Frogs and their amphibian relatives are being decimated by a deadly fungal infection that is contributing to the endangerment and extinction of hundreds of species around the world.
The disease, chytridiomycosis, is caused by the Batrachochytrium dendrobatidis Fungus that infects the keratin in the skin of amphibians. The infection has been found to cause almost 100 percent mortality in some species, while others are only mildly affected.
The fungus had spread to populations around the world, but so far Africa has been largely unaffected. According to a new study published in thejournal Frontiers in Conservation ScienceChytridiomycosis has been steadily increasing across the continent since 2000.
“This is a microscopic fungus that infects the skin of amphibians and is fatal to many species,” said Vance T. Vredenburg, a chytridiomycosis researcher and co-author of the limits in conservation sciences paper, and professor and vice chair of the Department of Biology at San Francisco State University news week.
“Indeed, the disease caused by this fungus, called chytridiomycosis, represents the worst case in recorded history of any vertebrate disease. Hundreds of species were affected (over 500), and many died in mass die-offs from invasion shortly after the fungus.”
chytridiomycosis kills frogs by infecting their skin and causing dander and other symptoms such as ulcers. Frogs and other amphibians absorb oxygen through their skin and perform crucial ion transfer across their skin layers.
“The best evidence of how it causes death suggests that the fungus interferes with essential functions of frog skin (essential ion uptake, respiration),” said Louise Rollins-Smith, associate professor of pathology, microbiology and immunology at Vanderbilt University news week.
Batrachochytrium dendrobatidis has been linked to the extinction of over 200 amphibian species including the southern gastric brooder, northern gastric brooder, sharp-snouted diurnal frog and southern diurnal frog in Queensland, Australia alone. The fungal infection is said in a 2019 study from the journal Science be responsible for the population decline of 39 percent of amphibian species worldwide.
It was first found in Queensland, Australia in the 1990s after several frog species were found dead. It is present all over the world but is most widespread in South and Central America, Australia and North America. The mushroom It is thought to spread via spores released into water by amphibian skin.
“There appear to be multiple pathways (pet trade, food trade – where frogs are consumed by humans or are accidentally moved in trades, e.g. bananas), but humans are involved in all of them. We don’t yet know how a landscape will develop, but we can follow it,” said Vredenburg.
“The fungus was first noticed in Australia in 1993 and described (giving a scientific name) in 1999. We used collections of amphibians in natural history collections to track the spread of the fungus around the world,” he continued. “We wrote this paper because while there are hundreds of scientific studies on this pathogen, few have been conducted in Africa, despite the fact that over 1,200 amphibian species are believed to be present there.”
The reason it took so long to fully conquer Africa is still a source of confusion for scientists.
“We do not know it [why Africa was spared until recently]. It could just be a coincidence,” said Vredenburg. “We know that the fungus spread in Australia in the 1990s, in California in the 1970s, in Mexico and Central America in the 1980s, and further south in the 1990s. In Europe it was later similar to Africa.”
Alternatively, the lower numbers in Africa could be due to a lack of data collection.
“It’s probably been in Africa for a while but went unnoticed because fewer people are looking for it there,” Rollins-Smith said. “There have been previous reports of its presence in South Africa and Madagascar.”
However, Asia is the latest stronghold in the global fight against the fungus, as the infection has so far only been detected in Indonesia, South Korea, China and Japan, and of the species in those regions only about 2 percent have been infected by people tested.
“Asia looks different. There is no die-off of the fungus, suggesting a longer evolutionary relationship between the pathogen and the host,” Vredenburg said.
Unfortunately, with no cure or vaccine available, there is not much we can do to prevent this infection from slowly spreading through the world’s amphibians, but some frogs are making a recovery.
“In many areas where species were thought to be extinct after these fungal epidemics, some individuals are being rediscovered,” he said. “In California, where two species of yellow-legged mountain frogs have been decimated by the fungus, one, the Sierra Nevada yellow-legged frog (Rana Sierrae) has reported back. In Yosemite National Park, this frog, whose populations collapsed in the 1980s, remains mildly infected, but populations are increasing.”
However, climate change could prove beneficial for the fungus. A 2006 study published in the journal Nature found that increased cloud cover as a result of climate change may cause cooler daytime temperatures and warmer nighttime temperatures, which would be more suitable for growth of the fungus, which thrives between 63 and 77 degrees Fahrenheit.
However, climate change can also negatively impact the fungus as it could cause hotter, drier, and drier conditions where the fungus needs a humid environment to spread via its spores and cannot grow above temperatures of 86 degrees Fahrenheit.
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