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An African clawed frog.

An African clawed frog, Xenopus laevis.

Photograph by Joel Sartore, National Geographic

Jane J. Lee

National Geographic

Published May 15, 2013

Apocalyptic, catastrophic, devastating: All words used to describe chytrid fungus infections that are wiping out amphibians around the world, including hundreds of frog and salamander species.

“It did a really huge number on an entire genus of frogs in Central America,” said Marm Kilpatrick, a disease ecologist at the University of California, Santa Cruz (UCSC). The fungus probably caused several species of this harlequin frog (Atelopus) to go extinct, he added. (Related:“Endangered Frogs Get Helping Hand.”)

Chytrid is also largely responsible for endangering California’s mountain yellow-legged frog (Rana muscosa).

"It's the single biggest threat to vertebrate diversity in the world," Kilpatrick said. (Related: "30 Amphibian Species Wiped Out in Panama Forest.")

The fungus, which seems to attack only amphibians, causes a thickening of the infected amphibian’s skin, preventing the animal from breathing properly and interfering with its electrolyte balance. The infection can eventually lead to cardiac arrest, although some frog species are better able to cope with it than others.

A new study delving into how this fungus spreads has now linked chytrid outbreaks in California—one of the more recent areas experiencing huge amphibian die-offs—to the spread of the African clawed frog (Xenopus laevis).

And the study’s implications could extend far beyond California, providing scientists with a potential road map showing how a devastating infection continues to spread around the world.

Until now, direct evidence of the chytrid fungus in African clawed frogs in regions of the world that have seen big amphibian die-offs has been missing, write the authors of the new study.

"I was surprised that nobody did this study before us, actually," said Vance Vredenburg, a conservation biologist at San Francisco State University and lead author of the new study, published May 15 in the journal PLoS ONE.

That could be because labs like his have been in crisis mode, scrambling to find a way to combat the fungus and save as many amphibians as possible, rather than trying to parse chytrid’s origins.

A Questionable Path

Chytrid's origins and how it spread have long been a big unanswered question for researchers, said Kilpatrick, who was not involved in the study.

Researchers in South Africa first proposed in 2004 that the African clawed frog was responsible for the spread of chytrid fungus around the world, said Vredenburg.

That earlier study suggested that the spread of chytrid was aided by the pet trade in African clawed frogs and by the animal’s widespread use as a research animal. Until the 1970s, the frog was also used in many hospitals as an indicator of human pregnancy; injecting the urine of a pregnant woman into the frog caused it to lay eggs.

Individual frogs that escaped or were released into the wild by hospital workers or pet owners may have carried the chytrid fungus, introducing the pathogen to new habitats around the world.

New Techniques

But a new technique developed by Vredenburg in 2011 allowed researchers to quickly evaluate whether amphibians, preserved as museum specimens, had the chytrid fungus or not.

Scientists could quickly swab the skin of an amphibian, analyze any DNA they picked up, and determine whether the chytrid fungus had infected the animal.

In what Vredenburg called the “old-school” way of testing for the fungus, analysis of a single tadpole required students in his lab to examine 200 skin samples under a microscope.

The new technique enabled Vredenburg’s research team to test 201 preserved frogs in the genus that includes the African clawed frog, collected from Africa and California between 1871 and 2010. The specimens, housed at the California Academy of Sciences in San Francisco, were all caught in the wild.

The researchers confirmed that wild specimens of the African clawed frog did indeed carry the chytrid fungus, yielding direct evidence of infection in this species outside of Africa.

That confirmation, combined with correlations between recorded instances of African clawed frogs around California and outbreaks of chytrid fungal infections, brings researchers one step closer to figuring out how this deadly infection became a global scourge.

Blame Game

But the blame for chyrid’s spread might not fall squarely on the African clawed frog, Kilpatrick cautioned.

The American bullfrog (Rana catesbeiana) is a popular source of frog legs served in restaurants around the world. And their movements can also be correlated to the spread of the chytrid fungus.

"The trade or movement of those two species has been responsible for the spread of [chytrid]," Kilpatrick said.

This pathogen could also have been present around the world in a nonlethal form, said Anna Savage, an evolutionary geneticist with the Smithsonian Conservation Biology Institute, who was not involved in the study.

Perhaps something in the environment changed, so that amphibians were no longer able to withstand the chytrid fungus, explained Savage, a former National Geographic grantee.

We’re still trying to compile basic information on this pathogen and how hosts, such as frogs, respond to it, she said. “So just knowing where it came from and how it spread, that’s really important information in terms of management strategies in dealing with this.”

Vredenburg is currently working on building a living library of beneficial bacteria that could help amphibians around the world combat chytrid infections. (Related: "Amphibian Bacteria Fights Off Deadly Fungus, Study Says.")

His team is working to isolate bacteria native to amphibian populations that help some species resist chytrid. They're hoping to culture those beneficial bacteria and dose any infected populations, giving their systems a boost to help fight off the fungus.

"I'm desperate," Vredenburg said. "I don't want to see any more massive die-offs of frogs. I'm done with that."

2 comments
Lance Richards
Lance Richards

I'm surprised at how little these scientist seem to know about African Clawed Water Frogs, I guess it shouldn't be all that surprising since most pet stores categorize them incorrectly and sell them to people who think they are African Dwarf Frogs which stay very small.  This is a large part of the reason why people let them go into the wild after their tiny little frog ends up being 9" long and devours anything that will fit in it's gigantic mouth.

