Science fiction has long explored the terrifying possibility that we are devoid of free will, and that some unpleasant creature could control our minds or turn us into plodding zombies. But mind control is not just a literary trope. It is also a common method by which parasites gain access to environments where they can grow, reproduce, and complete their life cycles.
Consider the fungus Cordyceps, which interferes with the behaviour of ants in tropical rainforests in such a way as to make them climb high into the vegetation, and latch onto a leaf to die. The fungus then reproduces by dropping its spores all over the forest floor, to infect more ants below. Similarly, a virus that infects gypsy moth larvae prompts them to climb en masse to the tops of trees to die. The virus then multiplies, and rains viral particles down on the forest floor.
These parasites make their hosts seek a higher elevation, which expands the reach of their infectious spores or particles. But other species can induce far more complex behaviours. Nematomorph worms, for example, infect crickets, and drive them to commit suicide by jumping into various water sources, be it a puddle or swimming pool. It is precisely in such aquatic environments that nematomorph worms reproduce and complete their life cycles.
And parasites' mind-control abilities are not limited to invertebrates. Consider the rabies virus, which is transmitted among dogs, humans, and other mammals by biting. To maximize its chances of spreading to another host, the virus actually alters its host's mind to turn it into an angry, slavering, biting machine that will chomp at anything it encounters.
Another species that can affect human behaviour is the protozoan parasite Toxoplasma gondii, the causal agent of Toxoplasmosis. T. gondii is extremely common, with an infection rate of 15-85% across different countries, depending on climate and diet. Whereas Brazil and France have infection rates of around 80%, Japan's is only 7.0%.
T. gondii can find its way to humans through farm animals such as pigs, cows, and sheep. And, as it happens, raw-meat dishes are more common in French and Brazilian cuisines. But T. gondii naturally targets cats, by way of rats whose behaviour it has altered. Namely, the microbe increases the likelihood of its host rat being eaten by a cat, by reducing the rat's natural fear of light (photophobia) and cat urine.
Humans, too, can experience alarming behaviourial changes after becoming infected by T. gondii. Infected men can become jealous, distrusting of others, disrespectful of established rules, and less risk-averse; as a result, they are almost three times more likely to be involved in a car accident. Infected women, meanwhile, can become either suicidal or more warm-hearted, insecure, and moralistic.
Moreover, there is evidence that a T. gondii infection could play a role in mental disorders. More than 40 studies have shown that people suffering from schizophrenia test positive for T. gondii antibodies, indicating that they may have been previously infected. And T. gondii has also been tied to dementia, autism, Parkinson's disease, and brain cancer.
How can these puppet-master parasites control the brains of such diverse invertebrate and vertebrate species? One possibility is that they can change the levels of neurotransmitters such as dopamine and serotonin in the brain. Neurotransmitters are ancient molecules that have been conserved through the ages of evolution, and they are known to influence behaviour.
Thanks to genomics and proteomics, we have begun to understand the role that neurotransmitters play in allowing parasites to manipulate host behaviour. When researchers analyzed the T. gondii genome, they found the precursor to dopamine synthesis, L-DOPA, suggesting that the parasite might be able to synthesize and secrete dopamine directly into a host's brain. This would explain why rats infected with T. gondii have higher levels of dopamine, and why dopamine inhibitors can suppress their parasite-induced behavior.
Parasites that infect invertebrates can also manipulate neurotransmitter levels. For example, the emerald cockroach wasp injects its cockroach host with a venomous cocktail that contains the neurotransmitter octopamine. This puts the cockroach into a sleep-like state, at which point the wasp drags it off to its lair and lays eggs in its abdomen.
And like T. gondii in rats, acanthocephalan worms (also known as spiny-headed worms) overrides the natural photophobia of their freshwater crustacean hosts. As the crustacean gravitates toward the surface of the water, it is eaten by a duck, at which point the worm completes its lifecycle.
Researchers have found that when uninfected amphipods are injected with serotonin, they spend more time near the surface of the water, as if they had been infected. And protein analysis of grasshoppers infected with nematomorph worms shows a change in the proteins that are involved in releasing neurotransmitters.
We are only just beginning to understand how these diverse puppet-master parasites can manipulate invertebrate and vertebrate behaviour. But we already know that pulling on the strings of neurotransmitters is one common method. If further research vindicates some of the more seemingly outlandish imaginings of science fiction, it wouldn't be the first time.
Robbie Rae is a lecturer in genetics at Liverpool John Moores University, United Kingdom. Project Syndicate, 2017.