Our World Is Filled With Fungal Spores. Some Should Frighten Us

There are many reasons to fear a new fungal pathogen—from the paucity of antifungal drug options to lack of vaccines and diagnostic difficulties in humans, to the potential for catastrophic crop and wildlife disease. Is it possible to contain an outbreak? Maybe. But the reliance of most fungi on the production of spores as a means of reproduction and distribution can, depending on the fungus—and the host—make a novel incursion virtually unstoppable. For all the current focus on fungal pathogens, there is little discussion of fungal spores.

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A fungus, depending on the species, can send millions if not billions of spores into the world. Released from a fungus’s reproductive structures (or fruiting bodies), spores carry instructions for a future Shiitake, puffball, bread mold, plant rust, or brain pathogen. Whether aloft in the air, between the toes of a bird, or in the crevasses of our shoes—spores are also how fungi move across towns, states, continents, and oceans. Different kinds of fruiting bodies release different types of spores with names like sporangiospores, conidia and chlamydospores—some are delicate and unable to live without a host, others are remarkably resilient. Chlamydospores from a banana-plant-killing fungus called TR4 can live for decades in the soil. Some melanized mold spores—those with dark pigment—can even survive the rigors of space travel. Massive spore production explains, in part, how the fungus that killed off the American chestnut tree managed such widespread destruction so quickly. If we want to understand the potential threat of fungi, understanding fungal spores is a good place to begin.

Atmospheric chemists estimate that some 50 million tons of spores popu­late the atmosphere on an annual basis. With every breath we take, we suck in tens, or thousands or millions of spores, many of which have been ejected, squirted, blasted, or simply dropped from dozens if not hundreds of different fungal species. Most will not harm us (and many are essential members of healthy ecosystems). For an infection to take hold, a spore must germinate and grow—and few fungi are able to do so inside of us. Of those that can— many cannot survive in those of us with healthy immune systems. Most won’t survive in the environment either. But fungi hedge their bets, sending off so many spores so that a few might land in a favorable environment. If there is going to be a novel fungal invasion there is a good chance it will begin with a spore—or a spore-producing fungus. And this is something we should be more prepared for, as the risk of new fungal pathogens—their movement and emergence aided by climate change and the unprecedented pace and magnitude of global trade and travel—is likely to increase over the coming years.

Imagine a Rhizopus stolonifer spore lands on a loaf of fresh sourdough. By the end of the day the bread mold has germinated. As tubular fungal cells penetrate the loaf they release digestive enzymes, breaking the carb-loaded substrate into nutrients it can absorb as food. In time fruiting bodies pop out dotting the bread’s surface. A single microscopic spore-producing “sporangium” of this mold (of which there would be hundreds if not thousands) will release tens of thousands of spores. The loaf is now a bioreactor. This is when we notice that Rhizopus has inhabited our bread. Those Rhizopus spores won’t bother most of us. (Although those with compromised immune systems can be vulnerable to “opportunistic” fungi, including Rhizopus.) Nor will most of the millions of existing fungal species; but several hundred species can infect humans. And some of those are important human pathogens. For these pathogenic fungi, says mycologist Christina Hull, “we are food.” Like the bread loaf to Rhizopus fungus, the fungi that settle into our lungs, or skin or brains is feeding on us.

Hull studies the infectivity of Cryptococcus neoformans spores. The fungus lives in the environment, and its spores may be found in soil, decaying wood, bird poop and in the air we breathe. When Cryptococcus infects humans, it grows not only in the lungs but can also infect the brain. Of the 19 fungi listed by the World Health Organization as top human health priorities in 2022, Cryptococcus topped the list. Hull says she studies spore infections in mice to help figure out what happens in humans. She adds that while a good deal more is known about spores made by plant pathogens (how they travel, how long they live, what they need to germinate) very little is known about those made by many human pathogens. Like most opportunistic fungal pathogens, Cryptococcus infects mainly those whose immune system is compromised. Globally, the fungus kills about 112,000 people a year, and is responsible for nearly 20% of AIDs-related deaths.

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The Coccidioides fungus which causes Valley Fever also spreads by soil-borne spores. In 2019, according to the U.S. CDC, there were just over 20,000 cases of Valley Fever. On average it kills 200 Americans a year. The spores made by this soil-dwelling fungi can survive in the soil in dry, hot places for years. When lofted into the air by an updraft from a wildfire or dust rising from a construction project these hardy spores can move – possibly even into cities where the infection is least expected. Coccidioides is particularly problematic because it infects those with seemingly healthy immune systems. Scientists worry that the changing climate will enable the fungus, which lives mostly in the desert Southwest, to spread east and north in the coming century.

One feature that sets Cryptococcus and Coccidioides apart from other environmental fungi, is that they tolerate our warm body temperature. Most fungi cannot. Our body heat along with our immune system is a formidable barrier to infection. Other spore-producing fungi able to infect us include strains of Aspergillus, Histoplasma, and Blastomyces—the fungus responsible for sickening dozens, possibly as many as 90 people, at a paper mill in Michigan in early 2023.

Spores carrying an emergent fungal pathogen may someday soon drift into the forest, field, or our own lives carried on the furious winds of a hurricane or perhaps a plant or animal shipped from one place to another. One hope for containing and preventing future fungal infections will be to better understand the resilience and movements of these tiny travelers.