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Fungal diseases that are decimating frogs could be more widespread than expected

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Deadly spores lurk in water and infect the skin of creatures they touch. Spreading on contact and then invading the body, this fungal disease causes ulcers and flaking so severe that the skin peels off in sheets.

The joints in the leg begin to lock, and soon after symptoms appear, the disease can lead to cardiac arrest and death.

Chytridiomycosis, the deadliest disease to afflict vertebrates in history, has wreaked havoc on amphibians for decades, including frogs, toads and salamanders.

The disease is not known to infect humans, but scientists warn that these outbreaks are crucial for understanding how fungal pathogens spread and learning to understand a mass extinction event plaguing our amphibians. friends.

The disease has already decimated amphibian populations in the Americas, Australia and parts of Europe, and the latest research has shown it may now be spreading across Africa. The disease can kill hordes of animals without scientists realizing it, warned Dr. Vance Vredenburg, a professor of biology at San Francisco State University and a research associate at the Museum of Vertebrate Zoology in the United States. University of California at Berkeley.

“There could be hundreds of species (in Africa) that could be put at risk by this pathogen,” said Vredenburg, co-author of a new study published March 15 in Frontiers in Conservation Science that reveals for the first time the ubiquity of chytridiomycosis in Africa.

For amphibians, the disease makes the Black Death that devastated Europe in the Middle Ages “look like a drop in a bucket,” Vredenburg said.

Chytridiomycosis is caused by a pathogen called Batrachochytrium dendrobatidis, or Bd for short. The disease has been a major contributor to the threat of extinction facing amphibian species around the world. About 41% of amphibians are currently endangered, according to the International Union for Conservation of Nature’s Red List of Threatened Species.

Understanding Bd and how it spreads has been a major career goal for Vredenburg. He began studying the pathogen in the late 1990s, tracking Bd around the world and observing its deadly impact. He remembers visiting the Sierra Nevada between 2004 and 2008, where he witnessed a particularly brutal epidemic and “saw thousands of frogs die before my eyes because of this disease”.

“Honestly, before this happened, I didn’t believe it,” he said. “Scientists did not believe that one fungal pathogen could (affect) hundreds of species. But in fact, the story of the nightmare is true. This unique pathogen has caused the greatest mortality of vertebrates ever recorded. »

Vredenburg said it had “changed the way scientists view disease and their ability to actually control wildlife populations.”

In places where scientists have observed Bd’s devastating impact more thoroughly, the “disease has caused the decline or complete extinction of more than 200 species of frogs and other amphibians,” according to the Wildlife Health Lab. from Cornell University.

Scientists previously thought that amphibians in Africa had been relatively spared the plague of Bd. But Vredenburg set out to see if the pathogen was present in museum specimens of amphibians from Africa and enlisted colleagues to overseas to collect live samples from the wild. He also delved into previous studies of the continent. In total, more than 16,900 animals were analyzed.

Signs of Bd in Africa were low – less than 5% – from the 1930s to the late 1990s, according to the study. Then the cases exploded.

Infection rates jumped to over 17% and again to almost 22% in the 2010s. The most severe outbreaks seemed to be happening in places where scientists had the most data, including countries such as Burundi, where infection rates exceeded 73%.

These rates are concerning, Vredenburg added, because they could signal that amphibian populations are disappearing en masse.

“Unless you really look, you might not notice they’re gone until they’re already gone,” he said. “We should really understand why this is such a problem in these vertebrates. They have been around for 400 million years.

The million dollar question for scientists is why there is such a sudden and dramatic threat to their existence, Vredenburg said.

It is difficult to determine how amphibian populations fight disease. Most frogs and their kin are nocturnal, so humans don’t always come into contact with them when they get sick. The pathogen is also fast-moving, killing infected people soon after symptoms begin. And amphibian bodies decompose quickly, Vredenburg added, erasing evidence of mass mortality before scientists could uncover the corpses.

However, Bd doesn’t always trigger a deadly outbreak, a positive — but puzzling — fact the researchers factored into the new study. The pathogen can be found in some populations of frogs that manage to survive, much like humans adapt to pandemics.

The amphibians that the study authors collected from Africa, Vredenburg noted, did not always show physical symptoms of the disease, although they tested positive for Bd. These populations could still die or they could have a natural defense against disease. And that’s part of the reason why Vredenburg urges further study of the presence of the disease on the continent.

Scientists have also been able to treat and immunize frogs in captivity, Vredenburg added, although trying to do this in the wild is nearly impossible. Frogs, of course, cannot coordinate the global distribution of vaccines the way humans can during a pandemic.

Scientists say humans should take certain steps to mitigate the spread of Bd. Vredenburg noted that the fungus can spread through the exotic pet trade if an amphibian is caught in one location and then released into the wild elsewhere. . Stopping this kind of trading can help thwart the spread of Bd.

Once Bd spawns in a given location, it spreads on contact. Frogs can contract chytridiomycosis from the pathogen by swimming in infested waters, where the fungus lives, or by rubbing against another infected animal.

Amphibian populations are already under threat due to habitat loss, noted biologist Dr JJ Apodaca, executive director of the US non-profit Amphibian and Reptile Conservancy, who did not participate in the news. study. Apodaca said the study offered valuable new insights into the spread of such a devastating pathogen.

The disease is “the straw that breaks the camel’s back” when it comes to amphibian conservation, he said. “When animals are stressed about habitat loss, all of these things come together, and then disease comes and ends it.”

Apodaca focuses on populations of frogs and other amphibians in the United States, but learning how Bd spreads in Africa helps understand the origins of the pathogen and the causes of outbreaks.

“My biggest wish would be for people to just understand that these issues exist,” Apodaca added. Threats like Bd will “have a big flash in the pan news event and then the next day is the next issue. … But all the while, our native wildlife, amphibians and reptiles are just getting hammered.


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