Zombies have captivated audiences in film and television for decades, yet they are not entirely fictional. In the natural world, there exist real-life parasites that can infect and control the behavior of their hosts in ways that can be both horrifying and fascinating.

These parasites employ mind control to alter their hosts' behaviours, compelling them to act in ways that facilitate the parasite’s own life cycle, reproduction, or spreading. Often, these changes run contrary to the host's natural instincts, frequently resulting in the host's demise.

While the terrifying clickers from the acclaimed video game and TV series The Last Of Us, as well as classic zombie films, may seem purely imaginative, they draw inspiration from actual organisms that manipulate their hosts in alarming ways. Let’s delve into some of the most notorious examples of these unfortunate animal and insect victims, along with their parasitic masters.

Deadliest Zombie Parasites in the World

Zombie Ants and Ophiocordyceps Unilateralis

Among the most infamous is the fungus known scientifically as Ophiocordyceps unilateralis. This parasite, colloquially termed the zombie-ant fungus, targets carpenter ants and other members of the Camponotini tribe found in dense rainforests and jungles.

Upon infection, an ant exhibits a compulsion to ascend toward the daylight, ultimately seeking out a suitable leaf to attach itself to with its mandibles. Here, it remains motionless for several days as the fungus proliferates throughout its body. Eventually, this fungal invader bursts through the ant's head, releasing spore-filled fruiting bodies that disperse to infect other ants within the colony.

Castrated Crabs and Sacculina Carcini

Another horror of the animal kingdom comes in the form of Sacculina carcini, known as the crab hacker barnacle. This parasite primarily infects the European green crab, which inhabits coastal regions of Europe and North Africa.

A female barnacle infiltrates the crab’s body, pushing a sac out through the crab's abdomen while extending tendrils through its entire system. This insidious manipulation leads to the atrophy of the crab's reproductive organs, rendering it effectively sterile. The crab becomes a mere mobile home for the barnacle, providing sustenance and protection. To reproduce, the barnacle develops its eggs within the external sac on the crab's abdomen, which the crab instinctively treats as its own offspring, offering vigilant care and protection.

Pulsating Snails and Leucochloridium Paradoxum

Among the most grotesque of nature’s parasites is the flatworm Leucochloridium paradoxum, also known as the green-banded broodsac. This parasite begins its life cycle in a snail but requires a bird to complete it.

Upon infection, the flatworm manipulates the snail’s circulatory system, growing tentacles that eventually invade the snail’s eyestalks. These tentacle-like broodsacs pulsate and wriggle, mimicking the movements of a caterpillar or worm, thus attracting birds. Sometimes, these broodsacs will explode from the snail's body, though if fortune favours the snail, the bird will extract them while leaving the snail alive, at least temporarily.

Dancing Fish and Euhaplorchis Californiensis

The fluke parasite Euhaplorchis californiensis has a complex life cycle involving marine snails, fish, and shorebirds. Most notably, it affects killifish, which inhabit wetlands and shallow bodies of water.

The parasite enters the fish through the gills, where it migrates to the brain, forming a cocoon around it. This manipulation causes the fish to behave erratically, moving jerkily and swimming towards the water's surface in a conspicuous manner. Such behaviour increases the likelihood of being spotted and consumed by predatory birds, allowing the parasite to thrive further in its life cycle.

Biting Ants and Dicrocoelium Dendriticum

The lancet liver fluke, scientifically known as Dicrocoelium dendriticum, showcases a remarkably convoluted life cycle. Its definitive hosts are typically ruminants like cows, although humans can be incidental hosts.

The cycle begins when ants consume cysts ejected by infected snails, leading to the emergence of juvenile flukes within the ant's body. One particular fluke targets the ant's brain, directing it to ascend a blade of grass and cling tightly. From this elevated position, the ant becomes an easy target for grazing ruminants, thus enabling the fluke to complete its life cycle.

Suicidal Rodents and Toxoplasma Gondii

Toxoplasma gondii, a microscopic parasitic protist, poses a significant threat to warm-blooded animals and is found across the globe. This parasite needs a feline host for its sexual reproduction, and it has developed a terrifying method for ensuring its passage into a cat's body: it alters the behavior of rodents.

When a rat or mouse becomes infected with Toxoplasma gondii, it experiences a reduction in anxiety and a peculiar attraction to cat urine, making it more likely to encounter its feline predator. Consequently, the rodent becomes less cautious and more susceptible to predation, thereby allowing the parasite to complete its life cycle.

Are There Zombie Parasites That Control Humans?

Interestingly, humans can also be affected by Toxoplasma gondii, the very same parasite that manipulates rodents. Some studies have suggested a potential link between Toxoplasmosis infection and increased risk-taking behaviour and impulsivity in humans, although more recent research indicates that this association is not particularly strong.

Are Animals Affected by Zombie Parasites Really Dead?

Despite their name, these parasites do not initially kill their hosts, as their evolutionary goal is to ensure that the host remains alive long enough to aid in the parasite’s lifecycle and reproduction. However, once this goal is met, the fate of the host can vary dramatically—some succumb to the parasite, others are consumed, and some face even more gruesome endings at the hands of parasitoid wasps.

Why Do Zombie Parasites Affect the Behaviour of Their Hosts?

Many parasites have intricate life cycles that necessitate specific hosts or multiple hosts at various developmental stages to reach maturity and reproduce. For fungi like Ophiocordyceps, the ultimate aim is to disperse their spores as far as possible, and controlling their host's behaviour is a crucial strategy in achieving this goal.