‘Zombie’ land: Meet the parasites that take over the minds of the hosts

A recent tweet created quite a stir after it showed a dead bug walking like a zombie. The insect, disembowelled and missing most of its organs, is clearly as dead as a doornail. Yet astonishingly, making its way through the forest undergrowth, walking on its legs. The annotation of the image tweeted reads, “Do you know? According to scientists a neuro parasite has taken control of brain of this dead insect and making it walk… Zombie!”

Do you know? according to scientists A neuro parasite has taken control of brain of this dead insect and making it walk…….. Zombie ☠️💀💀 pic.twitter.com/WBS8hNvH91

— Dr.Samrat Gowda IFS (@IfsSamrat) October 18, 2022

The walking dead bug in the video appears to be a cicada; nonetheless, the identity of the infecting parasite and the bug is unclear. Intriguingly, some lethal parasitic predators can turn their prey hosts into ‘zombies’ by taking over the control of the brain.

Neuro-parasitology deals with parasites that can control the host’s nervous systems. These parasites alter their host’s behaviour to make them puppets. By manipulating the host’s behaviour, the parasite moves from one host to another, providing an unhindered supply of nutrients to hatch eggs or enabling its spread. The tricks these parasites have evolved would shame Scott Sigler’s Infected trilogy.

Here are some of the examples of ghastly mind-controlling parasites we know of.

Zombie ant fungus

Deep inside the tropical forest of Brazil, if you turn the leaf and observe the underside carefully, you may accidentally find carpenter ants clinging to the central vein with their jaws clamped tight. The ant seems dead and makes no motion, yet it holds the midrib and does not fall down.

The ant is zombified. The mind and the body of the dead now belong to a fungus called Ophiocordyceps unilateralis.

When the spore of this fungus falls, the carpenter ant is infected. The spore germinates and grows inside the insect body. The fungal colony expands by draining the nutrients and consuming most of the organs and tissues. Within a week of infection, the ant is enslaved. Like in a trance, the bug flees the safety of its nest towards nearby vegetation. Like walking dead, the ant climbs precisely to a height of 25 centimetres, crawls to the underside of a lush green leaf and clutches the midrib with its mandibles.

Meanwhile, the tissue and organs become a sumptuous feast for the multiplying fungal colony that develops a network of filaments called mycelium, replacing most of the tissue. However, the brain and the nervous system are kept untouched. Nevertheless, chemicals secreted by the fungal colony activate the brain of the dead insect to send the right kind of signals commanding the bug’s muscles to move. Like in a daze, the zombie ant climbs up the vegetation.

Once the ant reaches the desired height, inside the zombie ant, the fungal cells multiply and develop a bulbous capsule full of spores. The matured fungal stalk burst open, raining down the spores onto the foraging ants below, infecting them.

The fungal cells begin life alone, but through a network of mycelium, the fungal colony cooperates. The brainless fungal colony commands a large creature’s brain and acts like a superorganism. Ant’s brain still drives the remaining muscles, but the fungus has the wheel.

Under the influence

While an uninfected fish keeps calm and dives deep to escape from the predatory shore birds, killifish infected with Euhaplorchis californiensis flatworms often surface. Further, they whirl in circles showing off their glittering back as if seducing the predatory birds. Researchers say the infected fish, 30 times more susceptible to falling prey than normal ones, is but an instrument in the hands of the flatworms.

Known as flukes, these flatworms live in the estuaries and salt marshes along the California coast. The first victim of flatworms is horn snails. Flatworms lay eggs inside the snails, and the offspring come out of the snail to freely swim around the water looking for its host, killifish. Once it finds the fish, the flatworm latches on to its gills, crawls towards the brain, and manipulates its behaviour. Under the charm of the flatworm, the fish attracts the attention of the shorebirds. Eaten by birds, the adult worm migrates to the bird’s intestine, where it lays eggs. The eggs come out through the birds’ faeces, enter the water body and are consumed by the snail.

Cast a spell

Lancet liver fluke, another type of flatworm, takes control of the brain of ants to reach their destination, the liver of grazing animals such as cows and sheep. Eggs and the faeces excreted by the hosts are first eaten by snails. Inside the infected snail, the eggs hatch and the newborn flukes are clumped with a protective layer into small slimy balls. The snail coughs up these balls.

Fatally attracted by the moisture of the slime ball, unsuspecting passerby ants consume them. The flukes come out of the encase and move their way up to the ant’s brain, where they release chemicals that control the ant’s movement. As if a spell is cast, the infected ants climb up and sit motionless on the tip of the blade of the grass. Grazing mammals eat the grass along with the fluke-laden ants. The fluke resides in the animal’s liver, where they mature and release eggs. The eggs come out of the faeces completing the life cycle.

Puppetmasters

The most gruesome tale of a behaviour-altering parasite is that of the Glyptapanteles species parasitic wasps. Female wasp species sting and injects their eggs into the Thyrinteina leucocerae caterpillar. After a brief blackout, the caterpillar recovers and resumes its regular life.

Meanwhile, the eggs hatch and the larvae grow by drawing nutrients from the caterpillar. In two weeks, 80 strong armies of menacing fully grown larvae emerge, ready for pupation. However, one of two larvae is retained inside, taking control of the brain and making the caterpillar’s body twitch violently. The vigorous movement deters the predators like birds. The half-eaten remains of the body lash back and forth, shooing away the predatory birds waiting to pounce upon the larva of the wasp. It protects the pupating larvae like the bodyguard. Over the period, the caterpillar becomes a slap-up meal for the pupating larvae.

