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In the fall of 1985, Scott L. Gardner found himself standing over his toilet bowl, fishing around in the squishy output of his empty bowels with a chopstick.
He had recently returned to graduate school in New Mexico from a three-month expedition to Bolivia, where he was studying parasites in wild mammals. His field site near the city of Trinidad had been sparse: portable microscopes, preserved specimens, and a local pig that oinked as it waddled between the tents.
Gardner felt healthy, but he decided to check himself for parasites, as one looks for ticks after a hike. He put a sample of his own feces on a microscope slide and peered through the glass. Immediately, he was shocked: Hidden within the sample were clusters of tiny, light-yellow eggs.
“I’m used to looking at that with regular animals,” says Gardner. “But I’m not a regular animal—I’m me!”
Gardner went to the student health center to get treated, microscope slide in tow. Sure enough, the doctor confirmed he had contracted Ascaris, a pencil-thin parasitic worm affecting around one billion people today, most of them in sub-Saharan Africa and Asia. Gardner was prescribed an antiparasitic pill, and the next morning, he pooped out his intestines’ inhabitant—all 12 inches of it.
Driven by curiosity, Gardner probed around in the toilet until “out came this noodle on a chopstick.” He dumped the specimen in a jar and brought it to work, disgusting and fascinating his colleagues.
Today, nearly 40 years later, Gardner is surrounded by noodles in jars at the University of Nebraska-Lincoln’s Manter Laboratory of Parasitology, where he has served as curator for three decades. Housed in a few modest rooms adjacent to a botanical collection and the floor’s only bathroom, the laboratory is the world’s largest university collection of parasites.
“You can compare us to a library,” says Gábor Racz, the Manter Lab’s collections manager. Each week, he catalogs and ships out specimens hailing from the Nebraska sandhills, the Caribbean islands or Mongolia’s grasslands, helping biologists around the world access parasites for research.
Racz and Gardner oversee around 170,000 cataloged vials, microscope slides and jars of creepy crawlies. But each item doesn’t contain just one specimen. Many parasites are so small or, in the case of worms, so jumbled up that it’s impossible to count them individually. A single vial can hold thousands of creatures, bringing the true collection closer to 17 million. Donations from researchers have made the number of slides and vials double every decade.
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Aside from a rotating cast of graduate students, the Manter Lab only receives enough funding to employ the two men. Besides, few biologists have the necessary training to manage a collection that encompasses all types of parasites. It’s like “asking someone to do mammals and echinoderms,” says Gardner. “That’s how far apart [the types of parasites] are.”
Gardner, 67, typically sports thick aviator glasses with a bright blue strap and various parasite-adorned T-shirts. Each year, when he teaches parasitology to a room full of University of Nebraska undergraduates, he begins with his chopstick horror story. His students groan, giggle and gag.
Gardner’s task—to pique young scientists’ interest in his field—is a daunting one. Parasites’ horrible reputation precedes them. For some, like Plasmodium (which causes malaria) and Leishmania (which causes leishmaniasis), their reputation is justified due to their devastating impacts on human health. Malaria kills more than 600,000 people a year, and leishmaniasis kills around 30,000.
But parasites are also diverse and understudied creatures that have evolved to flourish in nearly every animal and ecosystem on Earth. They keep ecosystems in balance, providing a natural control on host species while stimulating their co-evolution over many generations. For example, one group of the mussel Mytilus edulis evolved to tolerate infection from the small, parasitic crustacean Mytilicola intestinalis in the southeastern North Sea, while another group grew to resist it. This could cause the mussel to diverge into different species over time.
Parasites also change host species’ behavior, leading to oddities like the pop-culture-worthy zombie ants that mysteriously climb ten inches up plants, permanently lock their jaws into the plant, and allow their parasitic fungus (Ophiocordyceps unilateralis) to grow out of their body and rain spores onto unsuspecting ants below. And parasites keep food webs miraculously complex, forming hundreds of connections among themselves, other parasites and host species. Often, in an effort to travel between host animals, parasites will expose their hosts to new predators, like the tapeworm Ligula intestinalis, which grows so large it changes the buoyancy of the fish it inhabits, causing the fish to swim closer to the surface and get eaten by birds.
