Frogs are also struggling through their own pandemic that has several eerie parallels with COVID-19.
This green tree frog is a spotted leaf frog (Phasmahyla guttata) from Brazil.
Until COVID-19, few people alive today had experienced the chaos and destruction of a really bad pandemic, one that has at times ground businesses, schools and social lives to a near standstill and killed millions globally. But did you know that we aren't alone in being battered by a global infectious disease? Frogs are also struggling through their own pandemic that, according to biologist Kelly Zamudio, has several eerie parallels with COVID-19. Perhaps our own encounters with a pandemic will give us new sympathy for our slimy, bug-eyed friends.
A 2019 study in Science found that a chytrid fungus has contributed to declines in about 500 frog species around the world.
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Kelly Zamudio: So I think it was … still in February the stories that were heartbreaking out of Wuhan … I remember thinking to myself, 'Oh, no, here we go.' … and people at the time were saying, 'Oh, you know, just just another flu,' you know and … I remember telling my family especially … this is going to be big. … And so we were … hyper careful right off from the start … because my perception of it was influenced by … the frog pandemic.
Marc Airhart: This is Point of Discovery. I'm Marc Airhart. Today's guest is Kelly Zamudio, a geneticist who was especially cautious at the outset with the COVID-19 pandemic because she studies a pathogen that is causing another pandemic—for frogs. She looks at the natural genetic factors that help protect some frogs against it and hopes to help conservationists find ways to save frogs. Already about 6% of all frog species are in decline, or have even gone extinct, in recent years because of it. Zamudio aims to take action before still more species disappear.
MA: The global frog pandemic is caused by a fungus in the chytrid group called Bd. It's spread through water or moist soils.
KZ: So the fungus is an interesting one because it creates these zoospores, which are flagellated. They have a little flagellum, a tail that helps them move through the water ... So you can imagine as they emerge out of the skin of an infected frog, these zoospores get into the water very easily.
MA: And when these little swimming spores find a new host, they burrow into the frog's skin, sometimes kill the frog and then generate a new crop of zoospores, those fungus babies, to spread out into the environment again looking for new hosts.
KZ: it's basically an invasion, a breakdown of the skin barrier, by this fungus. The zoospore goes into the skin, it infects, and it starts creating something called a zoosporangium, which is like a little breeding chamber. And in there, it replicates and makes more and more and more and more zoospores. All these sporangia proliferating in the skin basically break down the integrity of the frog skin. And remember that frogs are aquatic, they're amphibious, right? And so a lot of the stuff that happens metabolically … is aided by the skin. So for example, oxygen uptake, you know, … the metabolic equilibrium … gets broken down. So people have actually shown that what the frog ends up dying of is cardiac arrest.
MA: Early in our human pandemic, Zamudio started noticing that COVID-19 has some eerie parallels with the frog pandemic. With COVID-19 -- caused by a virus that spreads mostly through the air – we HUMANS, who spend most of our time surrounded by air, are vulnerable. Likewise, frogs spend much of THEIR time in water and moist soils -- and they breathe, at least in part, through their skin, and so are vulnerable in water. Also, frogs infected with the fungus can have an asymptomatic case of the disease and yet still spread it to others.
KZ: A frog can be infected at low levels, and sort of tolerate that infection, and not show any symptoms, be asymptomatic, just as in a COVID patient that's asymptomatic, yet, they're still shedding these zoospores into the environment … So … that's asymptomatic transmission, just like with us. In frogs, there's another thing which is interesting, which is that some species get so infected that they become sort of what we call 'acute super spreaders.' So they're just … shedding so many zoospores into the environment over the process of their disease until they die, that they're basically just fueling increased transmission.
MA: These three factors — the fact that it spreads through the media we spend the most time in, asymptomatic transmission and super spreaders — have allowed both COVID and the pathogen that causes the frog pandemic to rip through vulnerable populations. And both were able to spread quickly around the world due to the global movement of goods and people. In 2019, Zamudio co-authored a major study that assessed just how bad this frog pandemic is.
