The Most Horrible Parasite: Brain Eating Amoeba

A war has been going on for billions of years that breeds well armed monsters, who struggle with other monsters for survival. Having no particular interest in us, most of them are relatively harmless, as our immune systems deal with their weapons easily. But there are exceptions. Naegleria fowleri is an amoeba that has not only developed a deadly taste for human brains but is also a match for our defences and stars in dramatic headlines.

What happens when this monster enters your body? Naegleria fowleri is an amoeba, a microbe with a nucleus, one of the smallest lifeforms on earth. It is a voracious hunter of bacteria and other critters that it devours whole and rips into pieces. Like many amoebae, it is able to transform into different stages that help it survive, but most of the time Naegleria fowleri is in its trophozoite stage, during which it looks like a squishy blob with tiny arms and hunts, divides and thrives.

Its natural home is in fresh water: ponds, rivers, lakes and hot springs. But unfortunately, it also feels happy in pipes, swimming pools, fountains or spas when they are not properly treated. The warmer the water, the more it thrives and multiplies. So in the summer, when humans seek to cool off and enjoy themselves, the chances are highest that both species will interact. Because this makes it hard to avoid, millions of people regularly have contact with the amoeba, especially in warmer climates, and many people even seem to have antibodies against it.

And this is mostly ok; you can even swallow it without consequences. Things turn bad when people dive or swim in water contaminated with the amoeba and water splashes high up into their noses. In a single drop of lake water, there are millions of viruses, bacteria and amoebae and that isn’t really a big deal. But Naegleria fowleri is different. Let us zoom into the nose of an unsuspecting victim enjoying a great summer day and see what happens.

First of all, the amoeba doesn’t really want to be inside your nose as it is not really looking for trouble, it just wants to eat a few bacteria. Instead, it is greeted by your natural defenses. Unfortunately for humans, Naegleria fowleri happens to be exceptionally good at generally flying under the radar of your immune system. For example, the inside of your nose is covered by mucosa, a slime filled with chemicals that kill or stun possible invaders or alert immune cells. But Naegleria fowleri is not particularly bothered by them and instead calmly checks out the scenery, mildly annoyed about the whole ordeal.

Now, if you are unlucky, the tiny critter stumbles over something that actually sparks its interest: nerve cells. Your nose is filled with a large network of olfactory nerve cells that pick up molecules from the outside and transmit their information to your olfactory bulb, the center of smell in your brain. To do their job, these cells talk to each other by releasing various messenger chemicals and recognizing them via specific receptors. One of the most important of these chemicals is acetylcholine. Through sheer evolutionary bad luck, Naegleria fowleri happens to have receptors that recognize acetylcholine. And it seems to attract them irresistibly, a little like moths that are attracted by light.

So as your olfactory nerve cells do their job, using plenty of acetylcholine to talk to the brain, Naegleria fowleri enters your tissue. It seems to follow the chemical signals upstream. Neutrophils, crazy suicide warriors, begin to attack the amoebae. Individually they have no chance against them as the invaders are large and pretty buff fighters, used to dealing with tough enemies. So the defenders swarm the intruders and kill them either by vomiting chemicals that punch holes into them or by literally ripping parts of them off and devouring them.

But the Naegleria fowleri train is still on track and while the Neutrophil attacks slow them down, they continue to follow the olfactory nerves to their final destination: your brain. This process can take between one and nine days and you’ll probably not notice anything during that time. Until the amoebae reach the olfactory bulb, the center of smell and entrance to your brain. Your brain cells are nothing more than helpless victims and they all release that wonderful acetylcholine. Naegleria fowleri initiates a massacre and releases an onslaught of various attack molecules.

Some of them are basically little bombs that rip holes into your cells on contact so their pieces can be eagerly consumed. But Naegleria fowleri is now multiplying - and it's also becoming really creepy. In a feeding frenzy, it can develop up to a dozen suckers called food cups, that look like giant eerie mouths. The amoebae engage your brain cells, suck them in and rip large “bites” out of them while they are still alive. Now things escalate quickly and the disease that will kill you sets in.

Alerted by the massacre, millions of immune cells, Neutrophils, Eosinophils and microglias invade the infected tissue. Which is a problem: your immune system is dangerous and not exactly a careful fighter. It's like burning down a forest to kill the wolves inside it. A really bad idea in the brain. They waste no time and attack the amoeba, using all the weapons available to them, from chemicals to trying to eat them alive. Neutrophils explode themselves to erect barriers spiked with deadly chemicals. A fierce battle ensues. Naegleria fowleri can actually fight back, itself attacking and killing many immune cells.

The immune system now throws everything it has at the invader but in vain. The complement system, tiny protein bombs that can kill intruders on their own, are easily disabled. Antibodies, usually one of your superweapons, are just destroyed or swallowed. A high fever that usually slows enemies down does nothing, as the amoeba actually thrives in the heat. All the while the amoebae continue to multiply, fight and devour your brain cells. A disastrous chain reaction is taking place.

One major thing your immune cells do when they fight is to cause inflammation, which directs large amounts of fluid from your bloodstream into the site of an infection. So as the battle rages on without a clear winner, more and more fluid enters the brain. At this point, the human will feel symptoms that quickly escalate. It all begins pretty vaguely, a headache, fever, nausea and vomiting. As the battle spreads rapidly through the brain serious symptoms appear, from confusion, inability to concentrate, to fatigue, seizures and hallucinations.

The brain swells up massively but can’t expand due to the bones surrounding it. So it compresses and disables the brainstem that controls things like breathing. Usually within a week the patient dies. Up to 97% of patients infected by the amoeba share this fate. In almost all cases, by the time an infection by Naegleria fowleri is recognized, the disastrous battle for the brain is already so far along that there is almost nothing to be done. Not only do we currently not have effective treatments, there are also an abundance of open questions about how an amoeba that usually enjoys its life in open water is able to overcome our immune system so effectively.

So how worried do you need to be about this horrifying killer amoeba? Well, not very. While the Naegleria fowleri is clearly extremely deadly and the infection truly horrible, there have only been a few hundred cases in the last few decades. You are way more likely to drown in a pool than to get infected. Not only does the amoeba need to be flushed high up your nose, it also needs to get a good grip and it also has to make its way through the first lines of your defences. Ultimately, Naegleria fowleri is neither evil nor a huge public health risk.

But every year some unlucky people have to deal with it. We still have so much to learn about it and until we find a way to treat it, Naegleria fowleri will continue to be this vague and horrifying thing, hunting in puddles and lakes and sometimes pools. Usually for bacteria. And very occasionally, for people.

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