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Some parasites can hijack the brains of their victims and cause them to behave in strange ways, but how they do it, and do we humans need to be worried?

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Imagine this: you’re minding your own business when all of a sudden, you’re mysteriously seized by the overwhelming urge to climb to the roof of your building after work and just stand there. All night long.

Come morning, you head back down and go about your day as if what you just did was totally normal. And then you do it again and again, every night. You may have heard of parasites that can hijack the brains of their victims and cause them to behave in strange ways like this.

But what’s even cooler than what these parasites can do to their hosts is the question of how they do it. It’s a field of science called neuroparasitology. And by studying these cases of mind control, scientists can gain deeper insights into how animals control their own brains.

One of the most direct ways to take over an animal’s brain is to move in and make yourself at home. Which is exactly what the Lancet liver fluke does to the ants it parasitizes. Ants are only one of this flatworm’s three hosts.

It needs to move between a grazing animal, a snail, and an ant to complete its life cycle. And while that might seem complicated, a lot of parasites have multi-host life cycles like this, which might be because some species are more abundant and easier to control. By moving through these intermediate hosts, the parasites can make it more likely they’ll find their way back to their main target, their final host, where the adult parasites live and reproduce.

As their name implies, adult liver flukes live in the livers of grazing animals like cows. Their eggs are excreted in the cows’ poop, and then eaten by a snail. The larvae develop inside that snail for several months, then travel to the snail’s respiratory system, where they’re covered in slime and excreted.

And ants apparently find these slime balls irresistible. Then the parasite needs this ant to be eaten by a cow or sheep, which traditionally don’t eat ants. They do eat grass, though, and that’s where a little mind control comes in handy.

After the slime ball is ingested, the flukes break out of the ant’s stomach. They aim for the ant’s head, but only the first to get there will make itself comfortable next to one of the ant’s cerebral ganglia, the ant’s version of a brain. It becomes what German scientists called the “hirnwurm”: a brain worm.

Specifically, it hunkers down at the base of the ant’s mandibular nerves, the nerves that command the ant’s mouthparts. And from this strategic spot, it can control the ant’s behavior in a bizarrely precise way. During the day, the infected ant carries on with its business.

Nothing to see here, just a normal ant doing anty things. But every evening when the temperature drops, it leaves the colony and spends the night camped out on the tip of a blade of grass. And it doesn’t just sit on the grass.

The ant, presumably under the control of that strategically-placed brain worm, bites down to firmly attach itself, ensuring it’s not knocked off. Since many grazers feed in the cool morning hours, all the ant has to do is patiently wait to be eaten. Then, if the ant is still alive when the temperature rises again, it climbs down and carries on as if nothing is wrong.

And this happens every night until a grazer finally gobbles up the ant. When that happens, the worm that had made it to the brain doesn’t actually get to enjoy the spoils of all its hard work. But the flukes that were hanging out in the ant’s body get to make their way to the mammal’s liver to become adults.

The brain worm has been too busy piloting its giant ant suit to develop, so it has to sacrifice itself for its siblings. Scientists are still trying to figure out exactly how the brain worm compels the ant, but it may employ tactics similar to parasitic fungi that also get ants to climb up things and bite down. Ophiocordyceps fungi infect ants and other insects and make them travel to a strategic spot so they can spread their spores.

Studies suggest the fungi secrete compounds that alter gene expression, targeting everything from the ant’s internal clock to its ability to smell in order to ensure they’ll be in the right place at the right time. And manipulating gene expression is a pretty useful strategy if you want to control movement behaviors; just ask the hairworm. The parasitic hairworm develops inside crickets, which live on land, even though as adults, the worms live and reproduce in water.

So they get their hosts to drown themselves. The hairworms produce a unique set of proteins to manipulate their hosts, including ones that look an awful lot like proteins normally found in insects. These induce changes in gene expression in the cricket’s brain, altering the levels of several proteins including ones involved in geotactic behavior, the way something orients itself in response to gravity.

There’s also evidence that the parasite affects phototactic behavior: the way something orients itself in response to light. And this ultimately means that the hapless cricket goes for a swim whether it wants to or not. These studies are helping elucidate the neuroscience of navigation, providing insights as to how animals big and small sense their world and make their way around.

Parasites can also manipulate their victim’s behavior more indirectly, by making them sick in a useful way. Many animals act differently when they’re ill, like eating less or being lazy, and these sickness behaviors are initiated by the immune system. So if you’re a parasite that wants your host to eat less for some reason, you don’t have to figure out what neurons or genes to manipulate, all you have to do is trigger the right immune response.

And one wasp has actually partnered with a virus to turn its host’s immune system against it. The wasp lays its egg in the soft underbelly of a live ladybug. After about three weeks of developing inside the bug, the wasp larva tunnels out and encases itself in a cocoon.

The ladybug then sticks around to guard this pupa, lying on top of it and twitching to discourage predators. And that probably happens because the mother wasp injects a virus along with her egg. The virus replicates inside the ladybug’s cells, especially brain cells.

But while the larva is still inside the bug, it somehow suppresses the bug’s immune system, so the infected cells are left alone. Then, when the larva leaves to pupate, the ladybug’s immune system kicks in to fight the virus, and inadvertently ends up helping it manipulate the bug’s behavior. Scientists think the “bodyguarding” behaviors are neurological symptoms that occur because of damage the immune system causes to the bug’s brain when it attacks the virus.

