Spored to Death

In March 2007 some wildlife biologists drove out to a cave near Albany, New York, to count bats. New York is home to six species of Chiroptera that overwinter in the state; the animals cope with the cold by hibernating, usually underground, in caves or abandoned mine shafts. Dangling from the ceiling by their toes, the bats descend into a state of torpor so complete it resembles suspended animation. Because they don’t move, hibernating bats are easy to census.

The count was supposed to be a routine affair—so much so that the biologists’ supervisor, Al Hicks, had remained in his office at the state’s Department of Environmental Conservation. When the biologists arrived at the cave, though, they realized something had gone terribly wrong. They pulled out their cell phones to call Hicks. “Holy shit, there’s dead bats everywhere” is how Hicks summarized their message.

The biologists didn’t know what to make of the carnage. They brought some dead bats back to Hicks. He noticed that some of their snouts were covered with a white powdery substance, as if they’d been snorting cocaine, but he didn’t know what to make of the situation, either.

“We were thinking, Oh, boy, we hope this just goes away,” Hicks later told me. “It was like the Bush administration. And, like the Bush administration, it just wouldn’t go away.” In the winter of 2008 more dead bats with powdered noses were found in more caves in upstate New York, and also in neighboring states. In Vermont’s Aeolus Cave, believed to be the largest bat hibernaculum in New England, the stench of dead bats was so overpowering that biologists decided not to even enter.

As it became clear that the problem wasn’t going to go away, researchers at the National Wildlife Health Center in Madison, Wisconsin, began to investigate. They succeeded in culturing the white powder, which they identified as a cold-loving fungus. The fungus, which has since been named Pseudogymnoascus destructans, or Pd for short, feeds on keratin, the protein that makes up, among many other things, human hair and fingernails. Bat skin contains lots of keratin, so bats infected with Pd can end up with tattered wings. They can also die long before that: the fungus causes them to wake up from hibernation, which, in turn, causes them to use up the limited fat reserves they depend on to survive until spring.

For reasons that, in the Covid era, are well known, emergent diseases are disproportionately dangerous. But even for an emergent disease, white-nose syndrome, as it’s become known, has proved extraordinarily deadly. Scientists believe that Pd was imported to New York from Europe, perhaps by an unsuspecting tourist. In the years since it was first identified, millions—perhaps tens of millions—of bats have died. At least a dozen species are susceptible, and populations of three of these—the northern long-eared bat, the little brown bat, and the tricolored bat—have plunged by 90 percent. Last year the United States Fish and Wildlife Service declared the northern long-eared bat to be threatened with extinction owing to the fungus. It has proposed listing the tricolored bat as endangered as well.

White nose has been called “the worst wildlife disease outbreak in North American history.” Despite more than a decade’s worth of research, no one knows how to combat it. There is no known cure for the syndrome or, at this point, any effective treatment. Pd reproduces via tiny spores that can survive for years without a host. These characteristics make it virtually impossible to eradicate and also highly mobile; the spores can be transferred from bat to bat, or from cave to bat and back again. From New York, the fungus has spread to thirty-nine other states and eight Canadian provinces. It is still on the move. Wildlife biologists fear that eventually it could reach Patagonia and kill bats there, too.

Pd has a distant cousin named Batrachochytrium dendrobatidis, or Bd, which afflicts amphibians. Bd makes Pd look like a piker. The frog-killing fungus affects more than five hundred species and probably has already driven many to extinction; these include the southern gastric-brooding frog, from Australia, which, remarkably enough, gave birth through its mouth, and the Rabbs’ fringe-limbed tree frog, from Panama, which was equally remarkable in that males cared for their tadpoles. Like Pd, Bd is drawn to keratin, and like bats’ skin—and, for that matter, humans’—frogs’ skin contains lots of the stuff. Amphibians rely on their skin to regulate their relationship with the outside world, so anything that damages it represents a mortal danger. Among the processes that Bd interferes with is electrolyte transfer. As a result infected frogs suffer what is in effect a heart attack. According to a report that ran a few years ago in Science, Bd is responsible for “the greatest documented loss of biodiversity” ever to have been caused by a pathogen.

Bd and Pd are just two of the disease agents Emily Monosson describes in her sobering new book, Blight: Fungi and the Coming Pandemic. Another is Cronartium ribicola, more commonly known as white pine blister rust, which has killed millions of five-needle pines in the United States and has nearly wiped out western white pines in their native range. A fourth is Fusarium odoratissimum, a strain of which seems likely to destroy the world’s banana crop. Fortunately for the fungus but unfortunately for banana lovers, the commercial crop consists largely of one susceptible variety, known as Cavendish. (Gros Michel, a banana variety popular in the early twentieth century, was killed off by a different strain of fusarium; by some accounts, this disaster was the inspiration for the song “Yes! We Have No Bananas,” which topped the charts in 1923.)

