The Fungus That Killed Frogs—and Led to a Surge in Malaria

A global fungal pandemic wiped out amphibians, destroyed biodiversity, and ultimately increased human illness. Now a second similar pathogen is on the way.

Karen Lips has never forgotten the silence. It was the early 1990s; she was finishing her PhD in tropical biology, and had come back to a research site in Costa Rica, a protected reserve high up in the mountains, after a short break. On her previous visit, the air had been full of the sounds of the frog species she was studying. Now, inexplicably, almost all the frogs were gone.

She was mystified and alarmed, but she arranged to move her research sites further south in Central America, into the mountains of Panama and eventually as far south as the border with Colombia. Wherever she and her colleagues went, though, they found a wave of death preceding them. “By the time we got there,” she recalls, “it was already too late.”

What Lips was seeing as a graduate student—she is now a tropical ecologist and a professor of biology at the University of Maryland College Park—was the arrival in the Americas of a fungal pandemic that had been sweeping the globe. Batrachochytrium dendrobatidis, a virulent spore-forming pathogen generally known as Bd, originated in Asia and probably spread for decades before its damage began to be noticed in the 1980s. Since then, scientists estimate, 90 amphibian species have been made extinct by the fungus, and more than 400 were severely harmed by it, losing up to 90 percent of their populations. Altogether, more than 6 percent of all amphibian species in the world were decimated or destroyed, a catastrophe that one research group has called “the greatest loss of biodiversity attributable to a disease.”

Over the years, Lips and other scientists were able to document what happened to the ecosystems that lost those frogs and other amphibian species: surges in populations of insects (which the frogs would have eaten) and drops in populations of snakes (which would have eaten the frogs). But what looked to ecologists like profound environmental disruption was invisible to most of society, because it occurred far from human habitation, in locations where surveillance is patchy and expensive.

Now, though, there’s evidence that the damage caused by Bd has rippled into the human world.

In the journal Environmental Research Letters, Lips and several other researchers report that the devastation of frog species in Costa Rica and Panama caused an unforeseen surge in human malaria cases lasting eight years after the pathogen arrived—likely because, with no tadpoles to eat their larvae, mosquito populations boomed. It’s the first published evidence that the worldwide amphibian die-off has had implications for people.

“This paper is a wake-up call,” says John Vandermeer, a professor of ecology and evolutionary biology at the University of Michigan, who was not involved in the study. “It makes the point that the problem is not just that we're losing biodiversity, and biodiversity is wonderful and pretty and beautiful. It’s that the loss of biodiversity does have secondary consequences on human welfare—in this particular case, human health.”

Though Bd swept through Central America from the 1980s to the 2000s, the analysis that demonstrated its effect on human health could be accomplished only recently, says Michael Springborn, the paper’s lead author and a professor and environmental and resource economist at UC Davis. “The data existed, but it wasn’t easily obtainable,” he says. Over the years, though, county-level disease records were digitized at the ministries of health in Costa Rica and Panama, providing an opportunity to combine that epidemiology in a particular statistical model with satellite images and ecological surveys revealing land characteristics and precipitation, as well as with data on amphibian declines.

“We always thought if we could link [the die-off] to people, more people would care,” Lips says. “We were pretty sure we could quantify changes in bugs, or frogs, or the water quality, or fish or crabs or shrimp. But making that connection to people was so difficult, because the effect was so diffuse, and it happened across such a large area.”

But precisely because Bd swept through Central America in a specific pattern, from northwest to southeast—“a wave that hit county after county over time,” Springborn says—it created a natural experiment that allowed the researchers to look granularly at Costa Rica and Panama before and after the fungal wave arrived. In the health records, they could distinguish that malaria rates were flat in counties (called cantons or distritos) before the Bd fungus tore through, then began to rise afterward. At the peak of the disease surge, six years from the arrival of Bd in an area, malaria cases rose five-fold.

And then they began to fall off again, beginning about eight years after the lethal fungus arrived. Researchers aren’t sure why, because most amphibian populations haven’t bounced back from the fungal onslaught. Though some populations appear to be developing resistance, most have not recovered their density or diversity. Since the fungus lingers in the environment, they remain at risk.

There’s a missing piece in the researchers’ analysis, which is that there is no contemporaneous data to prove that mosquito populations surged in a way that promoted malaria. The surveys they needed—of mosquito density during and after Bd’s arrival, in the 81 counties in Costa Rica and 55 in Panama—simply don’t exist. That makes it difficult for them to determine why malaria fell off again, particularly since frog populations haven’t revived. Springborn theorizes it might be due to human intervention, like governments or organizations noticing the malaria spike and spraying insecticides or distributing bed nets. Or it might be that ecosystems recovered even though the frogs did not, with other predator species taking advantage of the emptied niche to keep mosquito counts down.

But the fact that malaria rates came back down again doesn’t invalidate the findings’ importance. “For the most part, Bd has been a story of the consequences for amphibians, basically: Isn't it too bad to lose this charismatic group of organisms?” says James P. Collins, an evolutionary ecologist and professor at Arizona State University. (Collins has some connection to this research; he oversaw a grant that the National Science Foundation made to Lips in the 1990s.) “It’s been an embedded assumption that reducing the world’s biodiversity is bound to be harmful. Connecting the dots to real implications for humans is a nice piece of evidence for understanding the consequences.”

It’s important to have that evidence, because a second fungal wave is coming: a related pathogen called Batrachochytrium salamandrivorans, Bsal for short, that is lethal to salamanders and newts. Like the frogs and related amphibians decimated by Bd, salamanders are crucial members of wild ecosystems—and it happens that North America harbors about 50 percent of the world’s species, making them pillars of biodiversity for US woodlands and wildlife.

It’s been shown over years that Bd’s emergence was not due solely to natural forces. Instead, its spread across the world was turbocharged by international trade, as wild amphibians hitchhiked in cargo, and captured wildlife were sold legally or illegally into the enormous global market for exotic pets. International conventions on trading some species of wildlife were instituted in the 2000s in hopes of controlling the fungus, but genomic analyses published a decade later showed new strains circulating the world, indicating that fresh imports were occurring despite the bans. That’s relevant to the future of Bsal as well. To slow down that second fungus, the US Fish and Wildlife Service made it illegal in 2016 to import 201 species of salamanders. But experts have argued for years that federal resources for intercepting and inspecting even legally traded animals are inadequate.

The new evidence that Bd imperiled human health, combined with years of proof that it destroyed wildlife globally, might be enough to prompt further regulations to slow Bsal’s advance while it might yet be controlled. At the least, it serves as a warning of how difficult it can be to predict pandemics ahead of time—and how hard it can be to put the brakes on once they’re underway.

The Fungus That Killed Frogs—and Led to a Surge in Malaria

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