March 20, 2019 – To quote a certain Muppet, it’s not easy being green. And it’s not getting any easier thanks to the deadly fungus Batrachochytrium dendrobatidis that causes chytridiomycosis, a disease devastating amphibian populations globally. Work funded by Morris Animal Foundation, however, is helping scientists better understand the disease and its origins, and giving clues as to how its relentless spread might be slowed or even stopped. The study on chytrid’s origins was published last year in the journal Science.
“It’s everywhere. It’s in every continent except for Antarctica and that’s just because there are no frogs down there,” said Dr. Matthew Fisher, a co-author of the paper and mycologist at Imperial College London. “It’s managed to get its way into every corner of the world that appears to have amphibians, including some very curious places.”
Amphibians have been around for 400 million years. They were the first, bold vertebrates to emerge onto land and now number more than 7,000 species. But in the 1970s, scientists were caught by surprise when populations of these successful organisms began to crash globally, often in pristine, untouched environments. Researchers eventually ruled out deforestation, habitat change and overharvesting as root causes of these ‘enigmatic declines’ and focused on infectious disease.
In 1999, scientists discovered that a fungus, Batrachochytrium dendrobatidis (Bd for short), was the agent responsible. Bd has now caused the extinction of more than 90 known frog species. Hundreds more are in decline.
Bd kills amphibians by infecting their skin, through which they breathe and hydrate. The fungus hardens the skin, affecting the animals’ ability to regulate water and electrolyte levels, resulting in death. It’s not known for certain how the fungus is spread, but direct contact between animals or exposure to swimming spores in infected water are likely causes.
For the last decade, an international team of more than 50 researchers, from over 30 universities, combined their efforts to discover where this scourge came from. The team meticulously collected samples of the disease by clipping wild frogs’ toes before releasing them back into the environment. The process was painstaking.
“You typically have to sample 400 frogs to catch two or three chytrids,” said Fisher. “Then you’ve got to keep the chytrid alive until it’s back in a lab so we can grow and study it. It’s a huge amount of work to replicate that process across the planet.”
Fisher’s team sequenced more than 200 Bd genomes and compared them before finding four key lineages. While three were found all over the world, one was found only on the Korean peninsula. The finding, which came after receiving Asian samples for the first time from South Korean collaborators, ended a lengthy period of frustration.
“After sequencing enough chytrids, we knew what the original genome should look like, but we just couldn’t actually find it,” said Fisher. “It was a bit of a jigsaw, and the South Korean samples fit right into the center and it all clicked into place. It was definitely a eureka moment.”
The results further indicated that Bd likely spread around the world between 50 to 120 years ago, most likely through the global expansion of the pet trade. Since the 1940s, Fisher said, that included movement of animals out of Asia.
Researchers aren’t sure how amphibians transmitted the disease into wild populations. It may have been through infected individuals that escaped or were released into the environment, or even infected habitat water disposed of during cleaning. Frogs also can passively disperse infectious agents by stowing away on shipping containers, or even bunches of bananas, which take them to new, exotic homes.
Complicating matters is the complexity of the fungus itself. Its genome is highly elastic and able to mutate and adapt to many different environments. Fisher witnessed this firsthand while experimenting on chytrids they cultured and believes it’s the secret to its success.
“There’s a bit of an arms race that’s going on and these pathogens are really quite adept competitors,” said Fisher. “The fungus can recombine and mutate in novel ways so even if an amphibian does evolve an immunity to it, it’s likely to evolve in novel ways the frog can’t second guess.”
The study pinpointing the origin of chytrid already is having an impact on amphibian species preservation. Perhaps most stunningly, increasing numbers of countries, led by the United States, are closing their borders to imports of amphibians due to the huge risk of transporting new lineages of chytrid out of Asia and seeding new outbreaks.
Scientists also are learning from the pathogens themselves, and expect to be able to manufacture control methods, such as vaccination or manipulating an amphibian’s ecosystem, to attack chytrid while keeping amphibians safe.
One such victory came on Mallorca, a Spanish island in the Mediterranean Sea. Researchers gave amphibians there an antifungal treatment, which cleared Bd from the island. Fisher says that was a special situation given the geographic area; curing populations in larger regions will prove more challenging.
Morris Animal Foundation also supported a chytrid project in Madagascar, where frogs infected with Bd aren’t dying. Fisher’s team is trying to understand why so they can extend that protection to amphibians dying elsewhere in the world. This study includes investigating the potential of a novel gene-silencing technique to interfere with immunosuppressant activity of the fungus, possibly boosting infected frogs’ immunity to the disease.
“Amphibians need our help as they’re key players in terms of ecosystem dynamics, two for the price of one,” said Fisher. “As tadpoles, they graze ponds and serve as an important food source for insects, small birds and fish. After metamorphoses, fully grown amphibians control insect populations with their own feeding habits and serve as a food source for larger prey, such as reptiles, snakes and larger birds.
“It destabilizes an ecosystem when you remove the frogs and these impacts have already been shown in Central American rainforests where amphibian biodiversity is far lower than it used to be. You can’t expect to lose a very numerous set of species, such as amphibians, and not see increasing changes of dynamics and fragility in the network that depends on them.”
Learn more about how you can help support research that will save our amphibians globally. Become a supporting member of Morris Animal Foundation today!