A fungus that kills frogs around the world

On the slopes of Mont-Grand Matori in Guyana, dozens of naturalists wander the Amazon rainforest. They are guides or custodians of nature reserves, and members of the environmental police. Since 2012, they meet every year for a somewhat special hunt: they are looking for dendrobates. Frogs a few centimeters long in bright colors symbolize the Amazon. The scale of the task is staggering. It is necessary to inspect the wet shade of the dead forests where these animals live. When a frog is caught, a naturalist rubs a cotton swab against the shiny mucus on its skin. The gesture feels familiar, and for good reason: It’s a test PCR.

Using this technique, scientists from the National Center for Scientific Research (CNRS) From Cayenne looking for traces of parasitic fungi Batrachochytrium dendrobatidis where Caricature. This can cause a disease that prevents amphibians from breathing through the skin: onychomycosis. It has already caused the disappearance of ninety species of amphibians in the world. A study published in 2015 shows the presence of Caricature On the skin of many amphibians in Guyana, in all kinds of environments. In 2019, more than 7 % of the tested Guyana tree frogs were contaminated.

Far from Guyana, in the Pyrenees, Caricature It was first discovered in 2006 in the lakes of the Néouvielle mountain range. Then it infected a tenth of the individuals tested. Four years later, more than half. Starting in 2013, the number of amphibians decreased. Researcher Adeline Loyau studies these lakes at the Toulouse Ecolab. Since 2019, I’ve found about fifty chicks there: a sign of a weak boom in the booms ?

Onychomycosis has been observed since the eighties, parasitic fungi Caricature He conquered the entire planet in forty years. But pollution does not necessarily lead to disease. In Guyana, amphibians are positive for Caricature They are often healthy carriers. Elsewhere in the world, entire populations have been decimated. Why are some so weak? ? From the climate crisis to the disappearance of plankton and the role of amphibian farms, an overview of human activities harmful to frogs.

For hours, teams from nature reserves in Guyana have been examining the Amazon rainforest to monitor an epidemic of onychomycosis in dendrobates. © Claire Marine Saddles / Reporter

a “ Sam’s Cocktail ” Insecticides

Many scientific teams are trying to understand why Caricature It destroys some inhabitants while shunning others. Dirk Schmeller, who also works at Ecolab in Toulouse, explains that you have to think of illness as the top of the pyramid. Four factors make up the square base of the pyramid and contribute to the emergence of the disease: the host, its microbiome, its environment and finally the pathogen.

Some host species appear more vulnerable than others. Onychomycosis appeared in 2004 in the El Copé Natural Park in Panama. Of the 63 existing species, 25 have disappeared in a few years and 17 have declined sharply. About two dozen survived the arrival of the parasitic fungi. A recent impact study showed that out of the 500 species of amphibians affected by mycosis fungoides worldwide, only 12 are % of the population is showing signs of recovery with more than a third experiencing a significant decline.

Disease depends not only on the confrontation between the animal and the pathogen. The environment and all the microorganisms that live in it also play a role in the onset of symptoms. © Claire Marine Saddles / Reporter

However, significant differences can be observed within the same species. Dendrobates are particularly susceptible to mycosis fungoides: parasite name – Batrachochytrium dendrobatidis – Its name goes back to a staggering epidemic on an American farm. The health of dendrobates in Guyana shows that other factors explain the onset of the disease.

The microbiome, for example, has also been studied by Dirk Schmiller’s team. Consider the interface between the host and its environment. As in humans, the skin of amphibians is colonized by bacteria, fungi, and viruses. A complex balance is created and this micro-ecosystem can be protective. in France, Caricature It occurs in many environments but mainly decimates isolated amphibian groups. Their microbiome may be less effective at preventing the onset of disease, and scientists call them naive population ».

Another reason for weakness: pollution. Adeline Loyau and Dirk Schmeller recently discovered a Sam’s Cocktail » Pesticides in the Pyrenees lakes they study. The latter destroys some plankton. However, laboratory tests have shown that plankton can limit the reproduction of the fungi Caricature In the water. Thus, their disappearance can aid the progression of the disease.

A line of deadly strains appeared in amphibian farms

Likewise, environmental stress affects the health status of animals. Climate change brings its share of bouts of stress – droughts, for example. This worries Mathieu Chouteau, who specializes in development at CNRS From Cayenne: Nature is countless interactions between living things, everything is very tightly connected. Once the reaction is removed, it represents an uncontrollable sequence. » If stress damages the amphibian’s immune system, nothing can stop onychomycosis. Weaker, entire peoples get sick and die.

On the skin of this eggplant, mucus forms an ecosystem in itself, its own microbiome. The boundary between an animal and its environment is porous. © Claire Marine Saddles / Reporter

Story of the Caricature It is already linked to human activities. This mushroom belongs to a very common family. It is found naturally on the skin of several species of frog. A 2018 genetic study identified an emerging and diverse killer strain in amphibian farms. Contact with different strains of fungi would have favored their virulence, as contamination of very dense groups of farms is ideal for the emergence of variants.

However, thousands of amphibians are bred each year for food and research. By escaping, some spread disease-causing strains Caricature in nature. Mushrooms have continued to evolve and diversify in every environment they encountered. In 2013, new Shetrade Lethal to salamanders has been identified in Europe. It was submitted by horror fans from Holland. If it can be detected, it is because surveillance of amphibians has been greatly enhanced in the face of the ravages of mycosis fungoides.

Favored the reproduction of amphibians and an epidemic of onychomycosis. 90 species have already succumbed to this disease. © Claire Marine Saddles / Reporter

Between 2009 and 2012, a large epidemiological study was conducted in Europe. From Montpellier, Claude Mayaud and his team took samples taken from most of the reserves and nature parks in France. In 2011, they created the Alerte-Amphibiens website to collect reports on deaths and diseases. The data collected during the ten years of its existence is still being analyzed. A new platform should see the light of day soon.

We sent pictures of clinical signs that can’t be linked to anything, which means there are diseases that we don’t know about at the moment. Mass deaths in the Alps also allowed us to detect Rana virus, which has not yet been reported in France »Claude Maioud explains. Regarding Caricature, it is now present throughout the territory, like other pathogens that are still unknown. Scientists are watching the progress and occurrence of pollution, but they remain helpless in the face of mass deaths. Onychomycosis can be treated in captivity and in some very specific environments, not on the scale of a forest or lake. From Cayenne to Toulouse, via Montpellier, researchers agree on one point: the only room for maneuver in the face of damage from these emerging diseases is to enhance amphibian resistance by protecting their environment.

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