In a major coup, scientists have eliminated a deadly fungus on the Spanish island of Mallorca. Batrachochytrium dendrobatidis, which has killed off amphibians all over the world, was set to annihilate the endangered Mallorcan midwife toad. But by applying high-strength disinfectants and airlifting tadpoles to safety over 5 years, biologists completely eradicated the fungus from the mountain ponds where the species has been clinging to survival. “It's pretty incredible,” says Karen Lips, a herpetologist at the University of Maryland, College Park, who was not involved with the work. Though repeating the victory may not be easy, or even possible, for other species elsewhere, the findings offer rare good news in the fight against a pathogen that has been a catastrophe.
First described in 1998, B. dendrobatidis has pushed numerous amphibian species to oblivion around the world. Once infected, many amphibians develop a condition called chytridiomycosis, marked by ulcers, convulsions, starvation, or asphyxiation. Antifungal drugs can cure the sick, but there are no effective approaches to saving entire populations. In large part, that’s because the spores are always present in the wild, and the highly contagious fungus can infect more than 700 species. Desperate to staunch the losses, biologists are trying out possible vaccines and treatments with beneficial bacteria. They have even resorted to removing vulnerable species and keeping them in zoos.
For the Mallorcan midwife toad (Alytes muletensis), fungal infection was just one more stroke of bad luck. Endemic to the Mediterranean island of Mallorca, the toad evolved without predators. They were known only from recent fossils and presumed to have been wiped out after cats, rats, and snakes arrived with settlers many centuries ago. A few small groups were discovered in 1977 in Mallorca’s remote and inaccessible mountains. A reintroduction program in the 1990s expanded that number to about 25 populations, but in the process scientists accidentally introduced B. dendrobatidis, after the toads picked up the disease in a captive breeding facility.
Herpetologist Jaime Bosch of the National Museum of Natural History in Madrid, decided Mallorca might be a place to try a radical new approach—eliminating the fungus from the wild. Luckily, B. dendrobatidis was only present in five ponds on the island. Additionally, the ponds sit on bedrock, making them easier to disinfect. In summer 2009, Bosch and colleagues removed about 2000 tadpoles from two pools in the mountains—some by helicopter—and took them to a lab where they were treated with the fungicide itraconazole for a week. Months later, the researchers returned them to ponds that had been temporarily drained and dried by the sun to kill the fungus.
Unfortunately, by the following spring the treated animals had become infected with the fungus again. “It was frustrating!” recalls Bosch, who guesses that infected adults contaminated the pool when they returned to lay eggs. The situation was becoming urgent. In one population, the disease had caused the number of tadpoles to plummet from about 2500 to just 20.
Bosch intensified the assault. In 2012, his team drained three ponds in another watershed, then sponged a potent biocide, Virkon S, over the bottom of the empty ponds and the surrounding rocky crevices where adult toads live. As before, they removed the tadpoles, disinfected them in the lab, and brought them back to the ponds. This time it worked: The next year, all the tadpoles and toads that they found were fungus-free, they report this week in Biology Letters. The populations seem to be rebounding, and the tadpoles tested this summer had no trace of the fungus, Bosch says. He is optimistic that they will stay healthy, because it is unlikely other B. dendrobatidis–infected amphibians will reach the remote ponds.
Bosch says that environmental decontamination will probably only work for small, isolated ponds in dry climates—elsewhere, the fungus can be spread by other host species or can survive in wet leaves or bodies of water too large to drain or treat. But the approach could still help many amphibians in the Mediterranean. Lips hopes that the success will lead people to consider other kinds of isolated environments where B. dendrobatidis might be eradicated, such as urban parks or mountaintops. Bosch says: “We can’t stand by idly thinking that chytridiomycosis can’t be combatted!”
* Update, 18 November, 5:45 a.m.: This story originally said Mallorca was a small island. In fact its surface is 3640 square kilometers.