Confronting chytrid: the fight to save the world’s amphibians

Blue poison dart frog © Pria Ghosh

By |2020-12-17T16:32:43+00:00December 17th, 2020|Amphibians, Extinction, Species, Threats|Comments Off on Confronting chytrid: the fight to save the world’s amphibians

In the 20th century, a new infectious disease emerged and caused an unprecedented environmental catastrophe. Chytridiomycosis, a fungal skin infection, has compounded an amphibian extinction crisis driven by habitat loss, overharvesting and pollution, resulting in 40 per cent of amphibians being threatened with extinction. 

Today, over 20 years after the first chytrid fungus was described, we remain largely helpless when chytridiomycosis emerges in a new area, with no widely applicable mitigation measures and no treatment that offers long-lasting protection. As a last resort, many amphibian species have been saved by establishing ex situ captive populations – how can we return them safely to the wild?

A global amphibian ark

When I first stepped behind the scenes of a zoo’s herpetology department, as a student keeper, I knew that I would meet animals facing threats like pollution, habitat destruction, and a voracious pet trade. I wasn’t expecting to see amphibian after amphibian, tank after tank, being kept solely out of sheer, fierce hope. Safely cocooned in miniature, curated forests and streams are little communities, upon whose existence entire species depend. Members of The Amphibian Survival Alliance, a partner of Synchronicity Earth, are helping these families ride out the waves of a mass extinction event beyond their doors, hoping that one day their descendants will be safe outside the confines of these arks. The problem is that these animals are threatened by disease. That disease, chytridiomycosis, lives on in the soil of the forests and the waters of the streams long after its victims have gone; once it’s arrived – likely hitchhiking on a pet, a frog destined for a laboratory, or in some undisinfected water discarded down a drain – it is nearly impossible to eliminate. Their habitats may look pristine, but they cannot be made safe.

“The impact of chytridiomycosis on frogs is the most spectacular loss of vertebrate biodiversity due to disease in recorded history” – Skerratt  et al., 2007

Chytridiomycosis is caused by two species of microscopic fungi. Batrachochytrium dendrobatidis (Bd) was isolated in 1998 from the skin of a Blue Poison Dart Frog that died mysteriously in a zoo. Then, in 2014, Batrachochytrium salamandrivorans (Bsal), literally the salamander-devourer, was identified in the Netherlands. The arrival triggered collapses of fire salamanders, killing around 96% of the country’s population.

Fire Salamander Bsal mortality © Jaime Bosch

The scale and speed of declines these fungi inflict are unprecedented and, in the wild, unmanageable. Most people think of fungal pathogens as ringworm, or athlete’s foot but certain characteristics of fungi can make them perfect pathogens. They don’t need their host to survive, it’s no problem for them to hang around, maybe even for months, waiting for another victim to walk by; their genomes are large and flexible – individuals can swap genetic material with relative ease; and some of them are staggeringly unspecific in their infection targets.

Bd and Bsal have driven an estimated 90 species to extinction, and 200 more to the edge

 For a complex problem, creative solutions are needed

This year, humans have discovered what facing the rapid, global spread of a largely untreatable infectious disease feels like, and researchers have delivered the truly stunning achievement of creating a vaccine in less than a year. For amphibians, however, vaccination cannot provide the solution. A vaccination relies on triggering the immune system, and in many amphibians, chytrid fungi somehow avoid triggering any kind of immune response at all, making it impossible to vaccinate against. The host animal dies infested with fungus, but their cells don’t even recognise the invasion.

Midwife toad, Alytes obstetricans, mortality from a single lake in the French Pyrenees © Matthew Fisher

But, there is some hope. Creative and dedicated people all over the world have identified other possible pathways to a post-chytrid world. The disease has been eliminated in nature once; in Mallorca a Herculean effort was staged to remove all wildlife from infected ponds, treat every animal, even disinfect the rocks of the ponds themselves, and then return the clean water and wildlife. This saved the Mallorcan Midwife toad, but clearly isn’t wi