Parasitism, a relationship between two species where one benefits at the other’s expense, has evolved independently over 220 times in animals—more than any other animal lifestyle. Nearly half of all animals are parasites, with conservative estimates at 3.5 million species. Like the mammals, birds, insects and fish all around us, each parasite serves important functions in its environment—ones that, without the parasite, could spell widespread ecological change, if not disaster. For example, a widely supported idea called the “enemy release hypothesis” suggests that as parasites of invasive plants and animals decrease, the spread and abundance of the invasive species increase.
As Gardner, Racz and their colleague Judy Diamond wrote in their 2021 book Parasites: The Inside Scoop, “Getting rid of all parasites would not make the world a healthier place.”
Just as the fascinating and ecologically vital roles of parasites are coming to light, researchers realize many are in trouble. With warming temperatures and deforestation threatening biodiversity around the globe, the parasites on and inside other animals will soon feel the effects. As many as a third of parasites may be endangered.
Simultaneously, parasitology is confronting a loss within its own field. According to data provided by the American Society of Parasitologists, in 1973, its membership had grown to nearly 1,900 members from its modest start at 320 nearly 50 years earlier. As of its 100th anniversary this year, the society has dropped by 76 percent, with just around 450 members.
The reason for the drop is two-pronged: Older parasitologists are retiring or passing away, while few new researchers are entering the field.
“I started off in this [field] pretty young and I got to know all these people, and they’re all expiring,” says Gardner. “It’s so sad. But what can you do, right?”
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Gardner and his colleagues are trying to entice the next generation to study parasite biodiversity. They are sneaking parasites into their college biology curriculums when traditional textbooks leave them out. They are handing out parasite stuffed animals to children at museum events to make the creatures seem cuddlier. And they are running the world’s first ever parasite conservation program for a tiny tick so that conservation organizations will see parasites as creatures worthy of protection.
As the field shrinks around them, parasitologists hope to give their discipline a boost, ensuring that researchers will continue to study the overlooked creatures that call other animals home.
Scientists often call parasites the “dark matter” of the animal world: They’re unseen but incredibly important.
They’re also wildly diverse. Parasites can have segmented bodies, hard outer shells, many tiny legs, suckers, hooks, antennae, bristles or flagella; or they can be a single, tiny cell.
Parasites are unique in that they are “not linked by a shared ancestry but by a shared lifestyle,” explains Mackenzie Kwak, a parasite ecologist at Japan’s Hokkaido University. Think of it like herbivores: Cows and human vegetarians both avoid eating meat, but they’re not part of the same family or genus. Animals, plants, fungi and protozoa can all be parasites.
What defines a parasite is its reliance on a host species to get energy—not to mention free shelter and transportation. Parasitism is far from easy living; hundreds of millions of years of evolution have prodded parasites to find and manipulate other animals, just as those animals have evolved their own unique methods of survival.
Parasites are “almost like Pokémon,” says Carrie Cizauskas, a freelance science consultant with a background in disease ecology. “They’re like these weird little alien creatures that you can be like, ‘Ooh, this one has these capabilities and this kind of power.’”
Parasites are everywhere. Most animals host at least one species, with the average mammalian host harboring eight. In some ecosystems, the total mass of parasites is larger than the total mass of predators, despite parasites’ often microscopic size.
In fact, biologists can use the presence of parasites as an indication of an ecosystem’s health: The more parasite species an environment has, the more biodiverse it is, and therefore more resilient in the face of disturbances and threats.
Research also increasingly suggests that parasites play an essential, regulatory role in complex ecological networks. Their functions can be compared to top predators, like wolves or lions: Parasites keep populations in check so host species don’t outpace available resources, like food or prime habitat. Sometimes, parasites can indirectly alter the interactions between other animals. New England snails infected with a wormy parasite eat less algae than their uninfected counterparts, leaving more algae for other animals to munch on.
