It’s Halloween season, the liminal period when the general public seems to have a greater tolerance for stomach-churning content, so let’s talk about parasites, perhaps the most disgusting — and successful — form of life on this planet.
Even though I find parasites fascinating, there’s a skin-crawling feeling that is instantly triggered by the sight of a tick, lamprey or mosquito, to say nothing of hairworms, scabies and other unsettling creatures. I can’t help it. I evolved this trait for a reason and can’t molt it as easily as a rat flea sheds its skin.
We think of parasites in chiefly negative terms, as takers that literally suck the blood from us, nothing more than an evil scourge. The bedbug calamity recently reported in France is a perfect example, in which an alleged outbreak of the blood-sucking insects sparked international disgust (and may have also been part of a Russian disinformation scheme). If that was the Kremlin's work, it's playing on an age-old fear: many people agree that if we deleted all parasites from nature, there would be no love lost.
But nothing could be further from the truth. Parasites are the glue that holds all life on this planet together. The simplified web of life we learned about in elementary school starts with plants, that are eaten by bugs that are eaten by birds or rodents which are eaten by apex predators like hawks, wolves and humans. But almost every spoke in between those relationships is a parasite.
We haven’t come close to identifying the true number of parasites in the world. It’s a form of life that turns out to be more successful and more common than being a self-sufficient organism.
Any form of life can be parasitic, whether it’s fungi, bacteria, plants or animals. Several estimates suggest that parasites actually outnumber “free living” organisms by about 3 to 2. Beetles, for example, are likely the most biodiverse insects on Earth. There are more species of them than any other creature. But some experts disagree, saying that for every one beetle, there are an average of two to three wasp parasites. Many insect species are undescribed or unknown to science, as we haven't really done a thorough inventory of insects, especially flying ones, so we haven’t come close to identifying the true number of parasites in the world. This is only one example of how abundant parasites are. It’s a form of life that turns out to be more successful and more common than being a self-sufficient organism.
There are even hyperparasites: parasites of parasites, such as Aphelinus mali, a tiny wasp that turns woolly apple aphids (a parasite of apple trees) into “mummies.” There are even hyperhyperparasites, as described in this delightful report of “a fungus on a fungus on a fungus on a tree.” It’s parasites all the way down.
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Parasitism is a type of symbiosis, in which animals live in close relationship. Ah, but aren’t symbiotic relationships inherently beneficial? The plover birds that peck the scraps from crocodile teeth without getting eaten — that’s symbiosis, right? Technically, that’s mutualism, a subset of symbiosis in which both creatures get something from the deal. Next time you’re at a party, you can kill the vibe by unleashing this pedantic point. Everyone will be grateful (when you leave).
More importantly, parasites keep populations in check and resilient. Most successful parasites want to take from their host while giving back, so they don’t lose their primary food source. After all, it's kind of counterintuitive to bite off the hand — or neck or small intestine — that feeds you.
There is a theory that the relative lack of human parasites — which have been largely eradicated in the modern world — could be the underlying cause for autoimmune diseases like celiac.
Some parasites will even attack other parasites in order to protect their hosts or modulate their immune system to work in their favor. Of course, there are the murderous kinds of parasites, especially wasps (which are everywhere, though I appreciate that as well because wasps are freaky and cool), which kill their hosts. These are technically called parasitoids and are a little more specialized than your general parasites.
There is a theory that the relative lack of human parasites — which have been largely eradicated in the modern world — could be the underlying cause for autoimmune diseases like celiac, in which the body’s own immune system attacks itself. There may be some truth to this: Humans evolved for millions of years alongside our parasites, everything from tapeworms to pubic lice. Our immune systems have long been primed to fight them off. Suddenly, within the last century, we’ve made our parasites disappear.
But like a speeding race car with no brakes, our bodies still want to fight something, so they turn on themselves. This theory — sometimes known as the “hygiene hypothesis” — is so attractive that some people have taken to infecting themselves with gastrointestinal worms. (Somewhat off topic, this serves as yet another indictment of our ghastly health care system.)
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Clearly, parasites are more beneficial than most people will give them credit for. They serve as fundamental drivers of evolution and without them, our ecosystems would be out of balance. We not only should appreciate parasites more, we should actively try to protect them, just like we do with pandas and whales. Save the pygmy hog-sucking louse! Now that’s an environmental campaign sure to be popular.
One final thought: Are humans parasites? If Earth is one giant organism that we live on, as some scientists speculate in the "Gaia hypothesis," it’s clear that we are extracting resources from that organism like a leech sucking blood. This is more than just a metaphor. Every rain forest we bulldoze for cattle and every open pit lithium mine we carve out to power our Teslas is hardly different than a Cordyceps fungus turning its host into a zombie.
Personally, I think it seems more apt, both literally and metaphorically, to compare our relationship to the Earth to a cancer than to a parasite. The primary difference, arguably, is that, unlike a tapeworm, humans are conscious and intelligent enough that we can learn to coexist with the life around us. We could, at least hypothetically, learn to take what we need while giving more back, protecting our environment and preserving our future instead of destroying both of them.
An earlier version of this article originally appeared in Salon's Lab Notes, a weekly newsletter from our Science & Health team.
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