In the 1980s, conservationists ushered the planet's 22 last remaining Californian condors into captivity. They saved the birds, cared for them, fed them, and bred them. They also de-loused them and, in doing so, they killed off the last remaining condor louse—a harmless parasite that lived only on Californian condors. The condor population rose to over 400. The condor-louse population fell to zero. “It's a great example of a species that was knowingly, willingly, and thoughtlessly driven extinct by veterinarians,” says Kevin Lafferty, a parasitologist from the University of California, Santa Barbara. “I would hope we would act differently now.”
Lafferty is one of several scientists calling for conservationists to pay more attention to saving parasites. We view these organisms—these blood-suckers, free-riders, nutrient-drainers, and mind-controllers—with disgust and antipathy, and we're more likely to aggressively exterminate them than compassionately preserve them. But, in many cases, this kind of “taxonomic chauvinism” is a mistake.
Parasites play a critical role in ecological systems. They are abundant: In 2008, Lafferty traipsed through three Californian estuaries and found that the local trematodes—microscopic flatworms that specialize in castrating snails—outweighed all the resident fish and birds. They direct the flow of energy: A Japanese team found that trout get 60 percent of their diet from suicidal insects, driven to drown themselves in streams by mind-controlling worms inside their bodies. They keep populations of pests under control: All sap-sucking insects are targeted by some manner of parasitic wasp or fly that lays eggs inside their bodies. When we lose parasites, we lose ecological lynchpins rather than inconsequential oddballs.
We might also endanger other organisms that we're trying to save. In many cases, parasites are not threats to health, but co-evolved partners that help to calibrate the immune systems of their hosts. When wolves were denuded of mites and reintroduced to Yellowstone National Park, they became more susceptible to viruses. When humans are deprived of our resident microbes, we seem to run greater risks of allergies and asthma. “We maintain this diversity of parasites and organisms that we’ve evolved with, which fight off organisms that we didn’t evolve with or are exploiting us,” says Eric Dougherty from the University of California, Berkeley.
“The idea of parasite conservation is a litmus test for conservation,” says Lafferty. “There are many views for why we conserve species. Some feel it is their moral obligation to prevent extinctions caused by humans. Others argue for the intrinsic value of biodiversity, and many market biodiversity conservation based on its utility value for humans. Each of these perspectives results in a different list of what should be saved. But all of those lists so far lack parasites.”
Of course: #notallparasites. Many of them cause untold human suffering, including Plasmodium, which causes malaria, and the flatworms that cause schistosomiasis. “It’s hard to argue that we should be conserving pathogens that threaten human health,” says Dougherty. Lafferty agrees. “I believe in preserving all aspects of biodiversity, but I wholly support Jimmy Carter’s wish to see the extinction of guinea worm before he dies. I’m sure some people will argue on moral grounds, but I’ve yet to see someone volunteer to act as a host for the last remaining pair of guinea worms.”
Other cases are less clear cut. Take the hydatid worm, a tapeworm with catholic taste in hosts. It's often removed from wolves that are reintroduced into national parks, but it also helps the wolves by infecting and incapacitating their prey, including moose and other ungulates. Complicating matters further, the worm sometimes infects humans, causing over a thousand deaths a year in the tropics. “Can we ever really weigh the relative value of these things? No probably not,” says Colin Carlson from the University of California, Berkeley. “But if we think about all of the risks, we should keep the opportunities to conserve them on the table.”
Consider the black-footed ferret, a sinuous, masked animal that was declared extinct in 1979, before a group of survivors was found and saved. In the process, conservationists found two species of protozoan parasites called Eimeria. In a staggering burst of foresight, they decided to save these too, so the captive ferrets would develop appropriate immune responses to similar parasites when they were eventually released. Other researchers advocate the deliberate introduction of parasites to captive animals—lousing the Iberian lynx rather than delousing it.
Ignorance, more so than public perception, remains the biggest challenge to parasite conservation. The International Union for Conservation of Nature's Red List tracks the status of threatened species, but is heavily biased towards vertebrates and plants. “No one has taken the time to work out what fraction of parasites are threatened by global change,” says Carlson.
But even in the shadow of ignorance, there's still a lot we can do, as Dougherty, Carlson and others outline in a new paper. “The most fundamental idea, and it's a bit silly that we’ve missed this, is you don't destroy something if it’s doing okay,” says Carlson. “When we bring animals into captivity, we destroy parasites on the way. The starting point is to stop doing that.” That requires a different mindset—one that sees an animal not as an individual in an ecological vacuum, but as a bustling community of microbes and parasites that should be considered, assessed, and protected as one.
These inventories might even influence our decisions about which hosts to protect. “Maybe white-tailed deer don’t support much parasite diversity, and mule deer support three times as much. When we go about prioritizing mammals, we should have a list of host-parasite interactions for each species,” says Carlson. “We also need to think about what level of host we need to keep their parasites alive,” adds Dougherty.
His colleague Carrie Cizaukas tells me that the team is also working on several studies to nail down the biological traits that make parasites vulnerable to extinction, to help other scientists work out the most vulnerable species so they can prioritise their efforts, to work out how quickly different parasites might go extinct as a result of climate change, and to identify potential hotspots of parasite diversity.
Perhaps the most important step towards parasite conservation will be dealing with their chronic lack of charisma. Sperm whales evoke wide-eyed awe; Placentonema gigantissima, the nine-meter roundworm that lives in sperm whales, evoke narrow-eyed disgust. A giant panda can trigger a gut reaction of delight; that's hardly the case for Baylisascaris schroederi, the tapeworm that triggers gut disease in giant pandas.
“I was talking to an expert on feather mites,” says Carlson. “It turns out that the relationship between birds and these mites is really ambiguous for many species, and not even necessarily parasitic. But the labelling of that class of organisms as parasitic has slowed our understanding of their diversity. We'd like to remove some of the stigma attached to apparently parasitic groups, some of which might not as bad as we thought they were.”
This might seem like a fool's errand, but history suggests otherwise. Many microbes, including the algae that live inside lichens, the root fungi that provide bestow plants with nutrients, and the hordes of bacteria that digest our food and build our bodies, were initially viewed as parasites before scientists realized how much they benefit their hosts.
Likewise, conservation has a track record of attitudinal shifts. For the longest time, the very idea that species could go extinct, much less that they needed to be deliberately saved, was unheard of. And for people in the early 20th century, who saw top predators as competitors, enemies or sport, the idea of actively reintroducing wolves to national parks would have been a complete anathema. Likewise, Carlson is hopeful that parasite conservationists of the future will look back on today's attitudes with a similar roll of the eyes.
“I don’t think anything is set in stone,” he says. “Parasite conservation is very early in its development, but it’s absolutely on the table for a lot of conservationists already.” Perhaps the time has come for these grisliest of organisms to truly start capturing hearts and minds—and for sympathy, rather than just nutrients.