As a rule, fungi don’t like it hot. Most, Monosson reports, thrive at temperatures between 12 and 30 degrees Celsius, or 53.6 and 86 degrees Fahrenheit. Mammals generally maintain a body temperature above this range. (An exception are hibernating bats; when bats go into torpor, their body temperature can drop almost to the freezing point.) High body temperatures are costly because they require lots of calories to maintain. According to one theory, this is why mammals, which first emerged more than 200 million years ago, didn’t get very far for the first 130 million years or so; they were operating at an energetic disadvantage.

Then, some 66 million years ago, an asteroid the size of Manhattan slammed into the earth, ending the Cretaceous period. The impact did in whole groups of organisms, including, most famously, the dinosaurs, but the destruction was a boon for fungi. What’s become known as the “fungal infection–mammalian selection” hypothesis holds that mammals became the dominant group after the impact because cold-blooded reptiles were more susceptible to fungal pathogens. About a decade ago the author of this hypothesis, Arturo Casadevall, a microbiologist at Johns Hopkins’s Bloomberg School of Public Health, proposed that climate change would exert enough selective pressure on fungi that at least some would evolve to tolerate higher temperatures. Then along came Candida auris.

Candida auris is a yeast, which is a sort of one-celled fungus that behaves a lot like a bacterium. (Yeasts are a loose-knit group made up of fungi with very different evolutionary histories.) It was first described in 2009, from a sample taken from the ear discharge of a Japanese patient; hence the species name auris. Interestingly, the ear canal is cooler than the rest of the human body; it’s been suggested that this is how the fungus got, as it were, its toehold. Also in 2009 the fungus was found to have invaded the bloodstream of three patients in South Korea, only one of whom survived.

Over the next few years C. auris infections were reported in India, South Africa, and Kuwait. (In healthy people, the infection may cause no symptoms; to the elderly and the frail, it may prove fatal.) The distribution was confusing; usually with an emergent disease—see, once again, Covid—it is possible to trace the spread. With C. auris, new cases seemed to be arising more or less concurrently among populations that had not been in contact with one another. (Researchers eventually identified four distinct strains that, they wrote, had emerged “nearly simultaneously” on three different continents.) The fungus seemed particularly adept at spreading through hospitals. In June 2016 the Centers for Disease Control and Prevention issued a “Clinical Alert to US Healthcare Facilities.” Six months later the CDC reported that the first cases of C. auris had been identified in the US. Several infected patients had died, though it was unclear whether they had been killed by the fungus or by other illnesses; all had serious underlying conditions. In 2017, according to the CDC, there were 173 cases of C. auris in the country; last year there were fourteen times that number.

Under the best of circumstances, fungal diseases in humans are tricky to treat. This is because fungi are much more closely related to us than it may seem. Like people and indeed like all animals—but unlike viruses or bacteria—fungi are eukaryotes, which is to say their cells contain a clearly defined nucleus. The similarity in structure, Monosson explains, “makes fungal cells difficult to target and kill without damaging our own cells.” There are only three groups of antifungal drugs in use today, compared with more than a dozen types of antibiotics.

When C. auris was exposed to antifungals, the mystery of its origins only deepened. Disease agents usually develop resistance to drugs after being doused with them for decades; C. auris seems to have skipped a step. In the summer of 2021 there were two outbreaks—one in Washington, D.C., the other in Texas—that proved resistant to all three classes of fungus-fighting drugs. “C. auris is a novel human pathogen, and so no one can explain how it came by its remarkable drug resistance,” Monosson writes. Tom Chiller, the head of the CDC’s Mycotic Disease Branch, has called C. auris “a creature from the black lagoon.”

Casadevall has argued that C. auris is an example of just the sort of warming-induced adaptation he predicted. In a paper published in the journal mBio, he and two colleagues posited that the fungus has always been out there; only recently, though, under the pressure of climate change, did it evolve to flourish in the hothouse that is the human body. While Monosson, a toxicologist by training, doesn’t explicitly embrace this theory, she warns, “Over the past century fungal infections have caused catastrophic losses in other species.” So far, humanity has been lucky, but “our luck may be running out.”

Fungi make up their own kingdom, comprising an estimated 1.5 million species. They are an ancient and wildly diverse group whose members include the mold in your basement, the yeast that leavens your bread, the porcini in your pasta, the shrooms you tripped on in college, the polypores that grow like shelves on trees, and the mycobionts that, together with algae, form lichens. Relatively few fungi are pathogens; in fact, most aren’t even interested in living things. They are saprophytes, organisms that feed on the dead. (This, according to the “fungal infection–mammalian selection” hypothesis, is why they did so well after the asteroid impact—dead bodies were everywhere.)

