Parasites are going extinct. Here's why we need to save them.

They’re “gross and slimy and flaccid and wiggling.” But parasites can be just as important as more charismatic animals—and many may be on the verge of disappearing.

Published 11 Apr 2021, 06:12 BST
parasites
Larvae of the horsehair worm infiltrates a cricket and grows inside.
Photograph by Anand Varma, Nat Geo Image Collection

Growing up, Chelsea Wood dreamed of becoming a marine biologist and studying sharks or dolphins—the kinds of big, exciting animals that biologists call charismatic megafauna. Instead, during a college internship, she found herself peering through a microscope at the guts of a snail.

The snail was one she knew well. As a kid, she had often plucked Littorina littorea periwinkles off rocks along the shores of Long Island and dropped them into buckets to watch them crawl around. But she had never seen inside one. She cracked a snail open, teased out the soft parts, and under her magnified gaze saw “thousands of little white sausage-shaped things dumping out of the snail’s body,” she says.

The larva of the trematode worm infects the tadpoles of an American bullfrog.
Photograph by Anand Varma, Nat Geo Image Collection

The sausages were the larvae of the flatworm Cryptocotyle lingua, a common fish parasite. Seen through the microscope, each one had two dark eyespots, which made them surprisingly cute and charming. “I couldn’t believe that I’d been looking at snails for as long as I had and missing all the cool stuff happening inside them,” says Wood, now a parasite ecologist at University of Washington. “I just totally fell in love with them. I like to say that they got under my skin.”

Wood has since become a leader in a new conservation movement that aims to save the world’s uncharismatic minifauna.  

Nearly half of all known animals on Earth are parasites, Wood says, and according to one study, a tenth of them may already be doomed to extinction in the next 50 years due to climate change, loss of their hosts, and deliberate attempts at eradication. But right now it seems few people care—or even notice. Of the more than 37,000 species flagged as critically endangered on the IUCN red list, only one louse and some freshwater mussels are parasites.

By definition, parasites live in or on a host and take something from that host. This has made them the pariahs of the animal world. But not all parasites cause noticeable harm to their hosts, and only a small percentage affect humans. Scientists warn of dire consequences if we disregard the rest. Not only is there much can we learn about parasites and ways to use them for our own needs (such as medicinal leeches, still employed in some surgeries), but we’re also starting to understand that they play crucial roles in ecosystems, keeping some populations in check while helping to feed others.

Some experts say there’s an aesthetic argument for saving them, too. If you get past the ick factor and get to know them, you may find parasites’ pluckiness eerily charming. They’ve evolved ingenious means of survival, from the crustacean that becomes a fish’s tongue to the jewel wasp that paralyses part of a cockroach’s brain and then leads it to a nest by its antenna, like a dog on a leash.

An amphipod invaded by the larva of a thorny-headed worm.
Photograph by Anand Varma, Nat Geo Image Collection

“People think of parasites as gross and slimy and flaccid and wiggling, and that’s true some of the time,” says Wood. “But if you look at them under the microscope, they are just staggeringly beautiful.”

Of course, the modern conservation movement isn’t supposed to care about looks or charisma anyway, says Kevin Lafferty, an ecologist at the University of California, Santa Barbara. There are plenty of nondescript plants and homely, squishy, or creepy-crawly invertebrates that are protected. “None of those things are cute and cuddly,” he says. “The public doesn’t give a damn about them. But modern conservation biology still considers those important parts of biodiversity.”

A world of parasites

When we humans look at a landscape, whether African savanna or Australian coral reef, we see the other host species, like ourselves. But the lions and zebras and fish are just homes for most of the life hidden in front of us.

All told, 40 percent of known animals are parasites, and those are just the ones that have been described. Scientists think that’s only about 10 percent of all the parasites out there, leaving potentially millions more yet to be discovered. Parasitic wasps alone probably outnumber any other group of animals, even beetles.

Most species, it turns out, are parasitised by multiple others. Take humans: Despite our efforts to be unhospitable, we’re excellent hosts. More than a hundred different parasites have evolved to live in or on us, many of them now dependent on us for their species’ continued existence.

Parasites proliferate because every living thing is a smorgasbord of nutrients and energy, and being a top predator isn’t the only way to get a bite of that bounty. Parasites opt out of the arms race between predator and prey entirely, choosing an easier path. It’s clever, when you think about it, and it’s exactly why parasitism is so common. “Nature abhors a vacuum. If there’s an opportunity, someone’s going to evolve to fill it,” says Wood.

Parasitism has evolved as a way of life again and again, over billions of years, from the smallest and simplest microbes to the most complex vertebrates. There are parasitic plants, parasitic birds, a bewildering array of parasitic worms and insects, and even a parasitic mammal—the vampire bat, which survives by drinking the blood of cows and other mammals. Of the 42 major branches on the tree of life, called phyla, 31 are mostly parasites.