A stab in the back

It is well known that the female Koel fools a house crow into hatching its eggs. The Hymenoepimecis argyraphaga wasp found in the Costa Rican forest first induces an orb spider, Plesiometa argyra, to build a web to support its cocoon. Then it conscripts the spider to turn on itself to become a hearty meal for the wasp larva.

The expectant wasp looks out for the orb-weaver spider. The bite from the female wasp makes the spider temporarily paralysed and affixes the solitary egg in the abdomen. In about 10 to 15 minutes, the spider wakes up from the coma, unaware of the wicked rider. The egg hatches and the emerging larva makes a small hole in the spider’s abdomen and feeds on the blood for the next two weeks.

Once the larva is matured, it injects a cocktail of chemicals that changes the spider’s behaviour. Under the influence of mind-altering substances, the spider weaves a bizarre kind of web unlike it had built anytime before. The web is custom-made to encase the wasp larva in its cocoon while it metamorphoses. Once the net is completed, the spider moves to the centre of the web and remains immobilised while the larva gobbles it up. After 10 days, the wasp emerges out of this cocoon.

All that glitters

Not all the quirky behaviour in the host can be attributed to the mind manipulation by the parasite. Like any viral pathogen, the deadly rabies virus transmitted via dog and wild cat bites rapidly makes the cell in the host body into rabies factories, churning out thousands of copies. The rabies virus moves to the central nervous system and heads for the brain. The rabies virus attacks the hippocampus, amygdala and hypothalamus area of the brain. Unlike the body cells, the virus does not immediately get into devouring the brain cells but alters how these cells release neurotransmitters such as serotonin, GABA, and endogenous opioids. In this avalanche of neurochemicals, the mind is messed up. Animals infected with rabies are constantly anxious and thrash out at other animals out of fear. Humans infected by rabies become terrified of water and fresh drafts of air. Left untreated, the victims slowly lose their ability to sleep, slip into a coma, and die in a few days. Dreadful, but the virus has no identifiable advantage in making the host suffer macabre death. The toxins from the virus influence the brain’s function. Still, rabies does not mind-control its hosts and manipulates their behaviour to its advantage.

The same is the case of Trypanosoma. The parasite is one of the rare germs that can cross the blood-brain barrier and enter the brain of the humans. Trypanosoma alters the brain function, especially the hypothalamus plunging the victim into a depression-like mental state. If left untreated, the virus would become a monster in a few years, making the victim lazy and falling into a coma-like sleep. Typically, the germ is passed on to humans through the faeces of infected cats. That is why pregnant mothers are suggested to stay away from cats. Still, the pathogen cannot be said to manipulate the host to complete some essential aspects of its life cycle.

Humans have a robust immune system, so we resist most behaviour-altering parasites. Indeed human-animal interaction could cause destructive infection, but exaggerating fear is unhealthy.

Research

The ability of parasites to manipulate the brain and behaviour of the hosts is one of the most fascinating areas of biological research.

A study found that the Euhaplorchis californiensis parasite releases mind-altering chemicals in the infected killifish. Researchers took tiny pieces of different brain regions from infected and uninfected killifish. They measured the levels of neurotransmitters such as dopamine and serotonin. When a typical fish is stressed, the brain releases serotonin in a region of the brain called the raphe nuclei. However, the study showed that the chemicals from the parasite blocked the production of serotonin in the raphe nuclei of infected fish.

Further, the infection also lowered serotonin in the hippocampus while boosting dopamine in the hypothalamus. When the fish is under threat, there is a surge of serotonin that makes them freeze, helping them hide from the motion-sensing predatory shore birds. However, with lowered serotonin in raphe nuclei, the fish hardly tries to hide from the birds. What’s more, the elevated levels of dopamine make the fish swim aggressively, boldly surface repeatedly and go round and round, attracting the attention of the predator birds.

In an experiment, the researchers collected carpenter ants and deliberately infected them with Ophiocordyceps unilateralis fungus. Another set of ants was kept clean as the control. The infected ants invariably climbed the vegetation in the laboratory. They clutched to the leaf, a behaviour not exhibited by the uninfected ants. The researchers isolated the RNA from the zombie ants while biting and after they died. They also collected the RNA from the healthy control ants and the fungus grown outside the host. By comparing these four sets of RNAs, they found that during the manipulated biting behaviour, the fungus used 498 genes, significantly more out of 7,831 protein-coding fungal genes.

Further, it was found that specific genes of the fungus got hyperactive in the host, which produced LSD-like compounds and other chemicals that affected the serotonin and dopamine levels in the brain of the infected ant. Other proteins secreted by the fungus affected the ant’s movement; some others impaired the ability of the infected ant to communicate with its nest mates. The study also indicated that the secretions affected the biological clock, stress regulation system and movement of the muscles of the infected.

Neuro-parasitology is still nascent, and researchers do not yet fully understand the mechanisms of parasitological voodoo of the macabre world of behaviour-altering parasites. Only recently have scientists understood some precise mechanisms using modern investigation tools.

Intriguingly, the infected ants showed over expression of an ant gene that encodes for a protein found in the brain of alzheimer patients. Perhaps the study of the biochemistry of mind-controlling parasites could one day show us molecules that help treat alzheimer, dementia, schizophrenia and other mental illness. After all, penicillin, the first anti-bacterial agent, was isolated from a fungus.