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.
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