Parasites are particularly important in food webs. In one study on a California salt marsh published in the journal Philosophical Transactions of the Royal Society B, removing a single snail species decreased the food web’s stability by around 1,000 links between species. It’s not that the snail was itself so important, but that its removal caused the secondary extinction of 17 different species of parasitic flatworms. In turn, since the parasites moved between so many host animals during their complex life cycles, the food web lost those connections. For example, one of the most common worms in the marsh, Euhaplorchis californiensis, causes its California killfish host to shimmy and jerk around so it gets eaten more often by birds—meaning that the parasite’s removal also diminished the birds’ food source.
Globally, at most 4 percent of parasite worm species infect humans, though multiple parasitologists shared in interviews that the true estimate for all parasites is closer to 0.1 percent. Understandably, however, our perceptions of parasites are far from favorable.
“The way most people [in the U.S.] encounter them is in their dog’s [poop],” says Chelsea Wood, a parasitologist at the University of Washington. “That’s not really a thing that’s going to endear itself to you at first blush.”
Teachers and parents know the nightmare of lice, hikers are no strangers to ticks, and public health officials are on guard for giardia or toxoplasmosis. But causing human suffering is far from parasites’ whole story.
As the wide-ranging ecological effects of parasites come more clearly into view, many scientists hope our perception of them will change, too: from fear to wonderment.
“It might seem unimaginable that 40 years into the future, we’re all going to look at parasites and be like, ‘Oh, so beautiful, so magnificent,’” says Wood. “But I do think that’s what’s coming.”
Human parasitic infections in the Western world are far less prevalent today than at any point in history. Access to clean water is likely the biggest reason: Washing your hands and food are the first steps to parasite prevention. When people do get infected, antiparasitic drugs, surgical procedures or sometimes a change in diet can take care of the parasite. Gardner mused on how his longtime colleague, renowned parasitologist Robin Overstreet, was once sad that a meal of particularly spicy curry had caused him to poop out a parasite with which he had intentionally infected himself to show his students.
As parasite infections have decreased, so has parasite education. Medical schools often no longer mandate parasitology classes, with such courses falling into small concentrations like tropical medicine or infectious disease. Undergraduate biology programs do not require parasitology, and high school courses are practically nonexistent, according to Kwak, Wood and others. They say only veterinarians are still required to learn about parasites.
“Somehow, we’re giving people full-on degrees in biology when they don’t know anything about a plurality of animal life,” Wood says.
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A lack of parasite education leads many scientists to stumble into the field by accident. Wood, who originally wanted to be a marine biologist, was no exception. When her college didn’t have any marine biology labs, Wood wound up working with parasitic worms at a nearby university. The experience changed her career trajectory entirely.
“I realized that there was this whole sub-world, this whole alternate dimension that was present in nature and in all the animals that I had been interested in since I was little,” Wood says. “It was sort of like waking up from the Matrix.”
Others, like Chenhua Li, a parasitologist who now works in biotechnology, had never even heard of parasitology before her graduate studies in the field. “English is not my native language,” Li explains of researching Western graduate programs while living in China. “I didn’t recognize ‘parasitology’ as a word, but it started with a ‘P-A’ and ended with ‘ology,’ so I mistook that word as ‘pathology.’”
Li ended up loving parasites and pursuing a PhD.
If budding biologists don’t get a chance to learn about parasitology, established ones, too, aren’t always considering parasites in their research.
Utah State University ecologist Sara Weinstein worries that ecologists exclude parasites from studies where they might play crucial roles, since they’re often hidden inside more visible species. Cizauskas says they are bothered by how members of the biology community tend to read literature only about a particular specialty, remaining unexposed to other fields, including parasitology. They think that parasitology should not be an isolated discipline, since the organisms are, by definition, members of other biological systems.
Many parasitologists see a plainer reason that their creatures aren’t widely studied: They’re not cute. Since humans evolved to care for our young, research suggests we’re drawn to infantile features in other animals, like big eyes, round cheeks and stubby limbs. There’s even a German word for it: Kindchenschema, which literally translates to “baby schema.”
Unfortunately for parasites, says Kwak, they’re “not nearly as charismatic” as polar bears or puppies. And, he emphasizes, parasites aren’t simply unsightly—they’re often invisible. Some can only be seen with a microscope, and many make their livings inside other creatures.
