Our single-celled ancestors darted around the world’s vast ocean a billion years ago, propelling themselves with tiny flagella tails and feeding on primitive plants, algae, and one another. Around this time, two groups of these ancient creatures branched into what would become two of life’s most successful kingdoms. One group developed into animals. The other became fungi. Animals and fungi both breathe oxygen and replenish their energy by eating food. Their cells are similar. The two closely akin kingdoms have occupied the Earth through most of their histories in an awkward fraternal tussle. When environmental conditions change quickly, fungi turn into opportunistic parricides, attacking and feasting on their enfeebled animal kin. Deadly fungi are thriving today amid environmental tumult, wiping out nests of bumblebees, colonies of bats, and hundreds of species of frogs.
And they are coming for us.
Animals typically gulp down and then digest their food. Most fungal species have a different strategy: They stretch ravenous tentacles called mycelia into their meals, squirt out digestive enzymes, and slurp up the dislodged nutrients. Thanks to this feeding style, fungus acts as the world’s great decomposer. It breaks down dead plants and animals, freeing up and recycling organic compounds.
After BP’s Deepwater Horizon blowout, nematodes and other tiny animals virtually disappeared from oil-coated swaths of sand around the Gulf of Mexico. The tainted habitats are now teeming instead with molds, the same types of fungus that speckle shower curtains. The molds are breaking down the crude oil into carbohydrates that will be more palatable to animals.
As useful as these decomposers are, fungi don’t hesitate to feast on living cells when they get the chance. Fungi’s ability to switch effortlessly between different diets, sometimes eating living bodies and sometimes eating dead ones, boosts their pathogenic pestilence. And their greatest trick is their ability to shape-shift. Fungi can retreat to spore form and survive long periods without food. As spores, they can float vast distances through water or air. When conditions suit them, they can quickly grow long mycelia and burrow into flesh, alive or dead, or in some instances slip into living cells to feast on their prey.
These twin abilities—to subsist for long periods without eating and to change diets as needed—mean that fungal diseases are particularly dangerous. Bacterial and viral diseases burn themselves out when they kill their victims before spreading to new hosts. But fungi can wipe out entire populations of their hosts without jeopardizing their own existence.
During Earth’s greatest mass extinction, triggered 250 million years ago possibly by sudden climate change, soil-dwelling fungi rose from the ground to feed on forests that were weakened by environmental bedlam. “Less healthy plants are more prone to become infected by such fungi,” said Cynthia Looy, a biologist at the University of California, Berkeley who investigates how plants respond to environmental change. “Fungi can accelerate the demise of already stressed, unhealthy plants.”
During the comet-induced mass extinction that doomed the dinosaurs 65 million years ago, fungal spores suddenly saturated the world. Some researchers speculate that fungi dealt final deathblows to weakened dinosaurs, helping to give rise to the age of mammals. Fungi are poorly suited to growing in hot conditions, and mammals’ warm blood provides some protection against them.
During the past century, fungal diseases have felled great forests of elms, chestnuts, pines, and other trees around the world, overturning ecosystems and leaving grassy wastelands in their wake. The chytrid fungus Batrachochytrium dendrobatidis is thought to have wiped out hundreds of species of amphibians and has been fingered as the cause of frog die-offs worldwide. Scientists recently reported that the fungus is also infecting and killing crayfish. White-nose syndrome was discovered affecting a few unfortunate bats in New York in 2006; the fungus responsible for the disease has since killed more than 5 million hibernating bats in 21 states and four Canadian provinces.
Scientists have never before witnessed pathogens tearing such virulent paths of destruction through wildlife. Increasingly, humans are succumbing to fungal diseases, too.
Many people have immune systems that are debilitated by age, diseases such as AIDS, or therapies that keep organ-transplant and cancer patients alive. “This cadre of immunosuppressed patients is at major risk of fungal pathogens,” said Joseph Heitman of Duke University Medical Center’s Center for Microbial Pathogenesis. “And that group will grow.”
