Chytridiomycosis, the most deadly disease afflicting vertebrates in recorded history, has wreaked havoc on amphibians for decades, including frogs, toads and salamanders.
The illness isn’t known to infect humans, but scientists warn these outbreaks are crucial to understanding how fungal pathogens spread and learning how to wrap our heads around a mass extinction event plaguing our amphibious friends.
The ailment has already decimated amphibian populations in the Americas, Australia and parts of Europe, and the latest research has shown it may now be making its way across Africa. The disease may be killing off animals in hordes without scientists realizing, warned Dr. Vance Vredenburg, a professor of biology at San Francisco State University and a research associate at the Museum of Vertebrate Zoology at the University of California, Berkeley.
“There could be hundreds of species (in Africa) that could be imperiled by this pathogen,” said Vredenburg, coauthor of a new study published March 15 in Frontiers in Conservation Science that reveals the pervasiveness of chytridiomycosis in Africa for the first time.
For amphibians, the disease is making the Black Death that devastated Europe in the Middle Ages “look like a drop in the bucket,” Vredenburg said.
Why it matters
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 worldwide. About 41% of amphibians are currently in danger, 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 focus of Vredenburg’s career. He began studying the pathogen in the late 1990s, tracking Bd across the world and watching its deadly impact. He recalled visiting the Sierra Nevada between 2004 and 2008, where he witnessed a particularly brutal outbreak and “watched thousands of frogs die in front of my eyes due to this disease.”
“Honestly, before it happened, I didn’t believe it,” he said. “Scientists did not believe that a fungal pathogen could (affect) hundreds of species. But in fact, the nightmare story is true. This single pathogen has caused the biggest die-off of vertebrates that’s ever been recorded.”
Vredenburg said it has “changed the way that scientists view diseases and their ability to really control wildlife populations.”
In locations where scientists have more thoroughly observed Bd’s devastating impact, the “disease has caused the decline or complete extinction of over 200 species of frogs and other amphibians,” according to Cornell University’s Wildlife Health Lab.
Scientists previously thought that amphibians in Africa had been relatively spared Bd’s scourge. But Vredenburg set out to see if the pathogen was present in museum specimens of amphibians from Africa and enlisted colleagues abroad to collect live samples in the wild. He also pored through previous studies from the continent. All told, more than 16,900 animals were analyzed.
Signs of Bd in Africa were low — below 5% — from the 1930s through the late 1990s, the study found. Then cases exploded.
Infection rates jumped to more than 17% and again to nearly 22% in the 2010s. The most severe outbreaks appeared to be in places where scientists had the most data, including in countries such as Burundi, where infection rates topped 73%.
Those rates are concerning, Vredenburg added, because they could signal amphibian populations are dying out in droves.
“Unless you’re really watching, you might not notice that they’re gone until they’re already gone,” he said. “We really should understand why it’s such a problem in these vertebrates. They’ve 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.
The hidden threat of Bd
How amphibian populations are grappling with the disease is difficult to pin down. Most frogs and their ilk are nocturnal, so humans don’t always come into contact with them as they get sick. The pathogen is also swift, killing the infected shortly after symptoms begin. And amphibian bodies decompose quickly, Vredenburg added, erasing evidence of a mass die-off before scientists can discover the corpses.
However, Bd doesn’t always trigger a deadly outbreak, a positive — but perplexing — fact that researchers considered in the new study. The pathogen can be found in some populations of frogs that manage to survive, much as humans adapt to pandemics.
The amphibians that the study authors swabbed in Africa, Vredenburg noted, did not always display physical symptoms of the disease, though they tested positive for Bd. Those populations could still wind up dead, or they may have a natural defense against the disease. And that’s part of the reason Vredenburg is urging continued study of the disease’s presence on the continent.
Scientists have also been able to treat and immunize frogs in captivity, Vredenburg added, though it would be virtually impossible to attempt to do so in the wild. Frogs, of course, can’t coordinate global vaccine distribution as humans can in pandemic times.
Bd’s spread and impact
There are certain steps that scientists said humans should be taking to mitigate the spread of Bd. Vredenburg noted that the fungus can spread through the exotic animal trade if an amphibian is captured in one location and then released back into the wild somewhere else. Stopping those sorts of trade can help thwart Bd’s spread.
Once Bd appears in any given location, it spreads on contact. Frogs can contract chytridiomycosis from the pathogen by swimming in infested waters, where the fungus lives, or brushing up against another infected animal.
Amphibian populations are already at risk because of habitat loss, noted biologist Dr. JJ Apodaca, executive director of the US nonprofit Amphibian and Reptile Conservancy, who was not involved in the new study. Apodaca said the study offered valuable new insight into how such a devastating pathogen has spread.
The disease is “the straw that’s breaking a lot of backs” when it comes to amphibian conservation, he said. “When the animals get stressed from habitat loss, all of those things kind of come together, and then the disease comes in and finishes it.”
Apodaca is focusing on populations of frogs and other amphibians in the United States, but learning about how Bd is spreading in Africa helps to understand the pathogen’s origins and what causes outbreaks.
“My biggest desire would be that people simply understood that these problems exist,” Apodaca added. Threats such as Bd will “get some big flash in the pan news event and then the next day, it’s the next problem. … But meanwhile, our wildlife, our native amphibians and reptiles are just getting hammered.”