Zombies are a popular trope in film and tv, but there are real-world parasites that infect their hosts and control their behaviour in terrifying ways.
These parasites use mind control to get their hosts to behave in a way that helps the parasite complete its life cycle, reproduce or spread, and often in a way that’s against the hosts nature - usually leading to their death.
So, while the fungus-infected clickers of The Last Of Us or the zombies of classic films, controlled by a mysterious master, might seem like fiction, they are inspired by actual organisms. Meet some of these unfortunate animal and insect victims and their parasitic masters.
- What are parasites and how are they transmitted?
- The deadliest parasites in the world: 10 terrifying creatures you will want to avoid
- Cordyceps and Ophicordyceps: the zombie fungi made famous by The Last Of Us
Deadliest zombie parasites in the world
Zombie ants and Ophiocordyceps unilateralis
The eagle-eyed (and fans of The Last Of Us) among you might notice something familiar in the scientific name of this parasite - Ophiocordyceps unilateralis. Like the terrifying zombie-maker of that computer game and tv series, this is a fungus, and it does turn its host into a zombie that seeks to infect others.
In fact, it’s commonly known as the zombie-ant fungus, and it affects carpenter ants (and other members of the ant tribe Camponotini) in rainforests and jungles.
An infected ant will be compelled to climb up trees towards the light, find a suitable leaf and latch on tightly with its mandibles. Then it stays there, unmoving for several days, as the fungus spreads through its body, eventually bursting through the ants head with fruiting bodies which spread the spores far and wide, infecting other unlucky ants from the colony.
Castrated crabs and Sacculina carcini
Crabs look out, there’s a parasitic barnacle about. Sacculina carcini, also known as the crab hacker barnacle, is a type of parasitic castrator, which is about as horrific as it sounds. Its primary host is the green crab, found along the coasts of Europe and North Africa.
A female crab hacker barnacle will insert herself into the shell of the crab. Once inside, she pushes a sac to the outside of the crab’s shell on the abdomen, and spreads tendrils through the crab’s body taking over its digestive tracts and nervous system, atrophying its gonads and stops it molting. The crab essentially becomes the mobile home and servant of the barnacle, providing food, safety and a home.
To reproduce, the barnacle will develops its eggs in the external sac on the crab’s abdomen. Under the control of the barnacle, the host crab will behave as if the eggs are its own, carrying them around and protecting them from harm, with both male and female host crabs behaving in this way.
- Parasitic wasps and white butterflies: how this ‘zombie’ wasp invades a caterpillar’s body
- Meet the Cymothoa exigua parasite, the tongue-eating isopod
Pulsating snails and Leucochloridium paradoxum
One of the most revolting and horrific parasites is the flatworm Leucochloridium paradoxum or the green-banded broodsac. This parasite completes the first two parts of its life cycle in snails, but needs to get into a bird to complete the cycle.
To do this, it spreads tentacles through the circulatory system of the unfortunate snail host. Some of these end in broodsacs, and these grow up and into the eyestalks of the snail. These broodsacs are thick, wriggly, and pulsate, mimicking a worm or caterpillar, making them eyecatching for any birds that might be about. Sometimes the broodsac will explode out of the snail, but if the snail is really lucky, the bird will pull it out leaving the snail alive - at least for a while.
Dancing fish and Euhaplorchis californiensis
Euhaplorchis californiensis is a fluke parasite that requires three completely different types of animal to complete its life cycle; a marine snail, fish and shore birds. And like other trophically transmitted parasites, it affects the behaviour of its host to give it the best chance of making it onto the next stage of its development. In particular, it affects the behaviour of killifish which live in wetlands and shallow pools.
The larvae of Euhaplorchis californiensis enter the fish via the gills and make their way to the brain, forming a layer around it. This causes the fish to move unnaturally, shaking and jerking and moving towards the surface in an eyecatching way. The eye the parasite is trying to catch is any fish-eating bird that may be about, and this dancing behavour makes the fish much more likely to become a meal. Once it's in the bird, the parasite can complete its life cycle and release eggs.
Biting ants and Dicrocoelium dendriticum
The lancet liver fluke (Dicrocoelium dendriticum) has a very complicated life cycle. Its main or definitive hosts are ruminants like cows - though it can accidentally end up in humans. But to get there, it passes through snails and ants, and it’s in ants where things get really weird.
Ants will pick up the parasite when they consume infected cysts cast off by snails, which contain hundreds of juvenile flukes. Most of these will grow into the next stage of their life cycle in the ant, but one makes a beeline to the ant's brain, or more accurately to a cluster of nerves called the sub-esophageal ganglion. One here, it takes control of the ant, causing it to climb to the top of a blade of grass and latch on tight with its mandibles. This puts the ant in prime position to get eaten by ruminants as they graze, where it can complete its life cycle. Great for the fluke, less good for the ant.
Suicidal rodents and Toxoplasma gondii
The microscopic Toxoplasma gondii is a species of parasitic protist that infects warm-blooded animals and is present world-wide. It requires a feline host for sexual reproduction, and it has a terrifying way of getting itself into a cat so it can complete this part of its life cycle - it affects the brains of rodents.
When Toxoplasma gondii infects a rat or mouse, it causes it to become less anxious, less worried about predators and less averse to cat urine. This makes the rodent much more likely to encounter a feline, and much less likely to take evasive action. The result? The rodent ends up as dinner, and the parasite ends up inside the cat, where it wants to be.
Are there zombie parasites that control humans?
Humans may also be affected by a mind-controlling parasite - Toxoplasma gondii. This is the same parasite that causes behavioural changes in rodents, and some research has suggested that there is a correlation between Toxoplasmosis infection and an increase in risk-taking behaviour and impulsivity. However, more recent studies have concluded this relationship is weak.
Are animals affected by zombie parasites really dead?
These parasites don’t kill their hosts - at least not initially! The evolutionary goal of the parasite is to complete its life cycle and reproduce, so they need to ensure their host remains alive long enough to help make that happen. But after that… it’s often game over. Some hosts die, some hosts are eaten to pass the parasite on to the next host, and some hosts - like the ones under the control of parasitoid wasps - have a much more gruesome fate and are eaten alive.
Why do zombie parasites affect the behaviour of their hosts?
Many parasites have complex life cycles with certain stages of their development requiring a host, or in some cases more than one stage and more than one host, in order to reach maturity and reproduce.
For other parasites, like the …. Fungus, the ultimate evolutionary goal is to spread their spores as far as possible, so they control the behaviour of the host in order to make this happen.
