Parasites are going extinct in droves – and we should be very worried


In 1987, officials from the US Fish and Wildlife Service captured all 22 of the world’s remaining California condors in a last-ditch effort to save the species. Back at San Diego Wild Animal Park and Los Angeles Zoo, biologists liberally dusted them with an insecticide called carbaryl to remove any parasites. The condors emerged from the blizzard unscathed, and the captive breeding program was so successful that, by 1991, conservationists began returning birds to the wild. The California condor louse didn’t fare so well. With a few shakes of powdery pesticide, the parasite quietly went extinct. Few people noticed and even fewer cared. If saving this iconic bird meant losing a louse, so be it. Such attitudes still prevail. But perhaps not for much longer. While parasites can harm and even kill their hosts, there is also a growing appreciation of their value. Far from being organisms that must be eradicated at any cost, it is emerging that they play important and beneficial roles in ecosystems. They can even keep their hosts healthy. In fact, in a modern parallel to the California condor story, conservationists trying to save an endangered marsupial called the woylie have discovered that parasites can actually help. Despite this, many parasites are themselves endangered and their plight has been almost completely overlooked. Now a group of pioneering biologists aims to change that. They see parasites as an essential component of life on Earth and believe we should be protecting them. There is no doubt that parasites have a bad reputation. For most people, they are synonymous with disease. This reflects our daily interactions with them. We treat our pets for heartworm and ringworm or take precautions for ourselves against hookworm. We douse our crops in torrents of chemicals to prevent parasites from ruining our food. It is almost dogma that parasites are bad, end of story, says Kevin Lafferty at the University of California, Santa Barbara. Even biology textbooks focus on the negative. This is something conservation biologist Liz Nichols discovered as a graduate student. She is currently on sabbatical at the US State Department, but a decade ago while at Columbia University in New York she decided to investigate what students are taught about parasites. Working in the basement of the American Museum of Natural History with fellow postdoc Andrés GÓmez, she searched 77 English-language textbooks published between 1970 and 2009 and found that what little was mentioned about parasites portrayed them in a poor light. History reveals the danger of such blanket prejudice against a group of living organisms. Less than a century ago, many countries established programmes to exterminate apex predators, arguing that everyone would be better off if they didn’t exist. Wolves were extirpated from most of the continental US, and populations of big cats and bears around the world plunged. “More animals are being eaten from the inside out than the outside in”. However, ecosystems lacking top predators weren’t the peacenik utopias some had hoped for. Populations of prey animals, from antelope to zebra, exploded and then starved as their food sources ran out. In the 1960s, conservation biologists began a small but forceful movement to reshape how society views carnivores in a bid to return them to their rightful habitat. This quest is succeeding. Nichols saw similarities with parasites. Because they have been portrayed in such a negative way, no one had thought to ask whether they might have a beneficial role in the environment. Her own work on dung beetles had told her about nature’s interconnectedness, and she believes that conservation biologists have overlooked some of the strongest links in the natural world. Andrew Dobson at Princeton University agrees. “Parasites are as fundamental to an ecosystem as predators are,” he says. In nature, parasitism is the most common way to get food, shelter and other resources – so common that parasites far outnumber free-living organisms. For example, the 45,000 known vertebrates are home to more than 75,000 parasites. “There are more animals being eaten from the inside out than the outside in,” says Dobson. At least 70 per cent of food-web interactions are between a parasite and its host, says Carrie Cizauskas at the University of California, Berkeley. “Parasites are the glue that holds an ecosystem together.” Take the camel cricket. Like many crickets, it can be infected by a parasite called a horsehair worm. After the worm matures in the cricket’s body, it needs to reach fresh water to reproduce. Instead of waiting for the cricket to take an accidental bath, the worm compels its host to make a suicidal leap into a stream or pond, where it can then bore a hole and slither into the water. In doing so, the worm provides a crucial delivery service: crickets account for more than half the food eaten by trout in some areas, according to a study by Lafferty and others.

