher. "Only one lineage is a killer, and it has evolved very recently." Scientists had until now thought there was just one strain of Bd.

Farrer, Fisher and their colleagues also found that the start of the decline of amphibians around the 1970s coincides with the emergence of the amphibian trade.

"The age of the lethal BdGPL lineage coincides with the start of the amphibian trade in the 20th century, when we started moving many frogs and toads around the world," says Fisher.

"The horse has well and truly bolted, but to halt the further spread of this disease, we really need to increase global biosecurity," said Fisher.

The study was funded by the Natural Environment Research Council, the UK Department for Environment, Food & Rural Affairs, the Biotechnology & Biological Sciences Research Council and the European Research Council.

Further information

NERC Press Office
Natural Environment Research Council
Polaris House, North Star Avenue
Swindon, SN2 1EU
Tel: 01793 411561
Mob: 07917 557215

Dr Matthew Fisher
Imperial College London
Reader in Fungal Disease Epidemiology
Tel: 020 7594 3787
Mob: 07810 423915

Rhys Farrer
Imperial College London and ZSL's Institute of Zoology
Mob: 07853 246906

Notes

1. The paper, 'Multiple emergences of genetically diverse amphibian-infecting chytrids include a globalised hypervirulent recombinant lineage' was published in Proceedings of the National Academy of Sciences on 7 November 2011. Please contact the NERC Press Office for a copy of the paper.

2. The Natural Environment Research Council (NERC) is the UK's main agency for funding and managing world-class research, training and knowledge exchange in the environmental sciences. It coordinates some of the world's most exciting research projects, tackling major issues such as climate change, environmental influences on human health, the genetic make-up of life on Earth, and much more. NERC receives almost £400m a year from the government's science budget, which it uses to fund independent research and training in universities and its own research centres.

3. Consistently rated amongst the world's best universities, Imperial College London is a science-based institution with a reputation for excellence in teaching and research that attracts 14,000 students and 6,000 staff of the highest international quality. Innovative research at the College explores the interface between science, medicine, engineering and business, delivering practical solutions that improve quality of life and the environment - underpinned by a dynamic enterprise culture.

Since its foundation in 1907, Imperial's contributions to society have included the discovery of penicillin, the development of holography and the foundations of fibre optics. This commitment to the application of research for the benefit of all continues today, with current focuses including interdisciplinary collaborations to improve global health, tackle climate change, develop sustainable sources of energy and address security challenges.

In 2007, Imperial College London and Imperial College Healthcare NHS Trust formed the UK's first Academic Health Science Centre. This unique partnership aims to improve the quality of life of patients and populations by taking new discoveries and translating them into new therapies as quickly as possible.

4. Founded in 1826, the Zoological Society of London (ZSL) is an international scientific, conservation and educational charity, whose key role is the conservation of animals and their habitats. The Society runs ZSL London Zoo and ZSL Whipsnade Zoo, carries out scientific research in the Institute of Zoology and is actively involved in field conservation in over 50 countries worldwide.

Press release: 28/11

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