Although chytrid fungus has become extremely virulent in the wild, an entire species of toads has been saved from it. Rachel Nuwer finds out
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. colonises 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,” Trenton said. “It’s very worrisome.”
Saving wild amphibians
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, Trenton 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 they 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.
“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.”
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,” Deanna 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 Trenton believes that the new method could be tailored to more complex environments.
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,” Trenton said.
