A lethal and rapidly spreading fungus that is devastating amphibian populations across the globe may have originated in Korea, and spread internationally through the pet trade.
Those are the key findings from a genetic analysis conducted by scientists from 38 institutions and published in the journal Science.
Batrachochytrium dendrobatidis (Bd), also known as chytrid fungus, is causing population decline and in some cases local extinction in frogs, toads, and newts on most continents.
Classified as an emerging pathogen because of its sudden and recent appearance, its point of origin has long been a mystery. The first stab at an answer was published only in 2004, when a team of South African and Australian researchers tentatively identified the source as Africa.
The finding was based on a survey of 697 museum specimens of African clawed frogs (Xenopus laevis) that had been collected between 1879 and 1999. One specimen, dating from 1938, was found to have signs of chytridiomycosis, the disease caused by the fungus.
The researchers suggested that Bd had been around in South Africa for a period of time before spreading overseas, propelled by an international trade in the frog species – which was used for human pregnancy assays.
Within a couple of years, however, the conclusion was being challenged. In an overview published in the journal PLOS Pathogens, a team led by Erica Bree Rosenblum from the University of Idaho in the US reported that researchers had found limited genetic diversity in Bd samples taken from African frogs and rather a lot in North American bullfrogs (Rana catesbeiana).
While the researchers did not suggest that the fungus had its origin in North America, they did indicate that the results meant Africa should not now be considered the prime suspect. Again, Rosenblum and colleagues speculated that “global trade” played a significant role in its spread.
Now, the latest research continues to underline trade as an important mechanism for the spread of the fungus, but presents evidence that suggests Korea as the likely start-point.
A team led by infectious disease epidemiologist Simon O’Hanlon from Imperial College London in the UK collected samples of Bd from around the world, and then extracted and sequenced the genome of each. These results were added to existing sequenced genomes to provide a total collection of 234.
By comparing similarities and differences between the samples, the team identified four distinct Bd lineages. Three of them were present around the world, but the fourth was confined only to Korea. The Korean lineage contained much more diversity than the other three, implying that it was most closely resembled the common ancestor of all of them.
The research indicates that the killer strains of Bd diverged from the common ancestor very recently, sometime between 120 and 50 years ago. This coincides with a period of dramatic expansion in international trade, adding further weight to the idea that humans are important (if indirect) vectors for the fungus.
O’Hanlon and colleagues also point to the threat posed by a relation of Bd called Batrachochytrium salamandrivorans (BSal), which is infecting salamanders in Europe. It, too, is thought to have originated in Asia, and spread through the pet trade.
“Our research not only points to East Asia as ground zero for this deadly fungal pathogen, but suggests we have only uncovered the tip of the iceberg of chytrid diversity in Asia,” says co-author Matthew Fisher.