April 15, 2024

IIt was while we were sitting and talking in a hotel bar at the first world congress of herpetology that the world’s amphibian experts realized there was a problem: frogs, toads, salamanders and salamanders were disappearing in their thousands around the world and no one had don’t understand why.

Not a single talk at the 1989 Congress at the University of Kent discussed the strange disappearance of the world’s amphibians. But scientist after scientist had the same story: from Central America to Australia, they were disappearing.

The year before I had joined the Zoological Society of London as a veterinary pathologist. It was my job to find out why animals died. Soon after I started, members of the public started calling London Zoo with news that dozens of frogs had died without explanation in their garden. More and more of these reports started coming in. I started testing the dead frogs to find out what was going on as part of a PhD, and a ranavirus spread through frogs in England.

Although already known in the US, this was the first time a ranavirus had been found to kill wild frogs in Europe. I presented my findings, which led to an invitation to Australia to help with a new mystery. A Masters student was looking into a series of unexplained amphibian deaths in a Queensland rainforest.

In the late 1980s, reports began to come in from the public of frogs dying in British gardens. Photo: Graham Turner/The Guardian

The animals that died there appeared healthy: the tissues were intact, there were no parasites and they were tested for viruses and bacteria. Nothing. They were just dead.

But as I reviewed the evidence, I realized I had seen it before. During a visit to Melbourne Zoo a few years earlier, I was shown some tadpoles of an endangered Queensland species of frog. They thrived as tadpoles but died after turning into frogs. All the pathology reports found the frogs to be healthy – except that they were no longer alive – but for the presence of an unknown organism in the skin.

I looked at the skin of the frogs we examined from the Queensland rainforests with the master’s student. Under a microscope they had the same strange organisms I read about in the pathology reports at Melbourne Zoo. So we set up an experiment. We exposed a small number of healthy frogs to the infected skin. They all died – and they all grew the organism in their skin.

Cunningham set up experiments to see how healthy frogs reacted when exposed to the unknown organism. Photo: David Levene/The Guardian

At the same time, I knew colleagues in Panama were looking at the same problem. I told them to look at the skin of their dead frogs to see if they had the same infection. They have. We compiled our results and in 1998 we published themand announced to the world that a fungus – later named Batrachochytrium dendrobatidis – was infecting and killing amphibians worldwide. This attacked their skin and caused the frogs to have a sudden heart attack and die.

Other researchers have since verified our results and gone on to find several strains of the fungus. The deadliest strain appears to be only about 100 years old, probably transported by humans around the world, and it continues to wipe out amphibians.

So far, nearly 100 amphibian species are known to have disappeared in the past 50 years and hundreds have declined in number. One affected species I study is the mountain chicken frog – once common in the Caribbean – which is down to the last 30 individuals known to exist in the wild. I can survive it. For me, this disease is a reminder of the destructive impact humanity can have on the planet and its biodiversity. This disease probably wouldn’t exist without us. We must find a way to live in balance with the wonderful species we share the Earth with.

As told to Patrick Greenfield

Andrew Cunningham is the Professor of Wildlife Epidemiology and Deputy Director of Science at the Institute of ZoologyZoological Society of London

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