May 30, 2024


Proof of concept is a video series profiling the science and scientists behind some of the environment’s most unexpected research.

When Chelsea Wood was a child, she often collected periwinkle snails on the shores of Long Island.

“I used to pick them off the rocks and put them in buckets and keep them as pets and then release them,” Wood said. “And I knew that species very well.”

It wasn’t until years later that Wood found out that those snails were teeming with parasites.

“In some populations, 100 percent of them are infected, and 50 percent of their biomass is parasite,” Wood said. “So the snails I had in my bucket as a child weren’t really snails. They were basically trematodes [parasites] who commandeered snail bodies for their own purposes. And it blew my mind.”

Wood, now a parasite ecologist at the University of Washington, sometimes refers to parasites as “puppet masters,” and in many cases, that’s not an exaggeration. Some can mentally control their hosts, for example, causing mice to seek out the smell of cat pee. Others can shapeshift their hosts and physically alter them to resemble food. And their ripple effects can reshape entire landscapes.

An illustration depicts a parasitic nematamorph worm floating in a stream, while a cricket infected with the parasite stands at the stream's edge.
Estelle Caswell / Grist

For centuries, people thought of parasites as nature’s villains. They often infect humans and livestock. In fact, parasites are by definition bad for their hosts, but today more scientists are beginning to think of parasites as forces for good.

“I don’t think anyone is born a parasitologist. Nobody grows up wanting to study worms,” Wood said. “Somewhere along the way, I’d like to say, they got under my skin. I just fell in love with them. I couldn’t believe I had gotten this far in my biology education and no one had ever mentioned to me that parasites are incredibly biodiverse, ubiquitous, everywhere.”

On an overcast August morning, Wood took me to Titlow Beach in the state of washington, one of her team’s research sites. Back in the 1960s, one of Wood’s research mentors sampled shore crabs here. At the time, the area was very industrial and heavily polluted. But when researchers, including Wood, came back to collect samples half a century later, the beach had changed. The water was cleaner and the shorebirds had returned, but those weren’t the only promising signs: The crabs were now full of trematode worms, a type of parasite that jumps between crabs and birds.

A woman in a baseball cap kneels over rocks and water holding an orange bucket
A close-up of a woman's hands holding two inch-long crabs

Chelsea Wood kneels to search for shore crabs at a beach in Tacoma, Washington. She will later dissect the crabs to look for parasites. Jesse Nichols / Grist

The parasites were a sign that the local shorebirds were doing well, Wood explained.

As scientists have learned more about parasites, some have argued that many ecosystems may need them to thrive. “Parasites are a bellwether,” she said. “So if the parasites are there, you know the rest of the hosts are there too. And in that way they give an indication of the health of the ecosystem.

To understand this counterintuitive idea, it’s helpful to look at another class of animals that humans used to hate: predators.

For years, many communities treated carnivores as a kind of vermin. Hunters were encouraged to kill wolves, bears, coyotes and cougars to protect themselves and their property. But eventually people began to notice some big consequences. And nowhere was this phenomenon more apparent than in Yellowstone National Park.


In the 1920s, gray wolves were systematically exterminated from Yellowstone. But once the wolf population was eliminated from the park, the number of moose began to grow unchecked. Eventually, herds overgrazed near streams and rivers, driving out animals, including native beavers. Without beavers to build dams, dams disappeared and the water table dropped. Before long, the entire landscape changed.

In the 1990s, Yellowstone changed its policy and reintroduced gray wolves into the park. “When those wolves came back, it was like a wave of green that rolled over Yellowstone,” Wood said. This story became one of the defining parables in ecology: Predators weren’t just killers. They actually held entire ecosystems together.

“I think there are a lot of parallels between predator ecology and parasite ecology,” Wood said.

Like the gray wolves in Yellowstone, scientists are only beginning to recognize the profound ways in which ecosystems are shaped by parasites.

Take, for example, the relationship between nematomorphs, a type of parasitic worm, and creek water quality. The worms are born in the water, but spend their lives on land inside bugs, such as crickets or spiders.

At the end of their lives, nematomorphs must move back to the water to mate. Instead of making the perilous journey themselves, they trick their infected hosts into giving them a ride through a “water ride,” inducing an impulse on the part of its insect host to submerge itself in water. The insect will move to the edge of the water, consider it for a while and then jump in – to its own death, but to this parasite’s benefit.