While Monosson is worried about mycological menaces, Alison Pouliot, an Australian ecologist, has the opposite concern. Fungi, she argues, are too often feared. (There’s even a word for this anxiety—mycophobia.) Really, though, they deserve our gratitude and affection. “There’s a growing public penchant for fungi that suggests we are in something of a fungal awakening,” she writes hopefully in Meetings with Remarkable Mushrooms. Fungi are edging “their way from the margins to the mainstream.”

Pouliot concedes that fungi can be difficult for people to comprehend, let alone cozy up to. As not-plants and not-animals, they defy our expectations about how organisms should behave. In fact, they don’t even develop the way plants and animals do, which is by adding cells in layers. Instead, they stretch themselves out. Fungal spores send out what are known as hyphae—very thin cells shaped like hoses. Hyphae grow longer by means of an organelle called a Spitzenkörper, which neither plants nor animals possess. While individual hyphae are too small to see, many hyphae together can form a network, known as a mycelium, that looks a bit like a cobweb. If you’ve ever picked up a rotting log and found a collection of white threads underneath, you have disturbed some mycelia.

What we know as mushrooms are the reproductive structures some mycelia form under favorable conditions. “In making a mushroom, a mycelium absorbs water from its surroundings, inflating at a remarkably rapid rate,” Pouliot explains. “This explains the sudden appearance of mushrooms after rain.”

The importance of mycelia goes way beyond shiitakes or chanterelles. What are known as mycorrhizal fungi live in symbiotic relationships with plants. The plants provide the fungi with sugars, while the fungi’s mycelia act like root extensions for the plants, passing along nutrients and water. This arrangement, it’s believed, goes back more than 400 million years, and its significance is difficult to exaggerate: something like 90 percent of all land plants have been found to grow in association with fungi.

In recent years, mycorrhizal fungi have received a fair bit of attention because of the so-called wood wide web. Some scientists—most notably Suzanne Simard, the author of the best-selling Finding the Mother Tree (2021) and the model for Patricia Westerford, one of the main characters in Richard Powers’s novel The Overstory (2018)—argue that the underground webs formed by mycorrhizal fungi allow birches and beeches to communicate with one another and, what’s more, to come to one another’s assistance by distributing nutrients to trees in need of them. Other scientists who study fungi maintain that the notion that trees are socializing via fungal filaments is nonsense. Pouliot tries to split the difference, observing that many “astonishing discoveries” about mycorrhizal fungi have been made but that some experts question how the science has been “interpreted and communicated.”

Pouliot spends a lot of Meetings with Remarkable Mushrooms meeting with remarkable mushrooms. Ghost fungi, found primarily in southern Australia, are mushrooms that glow in the dark; Pouliot encounters some after tripping on a tree root near Rossiter Bay, on the country’s southwestern coast. Ophiocordyceps robertsii is a fungus that parasitizes moth larvae. Once the fungus’s spores get into a larva—no one is quite sure how—they send out hyphae that, in Pouliot’s words, liquefy “the caterpillar’s delicate innards via powerful enzymes.” When the fungus is done consuming the caterpillar, it sprouts a reproductive structure that grows out of the (by now dead) animal’s head. Pouliot finds one of these weird structures, which she describes as looking “suspiciously like a stick that might not be a stick,” in northwest Tasmania. Lobster mushrooms are fungi that have been parasitized by other fungi. The parasitized mushrooms turn bright orange and are said to be quite delicious. Pouliot spots them in the Cascade Mountains, in the western US.

Pouliot is concerned that fungi are overlooked even by conservationists. In part, she argues, this is because so little is known about so many species, and in part it’s because people feel so little connection to mycelia (though fungi are actually more closely related to us than plants). “To neglect fungi is to misunderstand how ecosystems function, and to overlook a fundamental element of biodiversity,” she writes.

Both Monosson and Pouliot make convincing cases, even though these cases strain against each other. Fungal pathogens are on the rise, probably because we humans move so many species around the world and possibly also owing to climate change. Pd has killed millions of bats; Bd has killed probably hundreds of millions of frogs. In March the CDC issued another warning about C. auris, which, it said, was spreading at “an alarming rate.”

At the same time, fungi are essential to the world as we know it. Without them, most of the continents would be barren; we wouldn’t have forests or grasslands or many of our crops. Fungi are also crucial decomposers that keep the wheel of life turning. Were it not for saprophytes, the dead would simply pile up.

Fungi sicken us and fungi sustain us. In either case, we ignore them at our peril.