The Problem with Parasites

In 2010, Chelsea Wood was conducting a biological survey of the Line Islands, a chain of atolls and coral outcrops a thousand miles south of Hawaii. Some islands are heavily populated, home to a robust fishing trade, while others have never been permanently inhabited by humans. Seizing upon the opportunity afforded by such a stark contrast, Wood, then a budding parasitologist pursuing her Ph.D. in biology at Stanford University, decided to compare the worms living in the organs of fish from the uninhabited islands with those from the heavily fished areas.

What she found shocked her.

There’s a common perception of parasites as harmful, a harbinger of an ecosystem in distress, that’s shared by the public, and even many scientists. “We associate parasites with badness and destruc - tion and decay,” says Wood, now an associate professor in the School of Aquatic and Fishery Sciences at the University of Washington.

“We expect that when we mess up ecosystems, parasites should then come in to colonize, but it seems like actually the very opposite is true.” Indeed, contrary to Wood’s initial expectations, the fish from the pristine, unpopulated islands harbored more parasites than their counterparts. “Certainly more species of parasites, and for some kinds of parasites, more individuals as well,” says Wood.

With her discovery, Wood joined a small number of scientists on the leading edge of an invisible biodiversity crisis: the decline of parasites. It’s a struggle that’s gone largely unnoticed, concerning organisms that are widely reviled, but the studies that do exist suggest that parasites are in big trouble: In 2017, scientists compiled data about 457 species of parasites in a study in Science Advances. The researchers then produced models predicting that by 2070, up to 10 percent of those parasites will have gone extinct from climate fueled habitat loss alone. On top of that, habitat destruction that harms host organisms could spell the downfall of the many parasite species that depend on them.

A world with fewer parasites, perhaps ironically, means a world with less healthy ecosystems, as parasites help keep the populations of their hosts in a delicate balance. But scientists like Wood are leading the charge to reveal the dire circumstances these species face — and to find ways to protect not just endangered parasites, but the entire ecosystems they inhabit.

Parasites are a broad group of organisms, connected by their strategy for survival: Their lives are intimately intertwined with other organisms, at a cost to the host species. They typically live in or on their hosts’ bodies for at least part of their parasitic life cycles, and rely on their hosts for key needs like nourishment or reproductive opportunities. For instance, tapeworms lack digestive tracts of their own. Instead, they live inside the intestines of their host species, where they siphon away nutrients.

When most of us think of parasites (if we think of them at all), we tend to conjure up animals like tiny intestinal worms and lice. But parasites come in all shapes and sizes. The world’s largest flower — a rotten-meat-scented blossom over 3 feet in diameter — is a parasite with no roots or leaves of its own. Brown-headed cowbirds that lay their eggs in other birds’ nests, microscopic Toxoplasma gondii protozoans that manipulate the behavior of mice so that they lose their fear of their feline predators, and 100-foot-long tapeworms in the guts of whales all belong to vastly different branches of the tree of life — but they’re all parasites, too.

Parasites aren’t just diverse, they’re practically omnipresent. “There is absolutely a parasite within 10 feet of you, if not inside of you,” says Wood. This revelation, which might be skincrawling for some, was an inspiration for Wood as a student. “I was stunned and totally in love,” she says. “I started to learn that there’s this whole world underneath the surface of everything that’s familiar.” Indeed, an estimated 40 percent of species on Earth are parasites, according to a study published in Proceedings of the National Academy of Sciences.

That parasitism has cropped up so many times in unrelated species — and thus evolved again and again — speaks to its success as an adaptive strategy. “Parasites basically lay around and let food come to them,” says Wood. “But parasitism does also involve some unique constraints that other organisms don’t need to face.”

For starters, parasites need to first find their hosts, a potentially daunting task for the many species that require different hosts at different points in their life cycles. As the ranges of their hosts shift with habitat destruction and climate change, this challenge can be compounded. And parasites’ difficulties don’t stop once they’ve found their targets; the hosts also fight back. “[Parasites] are living inside of a host organism that is bent on their destruction,” says Wood. “They have to deal with the host’s immune system and all the [host’s] adaptations to avoid and kick out parasites once they colonize.” The often-antagonistic relationship between parasites and their hosts begs the question: If parasites are indeed in trouble, why should anyone care?

It's undeniable that some parasites cause devastating damage to their hosts, including humans. Malaria, caused by a microorganism transmitted via mosquito bite, kills more than 600,000 people every year, most of them children. Intestinal parasites like hookworms and whipworms rank among the world’s most pressing public health crises. Beyond that, other parasites can harm livestock and crops that people rely on for survival. But taken together, parasites, while harmful to their hosts by definition, often aren’t even that harmful.

