**Warning for Graphic Pictures**
When most people think of parasites they likely think of something like a tapeworm. They are a long, wormlike parasite that lives in a host’s digestive track, usually completely unknown to said host. While the idea of having a 55 foot long worm living inside you may not be particularly appealing, tapeworms usually don’t seriously impact their host. In fact, they were once even marketed as a “no ill effects” fad diet because worms sometimes cause hosts to lose weight.
Though tapeworms may be well known, they are far from the only kind of parasite. And many parasites don’t go quite as unnoticed.
Parasites are an incredible category of creatures because they are so vast and varying. Some parasites are microscopic, others easily seen. Some are insects, some are fungi, some are fish. Parasites aren’t a single species or class, but more of a type of behavior. And while humans may find the idea of a parasite disgusting, some are actually beneficial to the ecosystem and many aren’t very harmful to their hosts.
But those aren’t the parasites I want to talk about.
Imagine a parasite that could not only wiggle into your body and feed on your insides, but could also hijack your brain, turning you into a literal zombie controlled by the parasite’s desires? It may sound like something out of a John Carpenter movie, but this is reality. And there isn’t just one species of parasite capable of this. There are many.
Schistocephalus solidus is a species of tapeworm with a complex life cycle. It hatches in the water where the larva are (intentionally) eaten by tiny crustaceans called copepods. The copepods are a favorite food of the tapeworm’s next host, the Stickleback fish. Once inside a stickleback, the parasite begins to grow. It causes changes to the fishes instinctual behavior, making it seek warmer waters (preferred by the parasite) and become more solitary. Once the tapeworm is matured enough it changes the fishes behavior even more, causing it to ignore all of its previous survival instincts and approach the waters surface, attracting the eyes of predatory birds. The Stickleback is eaten by a sea bird, and the triumphant tapeworm sets up shop in its new host where it mates, with the eggs being excreted by the bird back into the water.
Euhaplorchis californiensis is a worm with a very similar lifecycle. This parasite also starts life in the water, where it enters it’s first in a series of hosts, the horn snail. A side effect of its stay is the horn snail will become sterile, which is actually beneficial in controlling the snail populations. After a while the worm moves on to its next host, the Killifish. It enters the fish through its gills and plants itself in the fishes brain. Once there it begins to implement some important changes. Much like the previous tapeworm, this worm’s goal is to get its fish host eaten by a bird. It does this by controlling the fish and forcing it to dance and jump near the water’s surface. The poor Killifish is essentially turned into a suicidal zombie, and eaten by a bird so that the worm can continue its lifecycle.
Leucochloridium paradoxum is a truly horrific parasite. This species of flatworm enters an unsuspecting mollusk host and travels into the animal’s eyestalk. There it grows, inflating the stalk into a bulbous, pulsing mass. The parasite displays bright colors and patterns through the snails transparent skin, mimicking the appearance of a caterpillar. As if this wasn’t all disturbing enough, the parasite also takes away the snails inhibitions; snails parasitized by the flatworm were more likely to go to open, well lit areas where birds might see them. The light is the cue for the flatworm to begin to move, and attract its next host.
Myrmeconema neotropicum is a kind of nematode that infects south american ants. Ants pick up the parasite on food they bring to their larva. The young ants become infected before they can even leave the nest. The nematode lays eggs inside the ant’s gaster (essentially the butt). As they become adults the infected ants usually black gaster becomes translucent, revealing the red embryonic parasites. Because of the ants home (in the trees) and round red behind, it now eerily resembles a berry. Especially to passing birds. The ant becomes more sluggish the larger (and redder) its gaster gets, and somehow the parasite even convinces its host to hold its rump up while walking to be more obvious to passing birds! Once eaten the nematode finishes its lifecycle in the bird and is moved through dropped feces to a new ant colony to start again.
Ophiocordyceps unilateralis isn’t a worm like the previous entries in this list. Instead it is a type of fungal parasite. When an ant becomes infected by the spores of this fungus, it suffers from convulsions which cause it to fall out of its treetop home. The ant is then piloted by the fungus toward a more suitable habitat (for the fungus of course). The ant will go to a very specific height, in a very particular heat and humidity level, and find the northern side of a leaf. The fungus then compels the ant to bite and lock onto the leaf. The ant will never leave the spot, and the fungus will slowly consume its insides before bursting stalks out of the ants head to release spores and infect other ants. In Brazil and Thailand where this fungus lives, certain areas of the forest can be covered in the parasitized carcasses of entire colonies of ants. Luckily many ants have developed a way to sense an infected ant, which they will forcibly remove from the colony and carry far away to prevent further spore transmission.
Dicrocoelium dendriticum is a type of liver fluke. Like most parasites, its life begins in a pile of poop. A passing snail eats or trails over the feces, and the fluke enters its body. The snail’s body coats the flukes in slime as a defense mechanism and spits them back out, but this is all part of the plan. The fluke is now a tasty slime coated lunch for an unweary ant, who ingests the fluke. Once inside its host, the flukes next goal is to make its host someone elses lunch (noticing a trend yet?). But what this fluke makes the ant do is truly incredible. Each night the fluke takes complete control of the ant and steers it up the nearest blade of grass. There, at the top, it sits all night long, waiting. If the fluke is lucky and it’s plan works, a grazing animal such as a cow will come by and chow down on the grass (and thus the ant and fluke). If not, and the sun rises, the fluke gives control back to the now likely very confused ant. The fluke and ant will both die if they sit all day in the hot sun, cooking alive. But the next night the fluke will take control again. Once the poor mind controlled ant is ingested, the fluke swims to its new hosts liver and reproduces. The eggs are excreted, and the cycle starts anew.