They’re still working out the details of how, but the timing of the different stages of the infection and the onset of different aspects of the ladybug’s strange behavior line up perfectly. This kind of research can help scientists gain a better understanding of neurological diseases, especially how behavioral symptoms relate to immune responses. Weirdly enough, the ladybug is one case where the victim isn’t doomed.

Afflicted ladybugs sometimes recover completely from their ordeal. Their damaged nerve cells can regrow if they defeat the virus. For the wasp, that’s totally fine, since it’s already done what it needed to do.

And it didn’t even have to expend energy to make complicated, mind-altering chemicals. It got the virus to do the heavy lifting. But sometimes, if you want a job done right, you just gotta do it yourself.

The emerald cockroach wasp doesn’t take any chances. First, a female wasp temporarily paralyzes a cockroach with a quick venomous stab. Then, she uses her stinger to inject more venom directly into the cockroach’s brain.

She’ll even take her time and poke around to make sure she’s hitting just the right spot, parts of the roach’s ganglia involved in locomotor processing. The sting makes the roach calm and complacent. The wasp can then literally lead it by its antennae to her burrow, where its body is consumed by her young.

This zombie-like state is achieved thanks to a compound in the venom which dials down the excitability of the neurons, making them less likely to fire. It’s not that the roach can’t fight or run away, it’s that it no longer wants to. In the lab, affected cockroaches can still do things like fly in a wind tunnel and right themselves when flipped over.

But even as the larva eats its way through the cockroach’s organs, it doesn’t make any attempt to escape. It just waits patiently until it finally dies. Understanding the neurological mechanisms employed by these wasps can help scientists better understand how brains control decision-making.

Research like this can even give us fresh insights into the nature of free will. Of course, the idea of having your mind, or even just your body, controlled by another being is understandably terrifying. But these are all insects with small, simple brains.

It’s not like any of this could happen to us and our big, complex brains, right? RIGHT? Uh… Well… Ok, for the most part, no.

Our brains have lots of redundant connections which act as backup systems, and that means controlling a few neurons doesn’t have as big of an effect. It would be really hard for a parasite to control our behavior in a specific way like making us bite down on something, jump in a pool, or be patient while we’re eaten alive from within. Also, we have these wonderful things called skulls that prevent most parasites from getting easy access to our brains.

So if they wanted to turn on or off specific regions, they’d have to get all up in there like a liver fluke does to an ant. But to prevent that, we have a handy defense mechanism called the blood-brain barrier. That’s a layer of tightly packed cells separating the capillaries that feed the brain from other brain tissue.

It keeps junk and wandering parasites that are circulating in our blood from finding their way to our most vulnerable spots… well, most of the time, anyways. There is some evidence that one parasite can manipulate human behavior, although maybe not on purpose. It’s a protozoan called Toxoplasma gondii.

It’s able to sneak around that blood brain barrier by infecting the cells that line the brain’s blood vessels. And that’s exactly what it does in mice. Toxoplasma’s final hosts are cats, but to get there, they first infect small animals, especially rodents.

An unlucky mouse becomes infected when it accidentally consumes Toxoplasma eggs. The young protozoans find their way into the nerves and muscles and create cysts inside these cells. And in mice, they seem to make a beeline for the amygdala, a part of the brain involved in processing fear.

Though it’s not clear how, these cysts alter the rodent’s behavior, mostly by making them less afraid of cat smells. As you can imagine, that doesn’t work out so well for the mouse. But it turns out that if we spend some time around infected cats that are shedding eggs, the parasites can get inside our brains, too.

There, they can live for decades, and some scientists think they mess with our heads in much the same way. For example, some studies suggest that people infected with Toxoplasma are more reckless or extroverted, though such results are pretty controversial, and not all research has backed them up. But such effects, if real, are more like personality nudges than outright mind control.

So, a parasite-induced zombie apocalypse is probably not something to worry about. But neuroscientists are excited about all the new things we’re learning from these little puppeteers, especially about human brains. Understanding mind control at the molecular level can help neuroscientists better understand how neurons work in general, including ours.

And that could lead to a deeper understanding of how our brains work, or what precisely happens in mental illness. Someday, we might even figure out how to harness the chemicals these parasites use for medical applications, like to treat mood disorders. And totally not to take control of other people.

Probably. But there is one thing that neuroparasitology is already pretty useful for. And that’s inspiring some pretty creepy and gruesome horror stories.

And Halloween is coming up, so if you’re working on decorations for your zombie science lab haunted house, you’re going to need a classic horror portrait to tie the room together. All this week, we’re showcasing Skillshare classes we think you’ll like, and if you’re into art and horror, we think you’ll enjoy the class Paint a Horror-Themed Portrait taught by Damien Mammoliti. The class is fun and perfect for this time of year, but really all the lessons are pretty timeless.

As a digital artist, Damien shares the steps of his process to create professional, marketable art. No matter how spooky or not you want your portraits to be, you’ll learn a lot from this class. And Skillshare has over 20,000 other classes taught by experts in their fields.

Just click on the link in the description to take advantage of the offer from Skillshare to get two months of free access to all of their classes. And, if you do paint a horror portrait, may I suggest myself as your inspiration? *Rar!* [♪ OUTRO].