A Reprieve for Fungus-Battered Frogs - via Herp Digest

By Rachel Nuwerjan, Janauary 4, 2016, Science Daily

After a six-year effort, researchers on the Spanish island of Majorca have rid several groups of Majorcan midwife toads of the pathogen Batrachochytrium dendrobatidis — better known as chytrid fungus, or B.d. It’s the first time the disease, which is devastating amphibians worldwide, has been eradicated in a wild population.

“This is proof of principle that you can go out there and mitigate infections and that the method doesn’t need to be that complex,” said Trenton Garner, a biologist at the Zoological Society of London, who reported the findings with his co-authors in the journal Biology Letters.

Described in 1998 following mass die-offs of frogs in Australia and Panama, B.d. colonizes cells on the outer layer of an amphibian’s moist skin, causing the skin to thicken and interfering with electrolyte transport. The infection eventually leads to cardiac arrest.

An extreme generalist, the fungus infects many types of frogs, salamanders, newts and toads. It has been confirmed in 700 species on six continents. Several are presumed to have been driven to extinction, while many others have suffered catastrophic population declines.

Researchers do not know why the fungus is so virulent; around 1,000 chytrid species exist in the wild, but only two are known to infect vertebrates. “It’s not a biologically sound strategy for a parasite to cause its host communities to go into serious decline or drive its host to extinction, but that is the case for this fungus,” Dr. Garner said. “It’s very worrisome.”

Some zoos and research labs have managed to clear up the infection in captive populations, but until now no one has done so in the wild. The lucky recipient of this experiment is the Majorcan midwife toad, a species once thought to be extinct that was rediscovered in the 1980s in several isolated ponds in the island’s limestone outcrops.

A successful captive breeding effort allowed conservationists to expand the native frog’s range (despite its name, Majorcan midwife toads are not true toads). But in 2007, wildlife managers found that chytrid had sneaked into the wild populations — likely introduced via the captive-bred individuals meant to save the species.

In 2009, Dr. Garner and his colleagues began their efforts to clear the frogs of the fungus. They focused on the tadpoles, which readily pick up the infection in ponds where they live, but are not killed by it. When the tadpoles metamorphose into frogs, however, mass mortality ensues. The researchers removed thousands of tadpoles from five ponds and then drained the ponds almost completely, hoping that the sun’s warmth would kill the chytrid, which is sensitive to temperature.

Back in the lab on Majorca, they bathed the tadpoles in antifungals and kept them in captivity for months, until rains replenished the ponds. Crossing their fingers, the researchers returned the fungus-free tadpoles. But within a year, chytrid was back.

The researchers were not dissuaded. They tried again in 2012, but instead of just removing and treating the tadpoles, they also applied low concentrations of a common commercial disinfectant to some of the ponds and the rocky crannies around them.

The following year, tadpoles and frogs in the three ponds that were disinfected and whose tadpole residents were treated did not have any signs of B.d. Three years on, those three ponds are still fungus free, and the researchers have gone on to apply the treatment to the remaining two ponds as well.

“They set out to eliminate a major threat to the survival of a very special frog, and they were successful,” said David Wake, an amphibian specialist and evolutionary biologist at the University of California, Berkeley, who was not involved in the research. “It took years, but it worked — so far.”

The hope now is that this success will raise interest in mitigation efforts beyond Majorca. “The method is really cheap and easy to use, so why not try it in other places?” said Jaime Bosch, a senior research scientist at the National Museum of Natural Sciences in Madrid and one of the paper’s authors.

“We can’t just stand still and do nothing, watching amphibian after amphibian go extinct.”

Conditions on Majorca, however, are particularly well suited to such interventions, Dr. Wake said. Not only is the island isolated, but the frogs breed in satellite sites largely cut off from one another, and without other amphibian species hopping about that could reintroduce the pathogen or complicate tadpole treatment.

“Doing such a study almost anywhere else might be dauntingly difficult,” Dr. Wake said.

The special circumstances on the island, and the threat B.d. posed to the protected Majorcan midwife toads, justified the use of fungicides in the environment, said Deanna Olson, a research ecologist at the United States Forest Service’s Pacific Northwest Research Station, who was not involved in the work. But it’s a strategy that some other scientists see as extreme.

“That would be considered very controversial in a wider application,” Dr. Olson said. “Eradication of B.d. in the wild has been discussed, but implementation has been stalled due to the widespread effects of antifungals on an extremely important component of ecosystems: fungi.”

More work is needed to devise strategies for treating amphibians living in different settings and to address concerns about antifungals, but Dr. Garner believes that the new method could be tailored to more complex environments.

“Arguing that these methods could only apply with these chemicals and conditions on Majorca limits thinking,” he said. “There are other ways to mitigate fungal infections on large geographical scales, as we regularly show with livestock, agriculture and human pathogens.”

B.d., however, is just one of a suite of amphibian worries. A new species of chytrid fungus, recently detected in Britain and Germany, infects only salamanders and newts, but appears to be extremely virulent, causing rapid population decline. And a group of new viruses in Europe appears to kill all amphibians, and potentially reptiles, in its path.

Habitat loss, however, remains the biggest threat, while climate change exacerbates the problems caused by both disease and increasingly small ranges. “Amphibians are confronting a lot of issues now,” Dr. Garner said.

A version of this article appears in print on January 5, 2016, on page D3 of the New York edition with the headline: Reprieve for Fungus-Battered Frogs. Order Reprints| Today's Paper|Subscribe