“You don’t see a tapeworm walking down the street,” says Sarah Orlofske, a parasitologist at the University of Wisconsin-Stevens Point. “Most students have seen a bird, maybe they’ve seen a lizard, maybe they’ve seen a snake. But unless you’re, like, really keen on mosquitoes and ticks, most students don’t interact.”
If you can’t see the parasites, Orlofske continues, you “don’t relate to them”—and won’t study them. In her career, she has mentored over 80 biology undergraduates; only two of them initially came to her wanting to study parasites.
For many parasitology instructors, a partial solution could lie in changing biology curriculums to introduce parasites earlier. If high school students learned about parasites in introductory biology, they mused, students would be more likely to study them in college. Even at universities, however, biology textbooks largely leave out parasites; a 2011 study published in Biological Conservation found that 72 percent of English language conservation biology textbooks did not mention parasites or portrayed them negatively.
Some parasitologists aren’t waiting for curriculums to change. Orlofske has a “parasite of the day” in her college introductory biology class, and Gardner can’t help but incorporate his favorite worms into college zoology. Much of the time, says Gardner, professors will recruit future parasitologists by pulling aside their top students to suggest that they join their labs.
Kwak thinks parasitology could take inspiration from disciplines like entomology, which have amateur societies that naturally recruit people to careers studying insects. “You can’t just take a butterfly net out and catch ticks,” he acknowledges, but plenty of hands-on parasite activities are available for participants willing to get a bit messy. Some of his favorites include tick flagging (sweeping a cloth sheet attached to a pole along vegetation to gather ticks), scooping leeches out of ponds with nets, and dissecting fresh fish to find and identify their wormy inhabitants.
Orlofske keeps realistic expectations about peddling parasitology. “I think the future of parasitology, if it’s not convincing people completely to go to the dark side and call themselves parasitologists, is to make career-oriented professionals in other fields more parasite-minded,” she says.
If other disciplines pay attention to parasites, she thinks, the threats the creatures pose and face will worm their way to light.
For one world-renowned parasitologist, studying worms followed him into retirement. When Mike Kinsella graduated in 1969 with a PhD in parasitology from the University of Montana, he struggled to find a faculty position where he could discover new species of parasitic worms. He ended up back in school for pharmacology, then worked as a pharmacist for two decades to pay the bills. Upon retiring, Kinsella realized his love for worms hadn’t faded.
For the past 30 years, Kinsella has been a stay-at-home parasitologist, identifying species for zoos, wildlife biologists and fellow parasitologists from the comfort of his home desk in Missoula, Montana. Every week, he opens his mailbox to find packages of worms from different countries and host animals. Then he gets to work with two modest microscopes and volumes of parasite reference books. In total, Kinsella has described over 30 new species, published more than 200 papers, and had 20 species named after him.
Kinsella runs his personal parasitology lab, which he calls HelmWest—Helm for the helminth worms he studies, and west for its residence in Montana—all out of pocket. He washes and reuses slides 200 times and asks colleagues to send him chemicals when he can’t purchase his own. His wife, ever supportive, only “draws the line at dissecting things in the living room,” Kinsella says, acknowledging that she’ll sometimes let small mammals slide.
Despite this bare-bones set up, Kinsella is one of the world’s leading experts on nematodes, of which an estimated half a million species exist. He’s also 83.
Since parasite varieties are so numerous, parasitologists like Kinsella are sometimes the only expert on a specific group. When these scientists die, their intricate understanding of how to classify and connect species goes with them. “When I’m gone, I don’t know if there’s anybody left that can do what I do,” Kinsella says.
A 2022 study published in the International Journal for Parasitology found that for the past two decades, a small handful of productive scientists have found and described most new parasite species. These individuals are mostly in the later stages of their careers, with little turnover to younger scientists—a fact that, according to the authors, is “putting the whole field at risk of collapse.”
At the American Society of Parasitologists’ 100th anniversary meeting in June, the attendance was bimodal: a mix of older or retired scientists and college- and graduate school-age biology students brought by their professors. The gap in early career scientists was palpable, says Orlofske, who herself brought three students.
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Indeed, the society’s membership data reflects the same: In the 2023 presidential address, parasitologist Tamara J. Cook reported that only 44 percent of members were active scientists in America; the rest were students, retirees or international members. Of the American scientists, over a third work at primarily undergraduate institutions, where financial support for research is much lower than at larger universities.
