Biological zombies

How certain creatures can control the minds of their unwitting hosts…

Written by: Livia Lisi Vega

Art by: Cheng-Yu (Kou)

Ideas such as mind control,  or creatures left without free will by the actions of some kind of master or virus, might seem to only be possible in Hollywood films and TV shows. However, as many philosophers (such as Descartes in his Meditations) have stated, we can only imagine based on what we already know. Therefore, we shouldn’t be surprised to find out that there are creatures which do possess such  mind-controlling abilities.

These organisms are usually parasites that manipulate their hosts’ nervous system to make them carry out actions which benefit the parasite. This simultaneously marvellous, and terrifying, product of natural selection can force the host into taking counter-instinctive actions and often results in the death of the host, in order for the parasite to reach a specific developmental stage.

I have selected a few of the most interesting parasites to discuss, from those who infect the simplest of hosts to those capable of infecting creatures  as complex as humans.


Hairworms are part of the phylum Nematomorpha (a group of parasitic flat worms) which infect all kinds of insects. The most famous ones, Spinochordedes tellinii and Paragordius tricuspidatus are parasites of the family of insects formed by crickets, locusts and grasshoppers.

These parasites are capable of behavioural manipulation that leads to suicidal behaviour in the host; they hijack the host’s nervous system and change its responses to water so that, instead of avoiding it like all normal land insects, they will commit suicide by seeking and jumping into the water. This mind-control can be explained by the fact that the last developmental stage of the worm and its reproduction needs to take place in water.

The worm’s larvae are then ingested by another cricket/locust/grasshopper and start growing inside it. While developing, the worm’s larvae start altering the host’s protein expression in some way, depending on the infection stage.

Once adult, the worm starts producing specific molecules that target the host’s central nervous system (CNS). It has also been shown that the worms produce special molecules (Wnt family) involved in the development of the CNS, that show important similarities with those produced by the insects they infect, suggesting a possible and interesting case of molecular mimicry.


The Emerald Cockroach Wasp or Jewel Wasp (Ampullex compressa) preys upon cockroaches. The behavioural alteration of its host is one of the best-understood instances, and is also one of the best examples of host manipulation for offspring care.

Firstly, the wasp injects a toxin into the cockroach’s thorax so that its legs become paralysed. Then, a more precise sting is carried out by the wasp in order to inject a cocktail of chemical substances that are targeted to a specific area of the brain responsible for initiating movement.

After this, the wasp grabs the cockroach’s antenna and leads it to a preselected burrow where the wasp will lay its eggs. Incredibly, the cockroach follows the wasp in a docile manner as if it had already accepted its fate.

Once in the burrow, the wasp attaches its eggs to the cockroach’s abdomen and leaves, blocking the burrow’s entrance. The larvae start hatching from the eggs and start feeding from the host. Curiously, they know how to avoid vital organs, so that their host/prey stays ‘fresh’ as long as possible. The feeding process continues until they pupate inside the cockroach’s abdomen.

This process has been shown to be rather complex and time-consuming, with wasps targeting not only the CNS, but also specific peripheral ganglia that would help to achieve a local central paralysis. Furthermore, it has been suggested that the host-handling behaviour is genetically programmed rather than learned.

Parasites that infect humans?

The more we know about the intricate mechanisms used by these parasites, the more shocking nature seems. The idea that these ‘scary movie’ concepts might in fact be inspired by real life gets even more alarming when we discover that humans themselves are not free from these mind-controlling parasites; it seems zombies are not just a creation of science fiction after all…

Toxoplasma gondii is a protozoan that has a high prevalence of infection in warm blooded animals, including humans. This fascinating parasite can maintain itself indefinitely in any warm-blooded animal, but can only sexually reproduce in the intestines of cats.

There are different types of vertical transmission of this parasite, but one of the first studies was done in mice. To achieve the vital function of reproduction, T. gondii makes infected rats and mice lose their innate fear of cats. According to a Rockefeller University study, they are even sexually attracted to a pheromone in cat urine .

Infected rats and mice tend to show less fear to a spectrum of aversive stimuli; this makes them more prone to being eaten by a cat, facilitating T. gondii’s transmission.

Once inside the cat, T. gondii starts reproducing inside the intestinal cells. Eventually, infected cells rupture and liberate the oocysts in the intestine’s lumen where they are dislodged in the cat’s faeces.

Although humans are not the definitive host of the parasite, they’re also under risk of infection. In fact, scientists believe that around thirty percent of the human population may be infected.

Gondii infections in humans have been linked to several behavioural traits and disorders like schizophrenia or bipolar disorder. It appears that the cysts of this parasite play a role in manipulating neurotransmitter release from infected cells, especially that of dopamine. Accordingly, the steep increase of cases of schizophrenia during the 20th century has been found to coincide with the raise in popularity of domestic cats.

Furthermore, T. gondii infection has been also blamed for the state of dementia in people with end stage AIDS, since the severe immunodeficiency would enhance the effects of the parasite on the nervous system.

Additionally, different behavioural patterns have been observed in infected individuals depending on their sex and environment, with infected women being more sociable and trusting than uninfected women and infected men being more suspicious and introverted than uninfected men. It is believed that this is because T. gondii’s infection replicates the symptoms of anxiety and emotional strain in humans. Psychologically, anxious men tend to adopt an antisocial behaviour and women usually seek social acceptance and company.

Another more well-known parasite that alters human behaviour  is the protozoa belonging to the species Trypanosoma brucei, responsible for sleeping sickness.

This parasite’s life cycle starts in invertebrates and progresses to complex mammals whose fluids they devour. It is transmitted to humans through the TseTse fly and goes through several developmental phases before directly influencing the host’s behaviour.

Behavioural changes are a consequence of the parasite’s encephalic phase. Once it crosses the blood-brain-barrier and sprouts in the host’s brain tissue, it disrupts the host’s sleep/wake cycles and its alertness state. This is due to the particular predisposition of the parasite to sprout on the hypothalamus, the center of the brain responsible for regulating our mood and sleeping cycle, where it mainly alters the release of melatonin.

After some years, the infection will lead to tremors and ultimately to a prolonged sleep that can develop into a coma, which  usually ends with the host’s death.

The high number of parasites that have evolved  in order to infect their  host’s brain suggests its suitability for accommodating foreign organisms. With this in mind, is it possible that our thinking process is influenced by more than just our external environment?

For more information about zombie parasites, check out this video by National Geographic Live:

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