What's also surprising to me is the fact that they haven't caught on to the fact that this fungus not only affects amphibians as this article emphatically states but it drastically affects any fish that are in the water with them.  These frogs enjoy mainly shallow water though so any fish that are in there, generally get eaten pretty quickly due to their small size.  

I on the other hand, I have successfully, albeit accidentally cultured this fungus in a large fish tank with very large and aggressive fish present with the frogs.  The thing about these frogs is that they are one of the heartiest little creatures on the planet, they could survive in a cesspool for a significant period of time, it's truly amazing.  In their natural environment these frogs often times live their entire lives in a single puddle, they don't even need a pond and if by chance the puddle dries up, they can also hop a significant distance trying to find more water before drying out.

This fungus came very, very close to killing a beautiful and gigantic all white Koi fish that I just recently gave away after nursing it back to health.  Until I started reading about this fungus, I had no idea why some of my very expensive, very large & very aggressive fish were dying on me out of the blue until I took some samples & put them under a few different microscopes that I have.  I assure you, if exposed for an extended period of time, this still will kill anything that happens to be stuck in the water with these tremendous creatures.

The reason this problem has arose is that none of the frogs "captured" in CA actually originated there, all you need is a male & a female to be dumped in just about any water source that doesn't freeze over entirely and within two months, there will be hundreds of little frogs.  I've bred these frogs very successfully, currently i'm trying to breed the albino female with a non-albino male which would have worked already had my female not become morbidly obese since I've had her for so long.  It's amazing to watch them breed and to watch them go from egg to frog, it's like watching evolution right before you eyes, the only problem is that if you are not careful, you'll end up with far too many frogs to take care of or even get rid of in a responsible manner.  These frogs will do the nasty for 12 hours at a time, the male wraps it's arms around her waste and just holds on for dear life as she drags him at full speed, all over the tank for hours at a time.  In a single night, a female African Clawed Water Frog can easily lay 300-500 eggs but thankfully they will eat every last one of them within a day if you allow them to do so.

It's an awesome process and it's easy to see why they are so invasive because within 24 hours of the egg being laid & fertilized, it has already changed from a little round dot with a membrane around it, to a little white squiggly thing that can only swim very tiny distances before it sinks to the bottom and tries again.  Within a week they go from freshly laid egg to full fledge tadpole, the albinos are particularly interesting because as tadpoles they are not just white or translucent but they are entirely transparent.  So much so that you can actually clearly see the tadpoles heart inside of it's body and every time it pumps you can see the tiny little veins fill up with blood.  Easily inside of just one month the tadpoles with grow legs, lose their tales and become full fledged baby frogs or froglets.  If you breed them then this is the time to sell them because no one will want them once they get huge but that's the very problem at it's core, I live in MA so they cannot survive the winter here and are not a real threat to our ecosystem but I guarantee that if I were to let a male & female go into a small pond around April, that pond would be filled with hundreds, if not thousands of frogs by the time it got cold enough to freeze over but it would take a deep freeze to kill off all of them, that's how hearty they are and why they are such a threat, not to mention illegal in many states.

I find it sadly amusing that I am a complete amateur without a lab to work in, yet I seem to know a lot more about both, this species of frog as well as the different kinds of fungai that they carry.  Especially considering the places where you can most certainly find these frogs living, I'd bet dollars to donuts that large quantities of these frogs can be found throughout our sewer systems in all of the southern and southwestern states.  I would not be surprised at all if these things could easily survive in water that's contaminated with just about anything, even nuclear waste but if that were the case then I'd be terrified to see what they would become considering the biggest one I have already looks like Frogzilla.   

These are really cool, fun and very easy pets to have but don't be an a hole! If they get too big or you don't want them anymore for some other reason then there are plenty of pet stores that will be happy to take them off your hands because they require almost no maintenance at all, they will eat as much as you feed them but they can also go a very, very long time without any food or even a light or heat source so it costs nothing for pet stores to take them off your hands and they will profit off of them.  That being said, do the right thing if you are getting rid of these guys and don't let them go anywhere that doesn't get very cold in the winter or they will take over the entire body of water and possibly even spread to other bodies of water if close enough to one another.

These frogs can even survive in brackish or even full saltwater for some time before it dries them out so don't be surprised if you find them in estuaries or other places that connect to the ocean, they actually fair quite well in brackish water surprisingly enough.  

There's  no question though at least in my mind, that this fungus found on these frogs certainly spreads to any fish that are large & aggressive enough to survive the mere presence of these frogs, it takes a significant amount of time to manifest and will do so faster the more stagnant the water is.  I don't know if the fish can carry & transmit it to other critters but eventually it will kill even the largest & most powerful of fish.

Sorry for the diatribe, I just find that whenever I find stuff about these critters on the internet, they never seem to give great information about them.   Going on four or five years now, even when I had about 50 of them, I have never had a single frog die while in the water even when I had my biggest one in a tank with three massive & extremely vicious turtles (red eared & yellow bellied sliders), she still managed to survive their wrath but that was back before I really knew any better.  The only time in the last five years that any of these frogs have died has been entirely because they managed to jump out of the tank.  They are escape artists, especially if unhappy for some reason and the biggest frog is able to squeeze it's fat body through the tiniest of openings and boy can they jump when they want to.

William Cody
William Cody

Vance said he was surprised no one had done this study before, that's because the experts were buy blaming Global Warming

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