A True Horror Story: The Amphibian Chytrid Fungus Pandemic
“To see a frog die of chytrid is probably the worst experience I’ve ever had.”
- Anthony Waddle, conservation biologist, Macquarie University, Sydney Australia
“Selfishly, I think it’s really important that we recognize that fiction like ‘The Last of Us’ is a great avenue for people to understand fungal infections, as long as we can hamper down the fears and enjoy it as fiction.”
- Emily Hutto, Associate Video Producer and Editor, MedPage Today
The opening scene of the hit fictional TV drama “The Last of Us” features a mutant fugus that transforms humans into zombies. Prompted by the scene, Dr. Neil Stone, an infectious disease specialist, wrote on Twitter, “This has done more to raise awareness of the threat of fungal infection than anything the medical profession has managed over the decades.” If this is indeed the case, I’m striking while the iron is hot by telling a true horror story about a fungal infection that is devasting amphibians worldwide - chytridiomycosis.
.What is chytridiomycosis? The fungus Batrachochytrium dendrobatidis causes the disease “chytridiomycosis.” Both are mouthfuls to say and to remember. Fortunately, the fungus also goes by Bd, and the name of the disease it causes is commonly referred to as “amphibian chytrid fungus disease” (chytrid is pronounced KY-trid).
Bd is a waterborne pathogen that possesses zoospores that it disperses into aqueous environments. The zoospores use flagella for locomotion through water systems until they reach a new host, entering it via the skin. Infected amphibians are commonly found in a lethargic state, refusing to move when stimulated.
Bd grows best between 17 and 25C (63-77 F); higher temperatures can actually cure frogs of chytrid fungus disease. Researchers have shown that in nature the more time frogs spent at temperatures above 25C, the less likely they were to be infected with Bd.
Many experts consider Bd the most deadly invasive species on the planet. It is the main driver behind amphibian extinctions globally with profound effects on many ecosystems. While frogs and salamanders have been hit hardest by Bd, researchers in a 2019 Science review suggested chytridiomycosis played a role in the decline of at least 501 amphibian species during the past 50 years (90 of these species were confirmed or presumed to have gone extinct).
Recently, researchers mapped the geographic range of Bd; it spans much of the world. Currently, its impact is most readily seen in Central America, eastern Australia, South America, and western North America, but it is also rapidly spreading in Africa, the continent where it got its start.
When did Bd emerge? Despite the notion that chytrid fungi are new or “emerging” pathogens, they likely evolved from a fungal ancestor over one billion years ago. It wasn’t until 1993, however, that Australian Professor of Biology, Lee Berger, discovered that amphibian chytrid fungus disease was the cause of a massive mortality event involving several species of frogs. In further studies conducted on preserved amphibian specimens, researchers showed that Bd had been present in Australia since 1978 and that the disease may have originated in Africa as early as 1938.
The quest to save amphibians from Bd. Researchers claim that chytrid fungal disease may just be the largest fungal pandemic ever. (While a major tragedy for amphibians, Bd doesn’t infect humans.) Given the enormity of the destruction to amphibians, researchers are in earnest pursuit of improved measures to protect these marvelous creatures from this fungus, and conservation biologists are critically important in this quest.
Central to the approach of conservation biologists is their ecosystem perspective: “Everything is connected.” In a March 20, 2023, New Yorker article, “A Little Known Planet,” American journalist and author Elizabeth Kolbert reminds us that insect species are by far the most prevalent animal species on Earth—at least two million species. And like amphibians, they too are in dramatic decline (the so-called “insect apocalypse”). Given that insects are a favorite food of amphibians, it’s likely that conservation biologists are concerned about the impact of this double whammy (insect loss and chytrid fungus) on amphibian survival.
Has Bd spilled over into other animal species? More than 60% of emerging infections in humans are zoonotic, that is, they spilled over from other animals to humans (often involving an intermediary like mosquitoes or ticks). Researchers have not found the etiologic agent Bd that causes amphibian chytrid fungus in any non-amphibian species such as fish, reptiles, or any of our 5,400 more closely related mammal species. Nonetheless, the fear that such a transfer of a fungus toHomo sapiens is the underlying basis for the drama “The Last of Us.”
Interestingly, the fungus referred to as “Cordyceps” in the fictional drama is real; its official name is Ophiocordyceps unilateralis. But as reporter Giulia Heyward, pointed out in her January 30, 2023, NPR article, “The zombie fungus from ‘The Last of Us’ is real—but not nearly as deadly;” it sticks to insects like carpenter ants. And while in the movie Cordyceps is capable of mind control of infected humans (turning them into zombies), this never could happen as the human body temperature is too hot for the fungus to grow.
We Homo sapiens can relax about becoming infected with Bd. Nevertheless we should be mourning the devastation of amphibian populations worldwide.