Toxoplasma gondii has become well known by scientists and doctors alike, because it is one of the few parasites on this list confirmed to be transmittable to humans. In fact, this parasite can infect most warm blooded animals. Gondii may be a single celled organism, but it’s mind control abilities are eerily advanced. The ‘intended’ life cycle of Gondii is to be picked up by a rodent. From there the parasite rewires the rodents brain to be attracted to (instead of weary of) the smell of cat urine. The rodent will linger around areas trafficked by felines and likely be eaten, where Gondii will go onto its next stage. Occasionally however, a different host may pick up Gondii. It is estimated that nearly 50% of humans are infected by this parasite, and most will go about their lives totally unaware of it. But does it have an effect on humans? After all, a rodent/rat brain is far more advanced than most of the other mind controlled hosts in this list. Is it possible that humans could be controlled? So far no one is really sure. But scientists have found some interesting correlations between infected humans. They are more likely to have schizophrenia or bipolar, and more likely to get into car accidents. Whether coincidence or not, we may never know.
Sacculina carcini is a species of barnacle. While a barnacle might not seem particularly horrifying, this parasite is a crab’s worst nightmare. The female barnacle enters a crab host and sets up shop on its abdomen. The crab becomes infertile, unable to molt, and unable to regrow lost limbs. The barnacle prefers a female crab, but if it mistakenly enters a male, it’s no problem. The barnacle will essentially change the male crab into a female. The parasite will disrupt a male crabs hormones, causing physical changes to the the crabs body (widening of the abdomen) as well as behavioral changes (female mating dances). The reasoning is that the crab will now take care of the barnacles eggs as its own. The parasitic barnacles will remain with its host for the entirety of its life.
Hymenoepimecis argyraphaga is a species of wasp that attacks a Costa Rican spider. The adult insect finds a host and grabs it, temporarily paralyzing it with a sting, and lays an egg on its abdomen. Once the spider is released, it goes about its normal life for some time, likely thinking it has escaped safely. But the egg will hatch out a larva in the meantime, which latches like a leech to the spider and slowly drinks its blood. Once the larva has had enough, it injects a mind altering chemical into the spider. The spider is then compelled to build a very strange web, unlike any before. It makes one with thicker, stronger anchor threads. It likely doesnt realize that the reason for this is to hold the weight of its hitchhikers cocoon. Once built, the spider will settle in the center of the web, where it will sit calmly as it is killed and completely sucked dry by the larva. The larva builds its cocoon in the stronger, durable web.
Dinocampus coccinellae is a parasitic wasp that hunts down its host, a ladybug, and lays an egg on its abdomen. The larva that hatches will proceed to begin eating the small bits of the ladybug (including gonads when available), until it is ready to pupate. It paralyzes its host and creates a cocoon beneath its body, using the ladybug as a shield. The ladybug color wards off hungry predators like birds, and if an insect approaches the larva makes the host twitch to scare it off. After about a week the wasp emerges and moves on, leaving its protector behind. Unbelievably, in about 25% of cases it is possible for a ladybug to survive the hostage ordeal and awake from its paralysis.
Glyptapanteles is yet another kind of parasitic wasp. This kind seeks out a caterpillar whom it lays its eggs inside of. After a while the larva hatch inside the caterpillar. They feed on the host, avoiding its organs, and eventually chew out of its body, leaving exoskeletons behind to plug up the holes they emerge from. The reason for this is to keep the poor caterpillar alive; they aren’t done with it yet. The larva make cocoons nearby while the caterpillar, piloted by a remaining sacrificial larva, guards over the cocoons, spins silk (which should have been used to make its own cocoon) over them, and thrashes wildly at approaching predators to scare them off. Unlike the “lucky” ladybug of our last entry, the unfortunate caterpillar (along with its heroic larva pilot) will eventually starve to death. Broods with a caterpillar guard were far more likely to survive to hatching.
Ampulex compressa is a deceptively beautiful species also known as the jewel wasp. This wasp finds a very unlucky cockroach which it stings to paralyze. This is to make it easier for the wasp to administer the next, much more precise sting. The wasp aims for the roaches ganglia, a region that controls the escape reflex. Once administered the roach becomes a docile slave to the wasp, who leads it by the antenna like a leashed dog to its lair. Once inside, the wasp lays an egg on the roach, then entombs it in the burrow. The roach then sits and waits until the larva hatches and burrows into its body. The larva slowly devours the roach, eating it methodically in a way that will keep it alive the longest until it can form a cocoon. Finally the fully formed wasp emerges from the husk of its roach host.
Horsehair Worms are the umbrella term for several different species of similar parasitic worms that use an insect host. Some prefer crickets and grasshoppers, some mantids, and some spiders. But they all have one eerie trait in common; when they are ready to move onto their next (aquatic) stage, they compel their host to jump into the water. Some seem to make use of the hosts ability to detect humidity, some reflected light (such as on the waters surface). Scientists still know little about these parasites, but they theorize that the worms use chemical neurotransmitters to control their hosts. In the case of parasitized crickets they will effect the insects behavior, preventing it from chirping which can attract predators and expends energy. Some worms can be a foot long, filling nearly the entire body cavity of their hosts. Miraculously, if a host can make it to land, they can often survive the worm exiting them.
These were only thirteen examples of an untold number of different kinds of parasites capable of affecting the behaviors of animals and even controlling their minds. So far none have been shown to affect humans in a significant way, but parasites are still something that scientists know extremely little about. The idea that our behaviors may be being dictated by a parasite without us even realizing is chilling to say the least, and I have a feeling that in the coming years we will learn about even more nightmarish invaders.