Amphibians worldwide are under numerous threats, largely to do with habitat loss and fragmentation, pollution and others. However, the single most destructive factor in recent decades has undoubtedly been the global spread of the chytrid Batrachochytrium dendrobatidis, an aquatic fungus which causes in amphibians a fatal disease known chytridiomycosis. This disease has caused substantial population declines and local and species-wide extinctions across five continents, including in otherwise well protected and apparently pristine environments. The ensuing extinction crisis has been so severe that for a number of amphibian species, in particular in areas of high diversity and endemism such as high-altitude forests in Central America, the remaining hope for survival was to remove some of the last healthy individuals from the wild and take them into strictly controlled captive environments, now known as amphibian pods. Despite some successes in both captive breeding some of these amphibians and treating the fungal disease in captive collections, there was no realistic prospect of returning these individuals or their offspring to the wild given that the fungus persisted in the environment. Where individuals have been reintroduced, even after successful fungal treatments, they have generally quickly been re-infected once back in the wild. However, in a recent publication in Biology Letters, Bosch et al. (2015) describe just that, the first elimination of chytridiomycosis from a wild setting, albeit a comparatively structurally simple and contained one, represented by a small number of isolated ponds (5) on the island of Mallorca, home to the endangered and endemic Mallorcan midwife toad (Allytes muletensis).
The Mallorcan midwife toad has long been a poster species for the fight against chytridiomycosis in Europe. Listed previously as Critically Endangered and currently as Vulnerable, it has undergone very rapid declines as a results of this disease and has prompted very dramatic conservation actions. In the paper the authors describe an initial experiment where the tadpoles from known ponds with infection were removed and treated in captivity to eliminate the fungal infection before being returned to the wild. At the same time the ponds were drained and left to dry entirely in the hot summer Mediterranean climate. However, the subsequent infection rate showed no signs of decline until later, when the surrounding rocky terrestrial environment was also chemically disinfected as well as antifungal treatment of tadpoles and any adults that could be captured.
Two years after environmental application of the Virkon S chemical coupled with the itraconazole antifungal treatment of tadpoles carried over across years resulted in no evidence of infection detected in the subsequent year.
The experiment has been described as a “monumental achievement” given not just the very complex logistics of accessing, removing, treating and surveying tadpoles and disinfecting terrestrial environments in difficult field conditions and steep mountain gorges (tadpoles were taken by helicopter at some point!), but also the fact that this represents the first ever eradication of this disease from the wild. However, numerous questions remain about how this lesson could be used in other, more species rich and continuous habitat locations, given not just the complexities of targeting multiple amphibian species at the same time but also the use of the chemical disinfectant Virkon S in such large quantities to treat the terrestrial environment. Even the authors describe the use of the chemical as “controversial” but justify it by the urgency in treating the disease in Mallorca and the imminent threat to the very existence of the midwife toad there. Virkon S is a standard disinfectant solution used in farming environments and also the chemical of choice around the world for disinfecting equipment used by amphibian surveyors, such as nets, wellies and waders. However, the application in this field experiment in Mallorca was on a completely different scale and requires significantly more work in terms of understanding of impacts before being attempted elsewhere. Whether or not this experiment will lead us to a model for other sites and species remains to be seen but fundamentally, a very important milestone has been reached in this case and it is a rare glimmer of hope in an otherwise depressing context.
Finally, it is worth mentioning the other major factor threatening the conservation of the Mallorcan midwife toad- the introduced viperine water snake Natrix maura. The species is common in continental Spain and Southern France but was not found on the Mediterranean Balearic islands before human introduction. It predates both tadpoles and adults of the midwife toads and could potentially completely eradicate the small remaining populations. The Mallorcan midwife toads have another major threat to deal with before their long-term conservation is ensured but hopefully this island can provide another successful story for other sites.
Jaime Bosch, Eva Sanchez-Tomé, Andrés Fernández-Loras, Joan A. Oliver, Matthew C. Fisher, Trenton W. J. Garner (2015) Successful elimination of a lethal wildlife infectious disease in nature; Biology Letters. DOI: 10.1098/rsbl.2015.0874
Full copy of the paper can be accessed here.