amphibians, disease, human impact

Amphibians Facing Extinction: Fighting the Chytrid Fungus to Survive

A chytrid fungus has been ravaging amphibian populations worldwide for years, and the situation has only been getting worse. (Amphibians include frogs, toads, salamanders, and the lesser-known caecilians, which are tropical, limbless, worm-like critters.) Identified in 1998 as parasitizing and killing amphibians, the chytrid fungus species Batrachochytrium dendrobatidis (“Bd” for short) grows within the infected amphibian’s skin, thickening it, and ultimately leads to death by interfering with the amphibian’s normal salt balance. And Bd’s been hitting amphibians hard. In mid-2012, it was announced that 41% of all amphibian species are at risk of extinction. And it’s probably only going to get worse. Bd can easily be transmitted through water, can sit around in a pond for two months outside of a host, and, in controlled laboratory settings, can rapidly kill up to 100% of the test amphibians.

Chytrid fungus xenopus laevis
The African clawed frog (Xenopus laevis), a potential carrier for Bd that humans have transported all over the world for decades. (Image credit: Tim Vickers)

How did this happen? People are quite likely to blame. Bd has probably been in amphibian populations for some time (a museum specimen with Bd was found dating to 1938), but significant population declines have only been noticed since the late 1980s. One possible explanation is that Bd changed, becoming more deadly, due to human actions. Specifically, in 2012 a study revealed that the specific Bd that’s devastating global amphibian populations might have come about from two different Bd strains getting together and recombining, or hybridizing, to form a kind of “super Bd” strain. This is because different types of amphibians were being transported around the world, giving different Bd strains the opportunity to get acquainted. (In addition, the animals were likely stressed, making it harder for them to fight off an infection, and easier for the pathogens to thrive.)

People have been moving amphibians around the world for decades; for example, until the 1960s, the African clawed frog (Xenopus laevis) was globally used as human pregnancy tests. These specific frogs are also sold as pets and used in scientific research laboratories. And, as it happens, they’re one of a few amphibian species known to be resistant to Bd and can serve as carriers. So, we’ve been spreading potential carriers all over the place for decades.

Already it’s thought that Bd has caused 125 to 500 amphibian species to become extinct. An effective treatment for widespread use has yet to be developed, although keeping amphibians at high temperatures in controlled laboratory settings has worked to cure some amphibians (since Bd cannot grow at 82°F, and dies at 99°F).

So next time you find yourself thinking about releasing a pet into an area that it’s not normally found in, you may want to think again. Not only should animals that are not native to an area not be released there, but, ideally, foreign critters shouldn’t even be in any contact with the local, native ones. This is especially important to keep in mind for animals used in schools, since it’s super temping to release non-natives once they’ve outlived their usefulness in the classroom.


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