A new infection that emerged in the late 1990s has decimated amphibian populations around the globe. The disease, caused by a chytrid fungus known to scientists as Batrachochytrium dendrobatidis, affects more than 700 species of amphibians on five continents and has triggered widespread population declines, local extirpations, and even extinctions of entire species.

But researchers from Spain and the UK reported last week in the journal Biology Letters that, for the first time ever, they have eliminated the fungus from a population of amphibians in the wild.

The researchers worked for five years to rid Mallorcan midwife toads (Alytes muletensis) of the infection. The species lives only on the island of Mallorca and is considered Vulnerable by the International Union for the Conservation of Nature (IUCN).

The first step was to swab the mouthparts of tadpoles in ponds and ephemeral pools around the island to find out where the fungus was present. The researchers identified three infected ponds in the Torrent des Ferrerets drainage and two in the Cocó de sa Bova drainage.

They captured tadpoles from the Cocó de sa Bova ponds and transported them to the laboratory. There they administered an antifungal medication, itraconazole, daily for a week. They also drained the infected ponds and allowed them to dry out thoroughly during the arid summer months, then returned the tadpoles to the ponds after autumn rains refilled them.

The researchers had high hopes for this strategy because other studies have shown that the fungus disappears during dry periods, and the fungus was absent from nearby pools that naturally dry up during the summer.

But the tadpoles, which showed no signs of infection when they were returned to their ponds in the autumn, had reacquired the fungus by the following spring. The researchers think the tadpoles probably got reinfected from adult toads, which tend to hide out in inaccessible places and are hard to catch.

So, at Torrent des Ferrerets, in addition to treating tadpoles (and whatever adults they could catch) with itraconazole and draining the affected ponds, the researchers also applied a disinfectant called Virkon S to the rocks, gravel, crevices, and vegetation around each pool. This combination treatment cleared the infection and kept it from coming back. The toads in the Torrent des Ferrerets ponds have been fungus-free for two years now.

Later, the researchers confirmed this procedure’s usefulness by repeating it at the Cocó de sa Bova ponds. They were able to eradicate the fungus from the larger pond, though the smaller one still shows evidence of residual infection.

In the past, scientists have tried maintaining disease-free populations of various amphibian species in captivity as a kind of biological insurance policy, but eliminating the infection from wild populations has been a more difficult problem. Lots of researchers are interested in using beneficial bacteria to help control the fungus, but how to make this work varies depending on the species of amphibian and the microbiota of the surrounding environment. So that leaves antifungal chemicals as a simple, cheap, readily available strategy that could be used in many different locations.

Virkon S is a common laboratory disinfectant, but the researchers note that using it in a natural environment is likely to be controversial. They argue that the perilous conservation situation of the Mallorcan midwife toad justifies its use, but say future studies will have to look at its effects on other species and on the ecosystem as a whole.

It’s also worth noting that what qualifies as a “cheap, simple strategy” is relative. Compared to other efforts to eradicate the chytrid fungus, the antifungal-plus-disinfectant approach may indeed be most feasible. Yet draining and disinfecting ponds and hiking out of remote watersheds with tadpoles in plastic bottles represents a lot of effort. That the researchers have eliminated the fungus from a wild amphibian population is a huge step forward. But the lengths to which they had to go to do so show just how difficult a problem the world’s amphibian species currently face. – Sarah DeWeerdt | November 24, 2015

Source: Bosch J. et al. “Successful elimination of a lethal wildlife infectious disease in nature.” Biology Letters DOI: 10.1098/rsbl.2015.0874

Header image: Mallorcan midwife toad (Alytes muletensis). Credit: Jaime Bosch MNCN-CSIC.