Three orange and green frogs perched on a branch

The origins of a deadly disease that's killing amphibians

A deadly fungus has been devastating populations of amphibians all over the world - but no one knew where it began until now.

Chytridiomycosis is a disease capable of infecting many of the world's 8,000 species of amphibians (approximately 7,000 frogs and toads, 700 newts and salamanders, and 200 caecilians).

It has caused the decline and extinction of populations in Australia, Central and South America, the Caribbean, the Sierra Nevada mountain range in the US and the Iberian Peninsula.

The disease attacks an animal's skin, affecting its ability to regulate water and salts. It can also seriously damage the mouthparts of tadpoles.

Chytridiomycosis is caused by a chytrid fungus called Batrachochytrium dendrobatidis (Bd), and has become one of the most destructive panzootics (the animal equivalent of a pandemic) in the world.

New research into the fungus, published in the journal Science, was led by researchers at Imperial College London and brought together an international team involving 38 institutions, including the Museum.

The team traced Bd back to the Korean peninsula and dated its emergence to the early 1900s.

A black and white salamander

Chytridiomycosis affects hundreds of species all over the world. It can infect all three living lineages of amphibian: frogs, caecilians and salamanders (pictured above). Image: Shutterstock.com.

 

The researchers highlighted the need to tighten biosecurity across borders, including tighter controls on trade in amphibians as pets to ensure the survival of vulnerable species.

Dr Simon O'Hanlon, from the Department of Infectious Disease Epidemiology at Imperial, is the first author of the paper. He says, 'Biologists have known since the 1990s that Bd was behind the decline of many amphibian species, but until now we haven't been able to identify exactly where it came from.

'In our paper, we solve this problem and show that the lineage which has caused such devastation can be traced back to East Asia.'

Genetic analysis

The researchers collected samples of the fungus on amphibians from around the world and sequenced DNA from across the fungus's genome (an organism's total genetic material).

They combined the data with DNA data from previous Bd studies, including studies done on the Museum's amphibian collection. 

A scanning electron micrograph of a frozen spore of the chytrid fungus

A scanning electron micrograph of a frozen spore of the chytrid fungus. Image: Dr Alex Hyatt, CSIRO Livestock Industries' Australian Animal Health Laboratory (AAHL), via Wikimedia Commons.

 

The team identified four main genetic lineages of the fungus, three of which are found all over the world. A fourth lineage was found only on frogs native to Korea.

Cultures from this Korean lineage were found to contain much more genetic diversity than any other lineage. Deeper analysis of the Korean Bd showed no history of global outbreaks within their genomes. This suggests the Korean strains were native to the region, and most closely resemble the ancestor of all modern Bd.

The team were able to estimate when the current killer strain of Bd diverged from its most recent common ancestor. Their findings support the idea that rather than dating back thousands of years, as previously thought, the range of the disease expanded greatly between 50 and 120 years ago. This was roughly the same time as amphibians started being transported between continents, (both through commercial trade and by accident) on a large scale.

Controlling the amphibian trade

It is likely that the international trade in amphibians has directly contributed to spreading the fungus around the world. Panzootics like this are caused by the fungus being moved into new regions, infecting new hosts and starting disease outbreaks.

Researchers suggest there is strong evidence for much tighter controls on international trade in amphibians, especially from Asia, because of the high risk associated with transferring news strains of the fungus into new parts of the world.  

A grey caecilian

Museum experts and collaborators were the first to report caecilians infected by the amphibian chytrid fungus Batrachochytrium dendrobatidis. Image: Shutterstock.com

 

The team also highlighted the threat of another amphibian pathogen that began in Asia, B. salamandrivorans (or BSal). It affects salamanders in Europe, and its spread is linked with the global trade in pet amphibians from Asia.

Amphibians are also under threat from other diseases that are spread by international transport.

Dr David Gower, a Museum expert who contributed to the paper, says, 'The Museum has previously made small but important contributions to the understanding of Bd and chytridiomycosis. These have been through fungal surveys of amphibian specimens, both in the field and in its historical collections, and through the Museum's expertise in amphibian biology (especially caecilians).

'The new study represents a major advance in knowledge, by providing a detailed understanding of global Bd diversity, and a compelling hypothesis of the origins (in time and space) of this worrying panzootic. The new knowledge will focus future research into chytridiomycosis and help to refine efforts to tackle the conservation threats posed by the disease.'

The work of the Museum

Museum staff and their collaborators were the first to report caecilian amphibians infected by Bd. They have conducted infection surveys in the Indian Ocean, Africa and South America.

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