Importantly, this gap isn’t the case for the subfield of disease ecology, which concerns itself more with parasites’ ecological roles rather than their identities. Unlike taxonomy, which Kwak, himself a taxonomist, describes as “dusty and traditional,” disease ecology is booming in America in the wake of public interest in diseases like Covid-19 and Zika. Orlofske sees the field growing out of a natural instinct to protect ourselves, our pets and our livestock, while the funding for taxonomy “just has evaporated.”
As tenure-track ecology positions and conferences increase, the work of discovering, naming, connecting and cataloging parasites is petering out.
Tyler Achatz, a parasitologist at Middle Georgia State University, echoed his colleagues’ concern. As an early career scientist, Achatz, who described 25 new parasite species during his PhD, has watched the community of parasitologists he can consult dwindle around him. He worries young biologists like himself aren’t learning the necessary skills to identify and describe new parasite species.
“Unless people step up and start filling that gap, the knowledge is going to get lost,” he says.
Gardner, who has described around 50 new parasite species in his career—and has had three named after him—estimates that the Manter Lab’s collection holds thousands of new but un-described species, a number that would be astonishing in fields like mammalogy or ornithology.
And that number keeps increasing. Many parasitologists have spent their careers amassing private collections from their fieldwork, bequeathing them to the Manter Lab upon their deaths. Gardner and Racz have received donations of as many as 40,000 specimens at one time.
The Manter Lab mirrors a global trend. A 2020 paper estimated that, at current rates, cataloging every species of helminth would take 745 years—and that doesn’t even touch the numerous other kinds of parasites.
“Gábor and I are trying to do the best we can, but there’s no way to actually do it,” says Gardner, who estimates that the work of maintaining the growing collection would be best suited to ten people. “The task is daunting, and time is short.”
On a slushy winter afternoon, Racz stepped out onto the fourth floor of the University of Nebraska State Museum to a warm greeting from a white-haired museum employee. She was sitting down after a day of classroom museum visits but was eager to tell Racz about a child’s response to a parasite exhibition that he and Gardner designed.
“Someone asked what the giant snakeskin was,” she recounted, referring to a 100-foot-long plastic tapeworm suspended from the ceiling. “And I said, ‘It’s a parasite from a whale.’ And he said, ‘What’s a parasite?’”
This question is precisely what visitors are supposed to ask—and learn the answer to.
Pull-out drawers of taxidermy host animals, a simple microscope with an oversized wheel of specimens, and a ten-foot touchscreen table for interactive games round out the parasite exhibit. On one wall, snug in a jar, sits Gardner’s chopstick-scooped Ascaris.
The goal is simple: to show the public that parasites are more fascinating than fearsome. With children, specifically, parasite outreach will hopefully “spark some folks’ imaginations about career paths that are not dolphin training, but instead learning about what’s inside of dolphins,” says Wood of the University of Washington.
Around the country, parasitologists are adopting the tactic of creating colorful and interactive parasite activities and materials. At the University of Georgia, a program called EcoReach sends scientists to classrooms and science fairs to teach about ecology topics including parasitology. At the University of Wisconsin-Stevens Point, Orlofske runs an event where kids comb out glitter from stuffed animals’ fur to mimic ticks.
Wood is publishing a children’s book in fall 2024 called Power to the Parasites! Cizauskas, who is also an illustrator, is designing a parasite coloring book featuring scientific explanations of the creatures on each page. Li sells packs of Pokémon-esque cards highlighting parasites with unique qualities. One notable card describes Toxoplasma gondii, a parasitic protozoan that boasts “Mind Control,” because it attracts its rodent host to the smell of cat urine, where the rat spreads the parasite to felines. Another card features Leucochloridium paradoxum, a flatworm touting “Mimicry” by pulsating rapidly inside snails to attract birds searching for worms.
“Not only dinosaurs are cool,” says Li, who also commissions monthly parasite comics and has created a parasite personality quiz taken by 5,000 people.