Extinction threat

If parasites are as valuable as other components of ecosystems, they are also as threatened. In 2017, Cizauskas was part of an international team that tried, for the first time, to assess how global warming will affect parasites. Although climate change has been linked with the spread of diseases, the researchers found no indication that pathogenic parasites will be able to increase their ranges. Instead, the evidence points to between 5 and 10 per cent of parasites going extinct by 2070 as a result of climate-related habitat loss alone. Add in the loss of host species and changing environmental conditions, and as many as 30 per cent of parasitic worms are facing extinction within the next half century. The forecast is even worse for parasites such as lice and ticks that live on the outside of their host. For all this, only two animal parasites are listed on the International Union for Conservation of Nature’s Red List of Threatened Species: the pygmy hog-sucking louse and the European medicinal leech, and knowledge about the threat facing parasites is woefully lacking. “We have no idea how many parasites we’ve lost. They’re not cute and cuddly, so we’re not tracking them,” says Skylar Hopkins, an ecologist at Virginia Tech who has become a leading voice in the movement to protect parasites. Nor are scientists sure just how the loss of parasites will affect ecosystems as a whole. One small study, however, provides some worrying clues. A clam called the New Zealand cockle lives in many of the nation’s mudflats, using its muscular foot to burrow into the ooze. A fluke known as Curtuteria australis likes to embed itself in the cockle’s foot. If they become heavily infested, cockles lose their ability to hide in the mud and find themselves stranded. Research reveals that mudflats with more parasitised cockles supported a higher level of animal diversity. Organisms living in the sediment are less disturbed by burrowing shellfish. The exposed cockles are rich pickings for shorebirds and fish. And their shells offer a new habitat for barnacles and other aquatic species. By keeping cockle numbers lower and more stable, C. australis does a huge service to the ecosystem. As well as disrupting ecosystems, the loss of parasites could even be bad for their hosts. When scientists gave anti-parasitic drugs to Eurasian oystercatchers, they found that treated chicks were more likely to die than untreated ones. The researchers aren’t exactly sure why, but they think the lack of parasites may have interfered with the proper functioning of the bird’s immune system. Something similar is thought to happen in humans. Known as the hygiene hypothesis, some evidence shows that in our hyper-clean, low-parasite modern world, our immune system isn’t trained to deal with the worms and bacteria with which our species evolved. As a result, it overreacts to what should be harmless targets, causing allergies and autoimmune conditions. The presence of parasites helps to redirect the immune system away from the prolonged inflammation indicative of these conditions, says Cizauskas. The growing realisation that there is a positive side to parasites has persuaded some conservationists that we cannot simply ignore such species and focus only on more charismatic animals. Lafferty calls our current attitude “taxonomic chauvinism”. It is much easier to get donations to save cuddly pandas and majestic elephants than stink bugs and spiders, and parasites are at the bottom of the list. “But there’s an intrinsic value to every species. They’re all the result of millions of years of evolution, and they have adaptations that are remarkable,” he says. Dobson notes that saving any species means also saving its ecosystem – and you can’t save that ecosystem without also saving its parasites. But in the name of conservation, we intentionally kill parasites. The California condor louse isn’t an isolated example. Many parasites have co-evolved to live exclusively on a particular host and, in our efforts to save other species including black-footed ferrets and kakapos, we have deliberately driven some of their parasites to extinction. The answer isn’t as simple as forgoing anti-parasitic drugs. Although parasites aren’t the supervillains portrayed in many biology textbooks, clearly, they aren’t always heroes. As we know to our cost, some are deadly. Plasmodium, the parasite that causes malaria, kills millions of people every year. So do gastrointestinal parasites such as Giardia and entamoeba. But we also know that most parasites aren’t deadly. However, they are most likely to kill a host that is undernourished and stressed – conditions that a species on the road to extinction often faces. So, conservationists concerned about parasites must tread a fine line, as those trying to help the woylie have discovered.

Good infestations

The number of woylies has plummeted as a result of predation, habitat loss and climate change. Andrew Thompson at Murdoch University, Australia, suspects that these external stressors have somehow altered the marsupial’s relationship with its normal parasites, explaining why woylies captured for breeding programs are often besieged by multiple types of parasites, from intestinal worms to itchy ticks and lice. For many veterinarians, the instinct would be to blast them with strong, anti-parasitic drugs to eradicate these invaders. However, Thompson and his fellow conservationists have discovered that the right combination of parasites can actually improve a woylie’s chances of survival. Furthermore, they worry that giving captive animals too much anti-parasitic treatment might leave their immune systems ill-equipped to handle the normal range of parasites they encounter if they return to the wild. So, instead of trying to expunge woylie parasites altogether, the team doles out drugs to only the sickest animals, leaving healthier ones to use their own immune system to keep parasites in check. As a result, the conservationists aren’t just working to save one endangered species: a 2017 analysis identified 36 species that called woylies home. Eleven of these, including a previously unknown tick called Ixodes woyliei, live exclusively on woylies, and would blink out of existence if their host were to become extinct. The woylie efforts are part of a growing trend among conservationists to be more mindful of parasites. “We’re not just crazy nutcases – we want to save the host species, too,” says Hopkins. Teams trying to rebuild black-footed ferret populations, for example, are recruiting a single-celled parasite called Eimeria to help. Others want to deliberately cultivate parasites in captive Iberian lynxes. Finding the right balance in different situations won’t be easy, but if done correctly this new focus on parasite conservation looks like a virtuous cycle. It should help protect their hosts and entire ecosystems, and the parasites. Could this be the beginning of a makeover for some of the world’s most despised and misunderstood species? Nichols certainly hopes so. “What we need are children’s books about parasites, and pillows you sell for far too much money on Etsy,” she says. “Parasites are so stinking cool and gross.”

New Scientist, 2 November 2019