KZ: And we came up with this number, which is pretty shocking -- over 500 species for which there seems to be some evidence that chytrid was involved in some way in the decline. … just to put it in perspective, that's 500 out of nearly 8000 species of frogs. That's a big percentage of the frog biodiversity of the world. You know, if, if that percentage of mammals say for example, were threatened with extinction, we'd be losing our minds.
MA: About 90 of those 500 species of frog have already gone extinct, primarily because of this fungus. Zamudio points out that frogs play an important role in controlling insect populations that spread human diseases like malaria or destroy crops. They also play an important role in maintaining healthy ecosystems.
KZ: So frogs mostly eat insects, right? … And so what they're doing is taking energy from a really, really tiny little source, which are insects and moving it up … the food chain. And so what eats frogs? Birds, snakes, small omnivorous carnivores, right? So … they're the gateway between being able to take the energy from these tiny little insects that mostly are eating plants and moving them up the chain.
MA: And before they grow up, tadpoles eat algae in the water, which helps maintain water quality.
KZ: And so there's also some evidence that the ecological associations between organisms that live in the water is affected by … there not being tadpoles there to actually contribute to that.
MA: Coming up: how Zamudio is studying the frog pandemic — and looking for clues that might help end it. Also, one far-out idea for how to save the frogs.
MA: As a geneticist, Kelly Zamudio goes out into the field to find out how many frogs in a population are infected and with which strains of the chytrid fungus. And that process -- diagnostic testing -- turns out to be another parallel with our experience of COVID.
KZ: … we swab frogs, and then we can use a diagnostic test, it's the same test … it's basically a PCR test where you can ask, are there zoospores on the skin of this frog and … how much, given the standardized swabbing that we're doing?
MA: She also brings infected frogs into the lab to find out what genetic factors help them fight off an infection … or not.
KZ: Okay, what happens to this frog's immune system? What genes are getting up-regulated, you know, expressed like crazy in response to being infected, and on the opposite side, … what genes are actually getting dampened or downregulated, as we say?
MA: If members of her team can find natural genetic factors that help protect frogs, she envisions either breeding or genetically engineering frogs to be more resistant, and then using those breeds to restore populations that have disappeared.
MA: About a decade ago, scientists discovered that Bd, the chytrid fungus that causes the frog pandemic, has evolved different strains — some that are relatively mild and localized — and one really bad strain that went global.
KZ: So in Bd, just as with our coronavirus pandemic, there are lineages of Bd that have higher or lower transmissibility, more virulence or less virulence.
MA: The really bad strain, called the Global Panzootic Lineage, transmits more easily and is deadlier than the rest. It's another striking parallel with the virus that causes COVID, which has been rapidly evolving new strains, some of which spread more easily or cause more severe illness. That's made fighting COVID a bit like Whack-a-Mole. But the fact that the coronavirus and the frog fungus each have all those different strains might also hold a key to stopping the FROG pandemic.
KZ: If we can figure out in the genome of this fungus, what is causing this excessive virulence, we call it the Panzootic Lineage -- this lineage that went really crazy in terms of being highly virulent. If we can figure that out, we might be able to engineer a fungus that is less virulent, right?
KZ: So imagine that you engineer a pathogen that competes with the natural pathogen, but has a lower mortality on even susceptible species. And so then you could end up in a situation … where we were getting frogs highly infected with a lineage, but that lineage caused low mortality.
MA: That approach may sound a bit familiar. Right now, vaccine developers can create harmless versions of a virus and inject it as a vaccine to help our bodies recognize a more serious pathogen if we come into contact with it down the road. This is the big idea Zamudio has for helping the frogs.
KZ: … if they survive, then there's presumably some level of immunity that comes from having gone through the disease and … having survived it… think of it as a vaccine, a vaccine of frogs.
MA: That's our show for today. Special thanks to Kelly Zamudio.
MA: Point of Discovery is a production of The University of Texas at Austin's College of Natural Sciences and is a part of the Texas Podcast Network. The opinions expressed in this podcast represent the views of the hosts, and not of The University of Texas at Austin. Our website is at pointofdiscovery.org. If you like our show, be sure and tell your friends. We're available wherever you get your podcasts, including Apple Podcasts, Google Podcasts and Spotify. Our theme music was composed by Charlie Harper. Our senior producer is Christine Sinatra. Thanks for listening — and stay safe out there!