Still others, like science illustrator Mona Luo, want to capture public interest by subverting people’s parasite expectations. Her goal is to make the creatures “so beautiful that people don’t have time to think about how disgusted they are.”
Luo is known among parasitologists for her pun-filled parasite Valentine’s Day cards and was personally asked to design the logo for the 100th anniversary meeting of the American Society of Parasitologists.
“There is a lot of value in trying to cut these negative associations people have,” says Supraja Rajagopal, the coordinator of EcoReach, about parasites. They hope EcoReach can “get that idea in kids’ heads when they’re younger that this isn’t bad, this thing isn’t gross, or icky.”
The Manter Lab also dabbles in parasite paraphernalia. By Racz’s desk sits a cardboard box of parasite stuffed animals featuring cartoonish lice, ticks and the protozoan that causes Chagas disease. They’re leftover from outreach events at the University of Nebraska State Museum, and Gardner tosses extras to students who answer questions correctly in class.
“I think you can cuddle up to tapeworms, too,” says Gardner.
Sparking children’s and students’ interest in parasites are two fronts in the quest to revitalize parasitology. But their effects won’t be felt for decades, if and when some of those people grow up to be scientists.
That’s a problem given the more immediate threats facing parasitology: climate change and habitat loss.
“Parasites face a double threat,” wrote Kwak, Wood and colleagues in 2020 in the journal Biological Conservation. “They are directly vulnerable to extinction due to anthropogenic factors like climate change or invasive species, and indirectly vulnerable through coextinction.”
Many parasite extinctions will be silent, since we don’t know what creatures live in endangered hosts to begin with. Out of the millions of parasite species, only two have been assessed for the Red List, an inventory of threatened species maintained by the International Union for Conservation of Nature (IUCN).
Kwak, who serves as the co-chair of the IUCN’s Parasite Specialist Group, is on a mission to put parasites on ecologists’ radars with the first parasite conservation program. He studies the Ryukyu rabbit tick, Haemaphysalis pentalagi, which lives exclusively on a dark-furred rabbit species inhabiting Japan’s lush Amami Oshima and Tokunoshima islands. By learning about the endangered rabbit’s needs, as well as breeding captive populations of the rabbit and its tick, they will conserve both.
Kwak says it’s “two for the price of one conservation.” If successful, his study will show that parasite conservation is easy and cheap: Just don’t remove parasites from a host species you’re already saving. For the amount of money spent to save one or two large mammals, says Kwak, you could save “a thousand [parasite] species, many of which probably have much more significant value to ecosystems.”
Extinction isn’t in store for every parasite species; some may thrive in a warmer world. This can be a major public health concern, as warmer, wetter regions—perfect habitats for mosquitoes and ticks—expand into previously cooler ranges. Changing climates may also alter the behavior of host animals, making them more suited to contracting or transmitting parasites.
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With diverse hosts, life cycles and habitats, predicting the future of parasites is messy, if not impossible. But Wood sees existing evidence suggesting that climate change and habitat loss will harm far more species than they help. “Parasites might be among the most imperiled species on the planet, which cuts against most people’s understanding of what parasites are and where they’re going,” says Wood.
Parasitologists are in an accelerating race: to catalog the world’s parasite biodiversity faster than parasites—and their own community—disappear. This situation is called the taxonomic impediment.
“We’re obliterating biological diversity on the Earth as fast as we can,” says Gardner. “There’s not enough people who will take those [species] and describe them and understand what they are before they’re gone.”
Scientists often argue for studying biodiversity because we don’t know what any species might reveal, be it the missing puzzle piece to an evolutionary question or a new ingredient for a lifesaving drug. With parasites, we could learn which threatened species are keeping ecosystems ever-so-gently in balance, or which ones might be future human-killers.
We also might not. Still, some dedicated scientists are committing their lives to ticks and worms simply because they like them—and because they hope others, in time, might like parasites, too.
“There’s so much biodiversity everywhere, you know, right under [our] feet, even in the city, even in [our] own bodies, potentially,” says Cizauskas. “And that stuff is just as important as, say, a panda.”
Editors’ Note, July 31, 2024: This article originally cited the wrong journal for a study about a California salt marsh. It is Philosophical Transactions of the Royal Society B., not Zoology.