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Exploring the Darker Side of Everything

The Zombie Fungus That Takes Over the Brain

Written by Laura Davies

Ophiocordyceps unilateralis is a parasitic fungus that infects ants. Well, I say infects, what I actually mean is that it consumes ants from the inside out, replaces their tissue with fungus, controls them like a puppet master, kills them, and uses their corpses to launch another wave of spores. This probably isn’t an episode to watch while you’re eating.



As a fungus, Ophiocordyceps unilateralis thrives in dark, humid environments. In particular, the floor of tropical rainforests in regions like Thailand and Brazil. Its prey of choice are canopy-dwelling carpenter and bullet ants.

Seemingly as a way to avoid deadly fungal infections, these ants spend the majority of their time foraging in the treetops, 20 m above the ground. However, sometimes they have to venture down to the forest floor if there’s a gap in the canopy that’s too difficult to cross. As far as scientists can tell, because, you know, they’re insects, they do this pretty reluctantly, as whenever they reach the ground, they’ll spend no more than half an hour there before ascending another tree.

Unfortunately for the ants, this short window of time is all it takes for a fungal spore of Ophiocordyceps to land on and attach itself to their cuticle. At this point, the spore will germinate and begin its attempt to break through the exoskeleton. It uses two methods to do this.

Firstly, it secretes the enzymes chitinase, lipase and protease. These begin digesting and weakening the ant’s cuticle. Secondly, it forms an infection structure called an appressorium. This can exert enormous mechanical pressure, equivalent to the inside of the wheel of a Boeing 747. Pretty soon, it becomes strong enough to blow a hole through the cuticle. It’ll then send an infection peg past the ant’s defences, allowing the fungal material to flood in.



An ant’s circulatory system is known as its hemocoel and, once breached, lacks any defence against the fungus. The single Ophiocordyceps cells are free to travel, and they multiply quickly. This alone would be bad news for the ant, but the fungus has another, more creepy than a cuticle piercing infection peg, trick up its sleeve. The single cells can join up with short tubes called conidial anastomosis tubes, or CATs. These allow for both communication and the transfer of cell contents. In other words, they let the single cells begin working together as one horrifying superorganism.

At this point, the fungus could kill the ant outright. It wouldn’t take long for it to destroy neural tissue and consume the insect from the inside out. However, by this time, the ant would’ve made it back to its colony, and this would be suicide for the fungus.

You see, ants have to be pretty heartless when it comes to dealing with sick colony members as they lack a prophylactic immune system and have to rely on social immunity instead. This is just a fancy way of saying, if they spot any ant looking a little peaky, they’ll send a soldier to grab it and dump it as far from the nest as possible. Being disposed of like this would leave the fungus with no access to new hosts to spread its genetic material into.

Even if the infected individual managed to go undetected, it still wouldn’t be great for the fungus. This is because the ants nest high in the canopy, where the temperature and humidity are wrong for fungal growth. If the insect were to die there, it’d be almost impossible for the fungus to reach maturity and release its spores. One scientist in Brazil decided to test this and put infected corpses into several ant nests. None of them grew a decent stalk, and even if one had managed it, wiping out an entire colony would leave no tasty ants for the next generation of fungus to brutally murder.


Mind Control

Once the fungus has fully colonised the hemocoel, the brainwashing part of the story begins. In an ideal world, a predator requiring such specific climatic conditions for survival would have its own body and brain to control it. But, as fungi, fortunately, don’t, Ophiocordyceps just borrows that of its victims.

With headlines like “zombie ants” and “mind control fungus,” most people assume the next step for Ophiocordyceps would be to start invading the brain. However, cut up any ant dying from the infection and that would be the only place where you wouldn’t find fungus. The brain is essential to get it where it needs to go, so it can’t risk damaging it.

Instead, the CATs that I mentioned before, spread to completely surround the ant’s muscles and, in some cases, penetrate the muscle cells themselves. At peak infection, between 40 and 50% of the ant’s body will be fungus. Then, the cells begin secreting chemicals that force the individual muscles to twitch, contract, and spasm. This gives the fungus control over the ant’s movements, like an untrained puppeteer with a particularly ugly marionette.

As the infection progresses, the ant’s body is flooded with more and more chemicals designed to manipulate it. The fungus uses sphingosine and guanidinobutyric acid, compounds known to be involved in neurological disorders, and hypoxanthine, which damages neural tissues. These alter the ant’s motor neurons and remove any control the insect had left, trapping it as a prisoner in its own body.


Death Grip

At this point, the fungus can begin its mission to get the ant into the optimum position for growth and spore release. Controlling the movement of a whole body is tricky for a brainless fungus, and the infected ant will stagger around the canopy like a drunk. But, luckily for Ophiocordyceps, the destination is the forest floor, so falling from a few leaves and branches is just a quicker way of getting there.

Once grounded, the next job is to locate a prime murder site. This is a north-facing leaf, almost exactly 25cm from the ground, with a temperature of between 20 and 30 oC and humidity between 94 and 95%. A task that’d be next to impossible for most humans, but easily achievable for a fungus, whose only method of movement is to trigger individual muscle contractions in an ant.

Another important factor is that the death spot shouldn’t be too far from the ant’s original nest. This is because the fungus needs to make sure it’s in range of its next victim. This results in tens, or sometimes hundreds, of zombie ants ending up in the same area of forest, forming so-called graveyards with up to 26 bodies per square meter. Unfortunately for the remaining colony members, these are always in locations that foraging trails need to pass through. This creates a “sniper’s alley” for the foragers and a handy “deliveroo” type situation for the fungus.

