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After a six-year effort, biologists say they have for the first time managed to rid a wild toad species of a lethal fungal disease that threatens amphibians around the world.

Midwife toads on the Spanish island of Mallorca are now free of the chytrid fungus Batrachochytrium dendrobatidis, says Jaime Bosch, an evolutionary biologist at Spain's National Museum of Natural History in Madrid. His team reported their success in the journal Biology Letters on 18 November1.

But the successful treatment — which involved treating tadpoles with an antifungal drug and chemically cleansing their ponds — may not be widely applicable to the habitats of other amphibian species that are threatened by chytrid, the researchers and others say.

Toad doctors

The fungal disease is one of the greatest threats amphibians face across the globe: chytrid has already wiped out hundreds of species of frogs. Bosch and his colleagues in Spain and the United Kingdom first set out to save isolated populations of vulnerable midwife toads on Mallorca in 2009. The fungus was spreading on the toads’ skin, stifling their ability to breathe and manage their water balance, and ultimately killing them.

The researchers removed all the toads’ tadpoles and treated them in the laboratory with the antifungal drug itraconazole, while also draining and drying out their ponds in the hope of eliminating the pathogen. But after they were returned home – via helicopter – the first batch of successfully treated tadpoles was soon infected with the fungus again.

Solvin Zankl/Nature Picture Library

In 2012, the researchers tried again: this time, they drained and treated one set of ponds with Virkon S, an agricultural disinfectant made by DuPont. Tadpoles that were returned to those ponds a year later remained healthy, whereas those returned to ponds that were drained but not treated fell ill. After disinfecting the rest of the ponds, the researchers found no evidence of fungal infection two years on.

It is still unclear exactly where and how the fungus lingered in the untreated drained ponds, though Bosch and his team suspect that tadpoles were being reinfected by adult toads that remained tucked out of sight. The team sprayed disinfectant into nooks and crannies, so may have managed to reach the hidden adults.

“It’s pretty exciting that they were able to eliminate [chytrid] in multiple sites across the island,” says Karen Lips, a conservation biologist at the University of Maryland, College Park.

No cure-all

But Lips thinks that the treatment may only work in specific habitats. Mallorca is dry, with granite-carved ponds that flood seasonally, and few other species live there that could reintroduce the fungus. “Not many other places are geared to this approach,” Lips says. Exceptions include other isolated environments such as captive breeding programmes, zoos, laboratories and other kinds of islands — for example, urban islands and mountaintops. Still, she says, in places where species are endangered and costs are not a barrier, “I think this shows there are certain things you can do.”

To rid many amphibians of their fungal infection, it will be necessary to find another way, Bosch agrees. But the study shows that it may be worth trying the aggressive chemical intervention in some circumstances, he says. One of Bosch’s co-authors, Trent Garner of the Zoological Society of London, would like to see more mitigation efforts. “We spray for fungal infections in our crops every year,” he says. “Are there other things that we could use that could be applied environmentally and at a large scale?”

Doug Woodhams, an amphibian disease ecologist at the University of Massachusetts, Boston, hopes that interventions that don't involve extensive spraying of antifungal chemicals might also work. In his laboratory he is trying probiotic therapy, which introduces beneficial microbes to fight fungal infection — but he has yet to prove that it works in the wild2.

Meanwhile, in southeast continental Spain, which has a similar geographical landscape to Mallorca, Bosch and his colleagues are using their method to try to protect populations of the Betic midwife toad. Endemic to the region for millions of years, the toads now inhabit a landscape where humans have shifted water into artificial ponds meant for cattle. The toads now make these plastic-lined pools their home — a perfect setting in which to knock out the chytrid fungus.