Frogs, toads and other amphibians worldwide are under attack from a deadly fungus, which is wiping out the animals. But, encouraging new findings published this week show that the animals can learn to avoid the fungus and, with repeated exposure, even appear to become immune to its effects.
The fungal disease is called Batrachochytrium dendrobatidis (Bd). It spreads via water, and amphibian species that catch it develop a condition called chytridiomycosis, leading to thickening of the skin. This is bad news for amphibians, as they get a lot of their nutrients and even air through their skin, so many infected animals suffocate.
The fungus grows best at lower temperatures, meaning that amphibians that live at higher elevations or inhabit environments with a defined cold season are more vulnerable to attack.
To find out whether the animals can learn to fight off the fungus, or avoid it altogether, University of South Florida scientist Jason Rohr and his colleagues, writing in Nature, gave toads a choice test in which the animals could sit in one side of a test tank that contained the fungus, or the opposite side that was fungus-free.
When first placed in the tank the animals showed no bias for either position. But after a bout of fungal infection - from which the researchers cured them with a dose of heat exposure - over 75% that chose to sit on the fungus-free side of the test chamber climbed to 70%, showing that the toads can actively avoid the threat.
Next, to see whether the animals can build resistance to Bd, the team infected Cuban tree frogs and measured their immune responses before again curing them with a dose of heat exposure. This was repeated for each animal up to four times. Encouragingly, although the mortality rate was initially quite high with only 20% of the frogs surviving, by the time they were exposed for the fourth time, over half the frogs were surviving and their white blood cell count, a marker of immunity, had doubled and the amount of fungus that could be recovered from the skin was 70% lower.
Intriguingly, exposure to killed samples of the fungus could also be used to inoculate the frogs, protecting them against future exposure to the fungus for real. This lead the team to speculate that it might be possible to protect the animals in the wild by releasing dead fungal extracts into water courses populated by threatened species, almost like an environmental vaccine.
The animals that then do develop protective immunity, the researchers say, will also protect those that don't by a process called herd immunity - effectively the number of vulnerable individuals in a population becomes too low to sustain the transmission of the disease, so it fizzles out.
According to Rohr and his team, "the results presented here offer hope that other wild animal taxa threatened by invasive fungi might be capable of acquiring resistance and might also be rescued by management approaches based on herd immunity."
Well, the amphibians needed a bit of good news.