Cardioglossa melanogaster, a species of African frog David C. Blackburn
A deadly fungus that feeds on the skin of frogs and other amphibians has been rapidly spreading under the radar in Africa. Batrachochytrium dendrobatidis – Bd for short – has surged on the continent in the last two decades, raising concerns it could decimate amphibian populations in Africa as it has elsewhere in the world.
Bd causes a disease called chytridiomycosis, which leads to heart failure in amphibians and has been blamed for dramatic population collapses in the Americas and Australia. “We’re talking about hundreds of species that have been driven to or near extinction by one single pathogen,” says Vance Vredenburg at San Francisco State University.
Researchers think Bd originated in Asia, reaching every continent except Antarctica by the late 1900s. Yet its impact in Africa has remained relatively unexplored. Previous research suggests it has been on the continent since the 1930s, albeit at low levels. Some studies hint at higher infection rates more recently, but that could just be an artefact of researchers looking for Bd more now than in the past.
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To learn more, Vredenburg and his colleagues turned to museum collections of amphibians. Fungi and other parasites often get preserved along with the animals they inhabit, which allows researchers to use museum specimens for studying the history of infectious diseases.
The team took skin swabs from nearly 3000 specimens collected in Africa over the past century. They also tested the skin of 1651 live amphibians found in the wild, and gathered thousands of additional records from other studies of specimens collected between 1852 and 2017.
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Combining all this information, they found that Bd kept a low profile in Africa during the 1900s, consistently appearing in less than 5 per cent of animals tested. But that changed at the turn of the century, with prevalence soaring to around 20 percent across the continent in the early 2000s.
It isn’t clear what caused the increase, but one possible explanation is that trade and the associated movement of people and cargo spread Bd into new areas –as happened previously in other parts of the world, says Vredenburg.
The team has collected “an impressive amount of new data” to complement existing research, says Breda Zimkus at Harvard University’s Museum of Comparative Zoology. She says that many of the regions that show increases in Bd have also experienced declines in their amphibian populations – something the researchers suggest is no coincidence.
In Cameroon, for example, where the team’s data showed Bd prevalence hitting nearly 40 percent in the 2010s, numbers of once-common amphibians such as puddle frogs and long-fingered frogs have been falling rapidly.
The researchers also used the trends they found, along with existing data on Bd’s preferred climate and hosts, to predict where the fungus might go next. Parts of western Africa that have so far had no reports of chytridiomycosis could be particularly at risk, they showed.
Deanna Olson at the US Forest Service says she is pleased to see this kind of risk assessment applied to Bd in Africa. “These are tools that managers can use to identify the most important areas that might be needed for conservation planning… to prevent any further catastrophes for vulnerable species.”
Vredenburg says he hopes the findings will encourage more research on Africa’s amphibians. These animals are “highly understudied”, he says. “There’s probably a lot we could do [to help them] if we had more information.”
Journal reference:
Frontiers in Conservation Science DOI: 10.3389/fcosc.2023.1069490
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