Once the ant arrives at a suitable leaf, Ophiocordyceps waits until almost exactly solar noon, then forces the insect to sink its mandibles into a primary or secondary vein. And yes, the fact that a brainless fungus can tell time is potentially more disturbing than the fact that it can measure temperature, humidity, and direction. So, thanks to the team of scientists who spent weeks watching a bunch of zombie ants, night and day, to make us aware of that.

To ensure the body remains in the fungi’s chosen spot and to reduce the chance of it being dislodged by rain or predators, the ant then needs to be locked in place. To achieve this, the fungus releases chemicals that atrophy muscles and destroy mitochondria. This process reduces the energy available and ensures the mandible can never relax, resulting in a death grip on the leaf.



Now the fungus enters its most vulnerable stage. It needs to grow a stalk and release its spores, but because it’s stationary, it’s at risk of attack from insects and other fungi. To strengthen its position, hyphae are shot from the abdomen to secure the body to the leaf, fungal tissue grows into cracks and crevices in the weak spots of the carcass, and it fashions a protective coating to defend against microbes and other fungi. Finally, the ant’s put out of its misery and allowed to die. 75% of the cells in its head are now fungal.

Within a couple of days, by far the most disgusting part of the process begins. The stalk of the fungus erupts from the base of the now-dead ant’s head. Like an enormous antenna, it protrudes as far from the body as it can reach. Somewhere along the length of the stem, a large ball will form. This is essentially a massive fungal testicle, filled with spores. After a week or two, it’ll reach maturity, and the spores will be released, ready to shower down on the next victim and begin the cycle again. Delightful.

Ophiocordyceps unilateralis is such an effective ant killer that it could be capable of wiping out colony after colony. Fortunately for the ants, though, there is such a thing as an ant-eating-fungi-eating-fungi, or hyper parasitic fungi, if you want something very slightly shorter.

This is a type of fungi that can infect the Ophiocordyceps and castrate it, rendering it unable to produce viable spores. The little hero fungus is so effective at smashing fungi gonads that only around 6.5% of Ophiocordyceps are able to infect new hosts. This is obviously of no help to the ant, who is long dead before the hyper-parasitic fungi show up. However, it’s nice to know that it’s been avenged, with a bit of sterilisation thrown in for good measure.


Potential for Human Infection

As if the entire process wasn’t nightmare-inducing enough, let’s talk about whether or not Ophiocordyceps unilateralis could make the jump to humans and turn our innards into fungus. The idea has already been graphically illustrated for us in the game “The Last of Us,” where fungus turns humans into mutant mushroom “clickers,” if you want to see what that might look like.

Obviously, the game is a work of fiction, but, unfortunately, it’s not as unbelievable as you might think. In 1954, a batch of grain infected with a cordyceps fungus, similar to the zombie ant fungus, was delivered to a small French town. Consuming it induced hallucinations and manic episodes in the residents, resulting in one 12-year-old girl trying to kill her mother with a kitchen knife.

However, before you start stockpiling antifungals and smothering yourself in athlete’s foot cream, I should probably mention why Ophiocordyceps making the jump to humans would be extremely unlikely. Zombie ants aren’t a new thing. Thanks to the discovery of a fossilised leaf with the perfectly preserved and very distinctive dumbbell-shaped scar of a death grip, we know that Ophiocordyceps has been gruesomely killing carpenter and bullet ants for at least 48 million years.

In all that time, it hasn’t even made the leap to other ant species, never mind humans. The reason for this is that the mechanisms the fungus uses to assume control are incredibly specialised for the victim species. There have been a few cases in different ant types, but infection rates and spore viability were massively reduced.

More Parasitic Fungi

This isn’t to say, though, that it’s only carpenter and bullet ants who can fall victim to parasitic fungal infection. In fact, most species of insect have their own species of cordyceps specially adapted just for them.

For example, Entomophthora muscae forces flies to climb to a high spot and assume the death pose. This involves, straightening their legs, opening their wings, pointing their abdomens upwards, and forcibly ejecting spores to rain down on others.

In another case, Massospora cicadina causes part of a cicada’s abdomen to fall off and be replaced with a spore-producing plug of fungus. As the host is kept alive, the cicada continues to fly around, sprinkling spores like a flying salt shaker of death. They’ll even respond to the mating calls of both males and females to spread their fungal STD as widely as possible.

Finally, Ophiocordyceps Sinensis preys on ghost moth larvae, producing a fruiting stalk that erupts from their heads like the horn of a unicorn. Naturally, as it looks like a horn, it’s often harvested to aid sexual stamina in traditional Chinese medicine.

While using moth-eating fungi as Viagra might sound ridiculous, the medicinal benefits of cordyceps are potentially huge. After all, Penicillin is a compound produced by a fungus, and Ophiocordyceps unilateralis has already been found to contain antimalarial naphthoquinones and small-molecule agents with the potential for use against infection and cancer. So, maybe our next antibiotic is lurking in the abdomen of a cicada, the horn of a moth, or the carcass of a carpenter ant.

Unfortunately, the two leaders in the field of zombie ant research, Professor David Hughes and Principal Scientific Officer at the Center for Agricultural Bioscience International, Harry Evans, are raising concerns that these, potentially life-saving species could be lost. More research, requires more zombie ant bodies, but the habitats where these were originally found are being destroyed. When Evans returns to his old field sites, as part of what he calls the “World Ant Tour” he’s finding them cleared and the infected ants are nowhere to be found. Great for the ants, not so great for humans facing antibiotic resistance and cancer.


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