Genetic sleuthing has revealed the source the deadly fungus that is killing frogs and other amphibians around the world: the Korean peninsula.
The organism, responsible for the catastrophic decline of the Sierra Nevada yellow-legged frog and 200 other species, emerged from East Asia in the early 1900s and is spreading by the pet trade and the expansion of global trade, according to an international research consortium of 38 different institutions.
The evidence, published in Friday’s issue of the journal Science, bolsters the movement to ban the trade in amphibians from East Asia, including their sale as pets. The U.S. Department of Agriculture requires shipments of domesticated animals to be certified as disease-free, but the same safeguards don’t exist for wildlife.
“Until the ongoing trade in infected amphibians is halted, we will continue to put our irreplaceable global amphibian biodiversity recklessly at risk,” said Matthew Fisher of Imperial College London, a member of the research team, in a prepared statement.
FOUND IN 1970S
The discovery sheds new light the mystery of the fungus, first identified in the 1970s, which has been found in species ranging from the Bay Area’s Pacific tree frog and Western toad to the exotic jewel-colored jungle creatures of Africa and South America that decorate calendars and National Geographic magazine covers.
Amphibians have been around for hundreds of millions of years, surviving the great dinosaur die-off and other waves of mass extinctions.
But the fungus, called Batrachochytrium dendrobatidis, also known as “Bd” or the amphibian chytrid fungus, is closing their final chapter for many creatures. While some species show signs of resistance, others perish.
It kills by clogging their pores, deranging their blood chemistry and causing their tiny brains to swell.
Amphibians are a keystone of many ecosystems, and when they disappear, the environment changes dramatically. They’re struggling for other reasons, as well: loss of habitat, due to development; pesticides applied to farms and fields; invasive fish that gobble up tadpoles and global climate change that is warming ponds, creeks and lakes.
The research team, led by computational biologist Simon O’Hanlon of the Department of Infectious Disease Epidemiology at Imperial College London, analyzed the genome sequences of 234 samples of fungus, gathered from frogs around the world and previously published research.
This represents the largest, most comprehensive, global genetic database of the invasive pathogen.
They traced the ancestry of the fungus by counting mutations in its genetic material. As it has evolved over time, new strains became genetically distinct from older strains — and a genetic lineage was created.
All global strains of the fungus share genetic code with a specific strain called BdASIA-1, found on the Korean peninsula, strongly suggesting that the pathogen originated there.
While there had been a growing number of reports pointing toward Asia as the source of these amphibian pathogens, only recently has it been possible to do the genetic work needed to more clearly identify the source, said Karen Lips of the University of Maryland, a world leader in tropical biology, amphibian declines and conservation policy making.
She worries that we might see new lineages of the fungus introduced into the U.S., because so many different genetic lineages are circulating through the international amphibian trade. She also fears that the different strains might mix and mate, creating dangerous new hybrids.
Ecologist Reid N. Harris, director of International Disease Mitigation of the Amphibian Survival Alliance, agreed. “The high genetic variation of the Bd strain found in East Asia suggests the possibility that other pathogenic strains may exist in Asia and could be spreading around the world.”
While people have become increasingly aware of the threat posed by the trade of iconic species such as elephants, tigers and rhinos, fewer realize the risk that commerce poses to the future survival of species, such as frogs.
Amphibian trade is driven by three main industries: pets, food and preserved scientific specimens.
One creature — the Oriental Fire-Bellied Toad, native to Asia, including Korea — is widely exported, with about 3.5 million arriving in the U.S. between 2001 and 2009.
A favorite with starter hobbyists and advanced herpetologists alike, it is hardy and long-lived. The green toad has black spots — and, when threatened, it can flip over — revealing a dramatic red-and-black colored belly.
“These wild animals are infected with various parasites and pathogens, so when they get shipped to the U.S. or Europe, they are stressed and they are often put into close contact with other animals,” said Lips. “This is a situation that promotes the spread, and escape of diseases.”
Once the fungus is introduced, it can be spread by other species, like American bullfrogs, also widely traded. Bullfrogs are carriers of the disease due to their low susceptibility to infection. American bullfrogs are also exported to other countries.
Every year in California, the state Department of Fish & Wildlife allows the importation of more than two million non-native American Bullfrogs, with virtually no disease testing or quarantine enforcement, according to ecologist Kerry Kriger, founder and director of Laguna Beach-based organization Save The Frogs!
One study found that over 62 percent of bullfrogs imported into California are infected with the fungus, he said.