For most parasites, it’s not beneficial to kill or severely weaken the hosts they rely on. Kayce Bell, assistant curator of mammalogy at the Natural History Museum of Los Angeles County, studies parasites in chipmunks, including intestinal parasites called pinworms. “About two-thirds of all of the chipmunks I’ve ever examined have these pinworms,” she says. “And they’re almost always at a really low burden.” That burden typically clocks in at fewer than five pinworms per chipmunk, which isn’t enough to do any serious damage to the hosts.

Occasionally, Bell says, she comes across a chipmunk with a much more serious infestation: as many as 100 or more pinworms. “Something has gone wrong there,” she continues. “The chipmunk’s immune system failed to keep those pinworms in check.” Hosts’ ability to cope with their parasites relatively unscathed is often the result of an ages-long evolutionary arms race between host and parasite. Millions of years of intertwined evolution allow hosts to develop new defenses, prompting their parasites to develop the footholds they need for nourishment and reproduction — without tipping the balance and eradicating the hosts they depend upon.

What’s more, parasites play an important, if often poorly understood, role in entire ecosystems. Bell likens them to wolves in Yellowstone National Park. When wolves were eradicated there in the early 20th century, the ecosystem was upended in a variety of unexpected ways. “One of the famous examples is the willows getting browsed down to nothing because the wolves weren’t there to chase the herbivores off. The same can be true of parasites. If you remove something from an ecosystem, we cannot foresee what the consequences of that will be.”

Part of the challenge of ascertaining what a world with fewer parasites might look like is the simple fact that we don’t know much about the roles that they play in their ecosystems. “The crazy thing about parasites is that they’re everywhere, absolutely everywhere, ubiquitous, and yet, we have no clue how they’ve been faring over the past couple of decades,” says Wood. “Throughout history, people have been grossed out by parasites, and as a result, they have been woefully understudied,” she notes, and when they are studied, it’s typically from the perspective of getting rid of them entirely.

Still, scientists know enough about the importance of parasites to take their potential disappearance very, very seriously. For instance, a 2013 paper in Biology Letters showed the effects of dosing wild mice with the antiparasitic drug ivermectin, which targets nematode worms. As a result, the mice got fewer worms but became hosts to a greater number of protozoan parasites and developed gastrointestinal issues.

Plus, the benefits of a wide diversity of parasites can translate to whole ecosystems: A study in the Proceedings of the National Academy of Sciences that same year examined how wetland communities responded to a flatworm parasite, called Ribeiroia ondatrae, that causes harmful limb deformities in frogs. When there was a wider variety of parasite species in the ecosystem, the number of Ribeiroia infections dropped by 15 to 20 percent. “These findings provide evidence that parasitic and free living diversity jointly regulate disease risk,” the authors wrote. In short, a wider diversity of parasites seems to make host animals healthier.

Yet despite parasites’ importance, indicated by studies like these, scientists are often stymied by a lack of baseline data about the relationship between parasites and their ecosystems. “I made these discoveries in the coral reefs and then couldn’t look into the past.

There were no data to compare against,” says Wood, who notes that there were no historical records showing the gradual loss of parasites from the heavily fished Line Islands. But her mentors, parasitologists Armand Kuris and Kevin Lafferty, suggested another way to look at parasites over time: dissecting pickled fish in museum collections to look for the worms preserved in their stomachs.

“We could actually use these museum specimens as basically time capsules,” says Wood. When she arrived at the University of Washington as an assistant professor, she began a deep dive into the university’s fish collections at the Burke Museum of Natural History and Culture. The Burke Museum’s collection contains countless fish from the nearby Puget Sound, dating back to the 1920s, as well as older specimens from the Smithsonian Museum of Natural History, some of which date all the way to 1880. Focusing on eight species that were well represented across that time frame, Wood and her team dissected 699 fish that had been preserved in alcohol, finding more than 17,000 individual parasites

By comparing the parasites from the better part of two centuries, the scientists found an alarming trend. “There’s been a large-scale decline in parasite abundance in Puget Sound over the past 140 years,” says Wood. “That decline is concentrated among the complex lifestyle parasites [which hop between different host species throughout their lives].” Ultimately, parasites that rely on multiple species of hosts are at greater risk of something going wrong at some point in their life cycles, like how you’re more likely to run into flight trouble on a trip with lots of connections versus flying direct. If even one of the host species that a parasite relies upon disappears, then the parasite runs a risk of disappearing right along with it.

Pinpointing the reasons for an organism’s decline can be tricky. A wide variety of factors might have been at play to drive the downturn among parasite communities in the Puget Sound fish — habitat destruction, loss of host species, and changes in water quality, to name a few. But Wood and her colleagues think one major culprit may be primarily responsible: changes in seawater temperature caused by the climate crisis.

