“Found throughout the tree of life and in every ecosystem, parasites are some of the most diverse, ecologically important animals on Earth — but in almost all cases, the least protected by wildlife or ecosystem conservation efforts,” the plan opens, before laying out crucial steps we must take in the next decade to preserve parasites. These include ensuring legal protections for endangered louses and nematodes, building parasitism into the K-12 curriculum, and getting the public not just mildly curious but “enthusiastic” about parasites.
“Most people have only heard about parasites in the context of the ones that are harmful to human health,” explains Kelly Speer, one of the plan’s authors and a postdoctoral fellow at the National Zoo and National Museum of Natural History. “But you can’t have a healthy ecosystem without having parasites in it.”
Could this be a ploy to steer precious grant dollars to parasitologists? If so, it’s a long con. Cries for conserving parasites have rung out for decades — not that a lot of folks were listening.
“When a biologist sends something to an editor about being nice to parasites, the editor just figures, ‘What the hell? This guy’s a crackpot,’ ” says Donald A. Windsor, a biologist in Upstate New York who has published many seminal parasite conservation papers, such as 1997’s “Equal Rights for Parasites.”
Nowadays, we know of several reasons to cherish the humble parasite. The “hygiene hypothesis,” for example, attributes healthy human immune systems to past worm infestations. Parasitoid wasps are thought to save the U.S. agricultural industry billions of dollars each year by implanting crop pests with babies that then devour the insects alive. Charles Darwin said he could not “persuade myself that a beneficent and omnipotent God would have designedly created” such gruesome killers, but without them farmers would spray tons more pesticide, and who wants that?
Then there’s the fact that less than 1 percent of all known parasite species infect people. The rest are just out there, doing their own freaky things in freaky ways.
“They are regulators of population, they constitute a lot of the links in food webs, and they make up a lot of the biomass in ecosystems,” says Colin Carlson, an author of the plan and a global-change biologist at Georgetown University. “They’re sort of like dark matter — moving through ecosystems, connecting to things and having impacts, and we never see them.”
Take the trematode preying on the California killifish, a normally secretive bait fish that hangs out in western salt marshes. The parasite creates a cyst on its brain that completely changes its behavior.
“They swim close to the surface of the water, flash their shiny side upward, and make a complete spectacle of themselves,” says Chelsea Wood, a parasite ecologist at the University of Washington’s School of Aquatic and Fishery Sciences. “Infected fish are between 10 and 30 times more likely to be eaten by a bird than uninfected fish, and that means when you scale it up across the whole salt marsh that parasites are driving fish into the beaks of these birds.”
In the D.C. area, hikers might have the privilege of stumbling upon a cricket infected by a horsehair worm, an elegant parasite that stuffs itself into the insect’s body like a length of twine threaded into a peanut shell. The parasite hijacks the cricket’s brain, making it jump into water so the worm can continue its life cycle inside fish. In parts of Japan, these mind-scrambled crickets account for 60 percent of the diet of endangered mountain trout. “There are examples of this all over the parasite tree of life,” says Wood.
It’s hard to quantify the net benefit parasites provide to the world because, even in our modern era, we simply don’t know how many there are. It’s thought that about 40 percent of living animals are parasites, but only a fraction are identified even though they exist in or on almost every creature. “All vertebrates have parasites. So whatever you see outside of your window that’s running around — you see a squirrel, you see a seagull — they have multiple species of parasites in them,” says Anna Phillips, a research zoologist at the National Museum of Natural History.
That’s why the conservation plan calls for the rapid identification and classification of parasite species. It’s something Phillips has firsthand knowledge of, having helped discover a new kind of leech in Southern Maryland last year after making herself bait. “You roll up your pants legs and move around a little bit, and then if the leeches come to find you, that’s a pretty easy day’s work,” she says. “These are big leeches; they will come after you.”
The thought that parasites might not slither and burrow for perpetuity is painful for parasitologists. It’s not a baseless worry: These organisms are subject to increasing stress from habitat loss, climate change and co-extinctions when host animals die out and leave the parasites that depended on them stranded. “The reality is every time a Tasmanian tiger goes extinct, you’re not losing just the tiger and one species. You’re losing a suite of them,” says Kevin Lafferty, a senior scientist with the U.S. Geological Survey.
One 2017 study predicts we could lose as much as a third of all parasitic worm species by 2070. Some scientists have toyed with the idea of fighting parasite extinctions by bringing them back from the dead. “There’s a really interesting paper that asks if we’re going to bring back mammoths, will we have to bring back mammoth parasites, as well?” says Skylar Hopkins, one of the plan’s authors and an assistant professor at North Carolina State University. “Because if you bring back mammoths but they don’t have any of the ecological interactions they had when they existed, did you really bring back mammoths?”
To preserve parasites still with us, though, the plan calls for an ambitious public relations campaign. Mind you, we’re not talking about polishing the image of flesh-crawling Guinea worms, malaria-transmitting mosquitoes and other scourges of humanity. “I have no moral or scientific objection to driving the Aedes aegypti and Anopheles gambiae mosquito to extinction. Those would be landmark victories for global health,” says Carlson.
It’s all the others that could use more love, which might not be as implausible as it sounds. The “Alien” movies made blockbusters out of parasitic monstrosities, as did popular video games like “Half-Life” (headcrabs). There’s even a crabby-looking Pokémon with a parasitic mushroom on its back. “I mean, some parasite species are cute. Some have eye spots and smiley faces, and I think that’s adorable,” says Hopkins.
Wood believes tapeworms in particular are ripe for prime-time TV. “To the naked eye they’re just ribbons that are defecated out of vertebrates. They’re slimy and whitish-yellow and not very attractive,” she says. “But when you look at them under the microscope, tapeworms are spectacular. They have four tentacles that can be everted from the head like a gunshot very quickly, and each of its tentacles has backward-facing hooks on it that can be used to grab and hold on to substrates.” She adds, “We already have Shark Week. I think it’s time for Parasite Week.”
John Metcalfe is a reporter based in the Bay Area.