Yet we have barely begun to identify all the parasites, much less learn their lifestyles or monitor their populations. “That’s just not something that we’ve ever really prioritised,” says Skylar Hopkins, an ecologist at North Carolina State University. So a few years ago, Hopkins pulled together a group of scientists interested in parasite conservation, and they started sharing what they knew. In 2018 they presented research at the Ecological Society of America conference. Then, in October 2020, they published the first-ever global plan for saving parasites in a special issue of the journal Biological Conservation.

Hostile Planet: The Zombie Ant Graveyard
One of the more chilling aspects of the animal kingdom: an ant controlled by a parasitic fungi being led to its death. 

One of the things Hopkins and her colleagues have noticed is what they call the paradox of co-extinction. Since parasites by definition need other species, they’re particularly vulnerable to the phenomenon. Take, for example, the endangered pygmy hog-sucking louse. It lives only on another critically endangered species, the pygmy hog, which is disappearing from the grasslands it inhabits in the foothills of the Himalaya.

“There should be, potentially, millions of parasite species that are threatened, and probably a lot that have already gone extinct,” Hopkins says. “But the weird thing is that we’ve hardly documented any parasite extinctions.”

Wood says she has been hunting for historical data on parasite abundance for more than a decade, for any parasite—on land or in the water. “I’ve had my eyes peeled,” she says, and so far, she has found a grand total of two useful data sets: one from a research cruise in the late 1940s and the other in a lab notebook kept by one of her mentors.

With so little information, “we have no idea whether parasites are playing the same role now that they did in the past,” Wood says. “I think that’s a travesty.”

The poster child for parasite conservation, if there is one, is the California condor louse, an ironic victim of the conservation movement itself. In the 1970s, desperate to save the California condor, biologists began rearing the birds in captivity. Part of the protocol was to de-louse every bird with pesticides, on the assumption that parasites were bad for condors, though it’s not clear they actually were. The California condor louse hasn’t been seen since.

Similarly, the New England medicinal leech hasn’t been seen for over a decade, and overfishing has probably done in the marine fluke Stichocotyle nephropis, which depended on endangered rays and skates to complete its life cycle. Untold other parasitic worms, protozoans, and insects are presumed to have gone down with the ship, so to speak, as their hosts died out.

A world without parasites

While the demise of life’s hangers-on might seem like no big deal, or even something to strive for, ecologists caution that wiping them all out would probably spell planetary doom. Without parasites keeping them in check, populations of some animals would explode, just as invasive species do when they’re transplanted away from natural predators. Other species would likely crash in the ensuing melée.

Big, charismatic predators would lose out, too. Many parasites have evolved to move into their next host by manipulating the host they’re in, which tends to drive that host into a predator’s mouth. Nematomorph worms, for instance, mature inside crickets but then need to be in water to mate. So they influence the crickets’ brains, driving the insects to jump into streams, where they become an important food source for trout. Similar phenomena feed birds, fish, cats, and other predators the world over.

Even human health wouldn’t entirely benefit from wiping out parasites. In countries such as the United States, where we have eliminated most intestinal parasites, we have autoimmune diseases that are virtually unheard of in places where everyone still has those parasites. According to one line of thinking, the human immune system evolved with a coterie of worms and protozoan parasites, and when we killed them off, our immune systems began attacking ourselves. Some people with Crohn’s disease have even purposely infected themselves with intestinal worms to try to restore their guts’ ecological balance, with mixed results.

That said, scientists aren’t eager to save all the parasites. The Guinea worm, for instance, gets a hard pass from even the most hard-core conservationists. It grows to adulthood inside a person’s leg, often reaching several feet long, and emerges painfully through the foot. Former president Jimmy Carter’s foundation has set out to drive the worm to extinction, and few will miss it when it’s gone.

If anyone would want to get rid of all parasites, you’d think it would be Bobbi Pritt. As the medical director for the Mayo Clinic’s human parasitology lab, Pritt identifies parasites found all over the country and in every body part. A typical day could see her working with blood carrying malaria parasites, brain tissue full of Toxoplasma gondii, or toenail clippings with sand fleas that someone picked up walking barefoot on the beach.

Yet even Pritt has a soft spot for parasites. She writes a blog called “Creepy Dreadful Wonderful Parasites,” and she spends weekends studying the ticks outside her vacation cabin. As a physician, she backs the idea of eradicating parasites in places where they cause disease and suffering. “But as a biologist, the idea of actually going out and purposefully trying to make something extinct just doesn’t sit well with me,” she says.

Ultimately, the goal of promoting parasite conservation isn’t to make everyone fall in love with them. Instead, it’s to call a détente in our war against all of them, because there’s still so much we don’t understand about their value to ecosystems and maybe even to people. And if you’re not swayed by parasites’ usefulness, consider Kevin Lafferty’s take:

“If you are a religious person, you’d say they’re all God’s creatures; we should care about them all the same,” he argues. “And that’s kind of the approach that conservation biology has been taking, with one major exception. And that’s parasites.”

Erika Engelhaupt is the author of the book Gory Details: Adventures From the Dark Side of Science.

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