The story does not end there. In a way, the entire creek ecosystem relies on a worm trying to hitch a ride to the water. Fish eat the bugs that throw themselves into the water. In fact, one species of endangered trout gets 60 percent of its diet solely from these infected bugs. “So essentially the parasite is feeding this endangered trout population,” Wood said.

With less of the threat associated with hungry fish, the native insects in the stream can thrive, eat more algae and thus give the creek clear water.

Parasites make up an estimated 40 percent of the animal kingdom. Yet scientists know next to nothing about millions of parasite species around the world. The most important parasites that scientists have spent a lot of time studying are those that infect farm animals, pets and humans.

Many of these worrisome parasites, such as ticks or the parasitic fungus that causes valley fever, are expected to increase as a result of climate change. But no one really knows what climate change means for parasites, in general—or how any big change in parasites might reshape the world. “There is a general feeling that infection is on the rise, that parasites and other infectious organisms are more common than they used to be,” Wood said. “At least for wildlife parasites, there’s not really long-term data to tell us if that impression we have is real,” Wood said. “We had to figure out a way to get that data,” Wood said.

Wood had an unconventional idea of ​​where to look: a collection of preserved fish locked away in a museum basement.

A person holds a large glass jar full of preserved fish in front of a row of similar jars
Chelsea Wood holds a jar of preserved fish from the University of Washington Fish Collections.
Jesse Nichols / Grist

The University of Washington Fish Collections is home to more than 12 million specimens of preserved fish, dating all the way back to the 1800s. But the thousands of jars that stand on the collection shelves also contain something else: all the parasites that live in the fish samples.

“So many of the museum specimens were discovered that we hid at one time and then pulled off the shelf 100 years later,” Wood said. “It’s really remarkable to look back in time like you do when you open a fish from a hundred years ago. It’s the only way we’ll know anything about what the oceans were like, parasitologically, that long ago.”

A woman in a baseball cap looking into a microscope in a laboratory
Purple nitrile gloves holding tweezers dissecting a fish

Chelsea Wood dissects fish specimens in her lab at the University of Washington. Jesse Nichols / Grist

Wood and her team spent more than two years opening jars and surgically dissecting the parasites from the inside. Under microscopes, they identified and counted the parasites before sending everything back for future study. In the end, they found more than 17,000 parasites.

Looking at the number of parasites found in fish over time, the researchers found a mix of winners and losers, but there was one major class of parasites that unequivocally declined: complex parasites, the kinds that need several different host species to survive. This type of parasite decreased by an average of 10 percent every decade, the team found.

A chart shows trends from Chelsea Wood's 2023 study on parasite changes over the past century.  The horizontal axis shows whether different parasite species are increasing or decreasing.  Parasites with three or more hosts mostly decline.
Jesse Nichols / Grist

In Wood’s investigation, there was only one factor that perfectly explained the decline in parasites: It wasn’t chemicals or overfishing. It was climate change. It made a lot of sense: Complex parasites can only survive if everyone in the environment is one of those host species. If only one type of host is missing? “Game over. That’s it for that parasite,” Wood said. “That’s why we think these complex life-cycle parasites are so vulnerable: because things shift, and the more points of failure you have, the more likely you are to fail .”

Wood said that before this study, researchers had no idea that climate change was wiping out this important class of parasites.

A glass jar marked with a note reading, "Tagged fish were dissected by the Wood Lab in 2022 to determine parasite infection.  Internal organs were collected and returned."
Chelsea Wood holds a pot of fish her lab analyzed for a study published in 2023.
Jesse Nichols / Grist

“It’s probably a collateral impact,” she said. “We don’t even have a handle on how many parasites there are in the world, much less the scale of parasite biodiversity loss right now. But the early indications are that parasites are at least as vulnerable as their hosts, and possibly more so.”

Wood says that it’s important for people to understand that parasites play large and complex roles in nature, and if we ignore what we can’t see, we run the risk of understanding how the world really works. “We all have a reflexive aversion to parasites, right? We take drugs, we apply chemicals, we spray,” Wood said. “Our argument is that parasites are just species. They’re part of biodiversity, and they do very important things things in ecosystems that we depend on them for.”






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