A dangerous fungus has almost wiped out the toads. Some have learnt to survive

However, scientists have discovered a surprising exception. Some toads have managed to survive and recover their population, even though the dangerous fungus is still present in their habitat.

A study published in the journal *Nature Chemical Biology* has helped to explain why this is happening.

Why is the fungus so dangerous?

The pathogen is the fungus Batrachochytrium dendrobatidis (Bd). It infects the skin of amphibians, disrupting the body’s ability to maintain its water-salt balance. As a result, the animals often die.

Toads become particularly vulnerable after they transform from tadpoles into adults. It is at this stage that their skin becomes covered in keratin — a substance on which the fungus feeds.

What saved some toads

Researchers studied populations of midwife toads living near four lakes in the Pyrenees.

Near one lake, the animals continued to die off in large numbers. But near the other three, the toad populations gradually recovered, even though the fungus had not disappeared.

It turned out that the main difference lay in the skin’s immune defences.

In the surviving populations, the organism began producing protective antimicrobial peptides as early as the tadpole stage. Thanks to this, the young toads encountered the dangerous infection with a defence system already in place.

Over a thousand unknown protective molecules

Using modern mass spectrometry, the scientists discovered a veritable chemical arsenal on the animals’ skin.

They identified 1,152 different peptides, although previously only seven such molecules had been known to science in this species.

The study showed that in populations that managed to recover, the diversity of these defensive substances was significantly higher even before metamorphosis was complete.

Scientists have yet to determine exactly which of the peptides found directly destroy the fungus.

Why does the immune response kick in earlier?

Researchers do not yet know why the immune system develops earlier in some populations than in others.

Possible reasons cited include genetic characteristics, ambient temperature or the presence of predatory fish. If tadpoles have to transform into adult frogs more quickly, the immune system may simply not have time to fully develop.

Why this is important

The discovery shows that even species severely affected by a deadly infection are capable of recovering if their immune defences develop early enough.

Furthermore, the peptides identified may prove to be a promising avenue for further research. Many modern medicines have been discovered thanks to natural substances; however, these molecules are still a long way from practical application.

Background

The fungus Batrachochytrium dendrobatidis is considered one of the most devastating infections affecting wildlife. Over recent decades, it has caused mass amphibian die-offs worldwide and played a key role in the decline of hundreds of species of frogs, toads and salamanders.