Infections of Cryptococcus neoformans are rare among healthy people, but the fungus ravages those with compromised immune systems. It is spread primarily by the guano of pigeons and contracted by inhaling spores. More than 1 million immunosuppressed patients are infected annually around the world, according to figures from the Centers for Disease Control and Prevention. The worst affected sufferers live in sub-Saharan Africa, where mortality rates from the fungus among AIDS sufferers reach 70 percent.
As we tear up forests and turn over soil, we unleash spores from their slumber, including species that humans and other animals have rarely encountered before. World trade is helping strains of fungus spread and hybridize. And our wanton use of antibacterial medicine, including in farm animals, kills the microbes that could help keep fungus levels in check.
“The environment is changing quite dramatically,” Heitman said. “Logging, gardening, forestry, and other things that perturb the environment and move around soil or trees that are contaminated with the fungus are a major contributor.”
Fungus doesn’t just eat away at our organs and cells; it tucks voraciously into our food. Moldy bread can be a minor bother, but fungal agricultural pandemics have the potential to occur on a staggering scale. Researchers writing in Nature calculated that the known fungal pathogens could wipe out more than one-third of the world’s supply of major crops if severe epidemics struck simultaneously. And they point out that diseases such as rice blast and wheat rust are already having a major impact on agricultural productivity: “Our calculations show that even low-level persistent disease leads to losses that, if mitigated, would be sufficient to feed 8.5 percent of the 7 billion humans alive in 2011.”
This year, 39 people have died and 581 others have been sickened after mold spores infiltrated immunosuppressant medicine. The tainted drugs were injected directly into patients’ central nervous systems, where the fungus blossomed.
Doctors in desert regions of the Southwestern United States have reported a recent spike in cases of Valley Fever. In this painful and sometimes fatal ailment, a fungus that occurs naturally in the soil passes through the nose or mouth and settles inside the lungs. Once there, it sets down its mycelia roots and begins to eat. In 2007, 4,815 people were reported sickened by the fungus disease in Arizona. In 2011, 16,473 contracted the disease. It appears to be particularly abundant near decomposing animal carcasses. It might have originally specialized in decaying animals and then adapted to feeding on living tissue.
A fungus that once was limited to tropical regions has been spreading and killing people in the Pacific Northwest since it was discovered in 1999 on Vancouver Island. Cryptococcus gattii can cause pneumonia and meningitis, ravaging otherwise healthy sufferers. It has since been detected in Idaho, California, and farther north in Canada.
Researchers are racing to develop vaccines against some of the most deadly fungal pathogens. Anti-fungal medicines are readily available; they work by damaging fungal membranes and cell walls. But because we’re so closely related, potent medicines that damage fungal cells can also harm the human organs they were designed to protect.
The scourge of fungal contagions will continue to worsen, particularly among those with compromised immune systems. There appear to be no immediate threats that human fungal outbreaks will parallel the plagues afflicting bats and frogs. That’s largely because our body temperatures are warmer and less hospitable for fungus than those of amphibians and hibernating mammals. Also to our advantage, some of the most deadly fungal diseases afflicting humanity today are contracted after spores are kicked up from the environment and into our bodies—not spread from person to person, as was the case for the Black Death and Spanish flu, which were caused by a bacterium and virus, respectively.
But other fungal diseases do already spread from person to person, such as through sex, as in the case of yeast infections. If new strains of deadly fungi evolve similar abilities to jump directly from person to person, or from wild or farm animals to humans, new diseases could run rampant.
And some of the same scientists who speculate that fungi killed off the cold-blooded dinosaurs during a time of global cooling warn that global warming could help similar pathogens adapt to withstand our warm blood. If that happens, we will lose our best defense against fungal plagues, and our opportunistic kinfolk would be poised to overwhelm humanity with crippling bouts of their cell-sucking ruination.
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