There seems to be a link between overwintering and the disease's deadliness. Tadpoles in frozen-over lakes all huddle together in the deepest part of the lake where the water isn’t frozen – it usually stays around 4C. The disease tends to gather around the tadpoles’ mouths, and as these chilly tadpoles wriggle around they pass on the infection to other tadpoles they come into contact with. “At 4 degrees there’s not much the immune system can do about it,” says Schmeller. The Bd spores become concentrated over the years, leading to infections that are much more severe at high altitudes than at lower altitudes. When these high-altitude tadpoles finally metamorphose into toads, the disease overwhelms them and the effort is too much: they die.
To dig into the effects of the disease, Schmeller carries out experiments in the more comfortable conditions of his lab in Moulis. A concrete shed houses row upon row of small plastic water-filled tanks the size of office water cooler bottles, each home to a midwife toad at some stage in its development, monitored regularly for the disease and any changes in physiology or behaviour. Some of these experiments are done with Bd-infected water from the Aspe valley, hence Schmeller’s need for those donkeys.
Infected tadpoles apparently need to invest some energy in fighting the infection, and they are smaller when they reach the toadlet stage, says Schmeller. They’ve also seen that when they cool down infected toadlets to 4 degrees for just five or ten minutes, they die very quickly, while the ones that were not infected don’t. In other words, the animals can live with Bd until there’s an additional pressure, Schmeller explains. “This is something we need to understand better, especially under climate change scenarios,” he says.
What also needs to be understood is how to treat the disease. There have been various unsuccessful attempts; for instance, Bosch and Fisher’s teams have tried treating animals in Spain, the Western Pyrenees, and the Balearic island of Majorca with the anti-fungal agent itraconazole. While this can get rid of the disease, it often ends up killing infected toads as well. And in other cases, in Mallorca for example, early experiments to treat infected larvae cleared the disease, but once they were returned to their lakes or rivers, they became re-infected.
Over in the small town of Rascafria in the Penalara park, Bosch has built a breeding centre to try another another tack – repopulate the area with animals that have survived Bd outbreaks. Work has been slow going, and again results have been mixed, but he is beginning to see some success. “We are using the last surviving animals after 10 years of the outbreak to breed with them, and their offspring look more resistant,” he says. The big test will come when these animals are released into the wild.
That’s because we still don’t know how Bd arrived into these mountainous areas in the first place. In the western Pyrenees, there have been at least two separate infections, with several theories about their origins. The disease could have been unwittingly spread on the muddy boot of a hiker, or stuck to the feet of ducks or other water fowl. Or perhaps stocking lakes with fish, or hydroelectric pumping brought Bd in.
Another possibility is that the fungus was introduced into countries through trade: two of the main suspects being the African clawed frog (Xenopus laevis), which is used in biological research; and the North American bullfrog (Rana catesbeina), which is prized for its meat. Trade laws are in urgent need of updating, if Bd’s global spread is to be halted, says RACE participant Mark Auliya, a herpetologist and trade-policy expert at the Helmholtz Centre for Environmental Research in Leipzig, Germany. Pet fairs are big business, and exotic amphibians go for handsome sums. Yet these animals aren’t tightly regulated and could easily be carrying Bd or other infectious diseases.