Where Have All The Frogs Gone? New Study Illuminates Killer Fungus

Scientists have found a new clue as to why amphibians can't fight off a skin infection that threatens entire species.

Frogs in Peril

Joel Sartore

More than 80 percent of the world's sharp-snouted day frogs died off in the decade leading up to 2004. In 2002, the International Union for Conservation of Nature placed the little frog species with a wedge-shaped snout on its Red List of critically endangered species. Its hopping grounds were at the fringes of the rainforest and in the uplands of Australia.

Until they weren't. A 2006 study suggested the sharp-snouted day frog's situation was "the first case of extinction by infection of a free-ranging wildlife species from which it did not recover."

The deadly disease is chytridiomycosis, caused by the chyrid fungus, known as Batrachochytrium dendrobatidis. For something that's so pervasive and effective at killing off species, scientists know surprisingly little. And the sharp-snouted day frog has certainly not been alone in its dramatic population decline. The Panamanian golden frog, as well as the Northern and Southern gastric brooding frogs are also named among its victims, and it's been happening for a long time—more than 40 years. The fungus attacks the amphibian's immune system with unprecedented devastation. From 2002 to 2011, populations overall have declined by 3.7 percent each year.

It's been most baffling, given the amphibians' complex immune systems, not far off from the immune complexity of humans and other mammals.

The fungal infection causes frogs to become lethargic and crawl out of the water.

"There's been a big question in terms of why the amphibian immune system hasn't been able to respond to this nasty skin infection," Louise Smith-Rollins, an associate professor of pathology, microbiology and immunology at Vanderbilt, tells Popular Science. "The question is, if it's a failure to recognize the pathogen, what's the defect?"

Rollins-Smith has been studying this immune response for more than 10 years, and she and her team have found another clue as to why amphibians can't clear this fungus. This week in Science, a paper she co-authored brings in new information to understanding the answer to that question. The study, led by Vanderbilt graduate students J. Scott Fites and Jeremy Ramsey, shows that it may be the second line of immune defense where the breakdown occurs.

The first line of defense, antimicrobial peptides produced in the skin, seemed to be effective at producing an immune response. But during the next stage, something happened to stop the usual inhibiting response.

"It appears that the defect is that the fungus itself is able to release factors that target vulnerable lymphocytes and induce them to commit suicide," Rollins-Smith says. "Mediators that should be regulating and calling in the troops, they're stopped right there."

From 2002 to 2011, populations have declined 3.7 percent each year.

The toxic factors that may play a part in disabling the amphibian immune response. It's seen when the fungal infection causes frogs to become lethargic and crawl out of the water. And that could mean that though the fungus lies within the skin's layers, its having an affect elsewhere, getting into the circulation and having neurotoxic effects.

"Those of us who are amphibian biologists sometimes feel like we have to persuade the public why amphibians are important," Smith-Rollins says. But beyond the fact that they're adorable and a certain writer's favorite animal, amphibians and frogs specifically are pretty important to keep around. They play a part in the health of many different ecosystems as plant eaters, insect eaters, and prey.

Amphibians are also important for human health; they are important to biomedical testing, and their immune systems may hold clues to fighting human pathogens as well.