"We had good sea surface temperature data, which tracks with climate change. And what we found was very clearly that the parasite declines we observed were associated with changes in temperature,” says Wood, who published her findings in the Proceedings of the National Academy of Sciences in 2023. Climate change, she says, can alter living conditions more quickly than organisms can evolve to tolerate those changes.

“Any kind of change is a threat to those complex life cycles,” adds Wood, “because if any one of those hosts is being lost, that’s game over for the parasite.” However, studies like Wood’s are few and far between. “We just do not know enough about parasites,” says Bell. “We really need to focus on collecting data about the existence and occurrence of parasites and just some basic biodiversity work that’s really been neglected for a lot of parasite groups.”

So, in 2020, Bell, Wood, and their colleagues published a paper in Biological Conservation calling for a conservation plan for parasites. The paper puts forth a 12-point strategy, starting with figuring out which parasites are even out there. “The first part for me really is just this basic biodiversity discovery, that we need to be surveying for parasites, describing what’s out there,” says Bell. “Alongside that is archiving those [parasite specimens] in museums. Put this material in collections where people can find it.”

While many natural history museums already have parasites in their collections, they’re rarely centralized in a way that highlights parasitic diversity. Plus, many of the parasites in museum collections arrived there entirely by accident, like the worms that happened to be preserved in the fish in Wood’s Puget Sound survey. The parasite conservation plan calls for more intentional, concerted efforts to survey the world’s parasites and collect them for future study

The next step, says Bell, is to carry out assessments that help answer questions like how different parasites are doing, which ones might be endangered (or thriving), and what types of threats pose the greatest risks to certain species. “Once we know what’s out there, we’ll have more tools to try to understand how they are faring."

Skylar Hopkins, an ecologist at North Carolina State University and another author of the 2020 conservation plan, is one of the scientists tasked with developing the tools to protect parasites. In December 2022, the International Union for Conservation of Nature (IUCN) officially established a Species Survival Commission (SSC) focusing on parasites, with Hopkins as co-chair.

“There’s still such a long way to go [in parasite conservation], and a big step forward was the formation of this IUCN group,” says Hopkins. “We really needed an international group to coordinate some of these communication efforts.” In the first year of the SSC for parasites, Hopkins says the group has focused primarily on seeking and securing grants to fund parasite conservation work.

Mackenzie Kwak, Hopkins’s SSC co-chair, is a leader in this real-world effort to monitor parasite numbers and study their role within their ecosystems. “Basically, we wanted to create the world’s first conservation program for parasites because it’s one thing to identify their decline, it’s one thing to write their obituaries,” says Kwak, a research fellow at Hokkaido University in Japan. “It’s another thing entirely to try and actually save them.”

One possible standard-bearer for parasite conservation could be the Ryukyu rabbit tick — a species that attracted Kwak’s interest when he first arrived in Japan. These ticks are only found on a few small islands in southern Japan, on small, dark-furred Amami rabbits. The rabbits’ range has shrunk over the years, and the rabbits have also been preyed upon by invasive mongooses that had been introduced to curb viper populations in the area. In short, as the rabbit has become endangered, so has the Ryukyu rabbit tick, which appears to only live on that single host species.

Since 2023, Kwak and his colleagues have been monitoring the tick populations by examining Amami rabbits struck by cars, assessing their medical condition, and removing all the ticks on them for study. By examining the roughly 100 Amami rabbits hit by cars each year, as well as ticks collected from the forest floor and in soil samples from rabbit burrows, the researchers are beginning to get a better understanding of the tick’s still-unclear ecological role, and how it interacts with its host. Kwak and his colleagues even found that the rabbits rebounded after the removal of the invasive mongooses, which means that the ticks will likely recover, too.

“You get basically what I think of as a conservation dividend,” says Kwak. “If you save one species, you’re actually saving two.” This two-for-one conservation model is likely to be fruitful, says Kwak, because most species have at least one specialized parasite that only lives on them.

“Almost anytime I look at an endangered animal in any kind of detail, I find very often new species of parasites,” he adds.

While the Ryukyu tick conservation program could serve as a flagship for parasite conservation overall, Kwak notes it’s about more than just saving individual species. “At a broad level, we have to be saving ecosystems,” he says. Healthy parasites require healthy host populations, and for those host populations to be healthy, a wider web of animals, plants, fungi, and microorganisms must be healthy too.

“We’re just gathering the information to suggest that parasites are really important linchpins in their ecosystems,” says Wood. “They keep a cap on the abundance of their hosts, so that those hosts don’t become overabundant and lurch their ecosystems out of balance.” Parasites, in their often-invisible role in regulating their ecosystems, are thus uniquely important to this holistic approach to conservation. Saving them could be the key to preserving the diversity of life on Earth as we know it.