How brain parasites change their host's behaviour
Which of these entities has evolved
the ability to manipulate an animal many times its size? The answer is all of
them. These are all parasites, organisms that live on or inside another host
organism, which they harm and sometimes even kill. Parasite survival depends on
transmitting from one host to the next, sometimes through an intermediate
species. Our parasites elegantly achieve this by manipulating their host's behaviour,
sometimes through direct brain hijacking.
For example, this is the Gordian
worm. One of its hosts: this cricket. The Gordian worm needs water to mate, but
the cricket prefers dry land. So once it's big enough to reproduce, the worm
produces proteins that garble the cricket's navigational system. The confused
cricket jumps around erratically, moves closer to water, and eventually leaps
in, often drowning in the process. The worm then wriggles out to mate and its
eggs get eaten by little water insects that mature, colonize land, and are, in
turn, eaten by new crickets. And thus, the Gordian worm lives on.
And here's the rabies virus, another
mind-altering parasite. This virus infects mammals, often dogs, and travels up
the animal's nerves to its brain where it causes inflammation that eventually
kills the host. But before it does, it often increases its host's
aggressiveness and ramps up the production of rabies-transmitting saliva, while
making it hard to swallow. These factors make the host more likely to bite
another animal and more likely to pass the virus on when it does.
And now, meet Ophiocordyceps, also
known as the zombie fungus. Its host of choice is tropical ants that normally live
in treetops. After Ophiocordyceps spores pierce the ant's exoskeleton, they set
off convulsions that make the ant fall from the tree. The fungus changes the
ant's behaviour, compelling it to wander mindlessly until it stumbles onto a
plant leaf with the perfect fungal breeding conditions, which it latches onto. The
ant then dies, and the fungus parasitises its body to build a tall, thin stalk
from its neck. Within several weeks, the stalk shoots off spores, which turn
more ants into six-legged leaf-seeking zombies.
One of humanity's most deadly
assailants is a behaviour-altering parasite, though if it's any consolation, it's
not our brains that are being hijacked. I'm talking about Plasmodium, which
causes malaria. This parasite needs mosquitoes to shuttle it between hosts, so
it makes them bite more frequently and for longer. There's also evidence that
humans infected with malaria are more attractive to mosquitoes, which will bite
them and transfer the parasite further. This multi-species system is so effective,
that there are hundreds of millions of malaria cases every year.
And finally, there are cats. Don't
worry, there probably aren't any cats living in your body and controlling your
thoughts. I mean, probably. But there is a microorganism called Toxoplasma that
needs both cats and rodents to complete its life cycle. When a rat gets
infected by eating cat faeces, the parasite changes chemical levels in the
rat's brain, making it less cautious around the hungry felines, maybe even
attracted to them. This makes them easy prey, so these infected rodents get
eaten and pass the parasite on. Mind control successful.
There's even evidence that the
parasite affects human behaviour. In most cases, we don't completely understand
how these parasites manage their feats of behaviour modification. But from what
we do know, we can tell that they have a pretty diverse toolbox. Gordian worms
seem to affect crickets' brains directly. The malaria parasite, on the other
hand, blocks an enzyme that helps the mosquitoes feed, forcing them to bite
over and over and over again. The rabies virus may cause that snarling,
slobbering behaviour by putting the immune system into overdrive.
But whatever the method, when you think
about how effectively these parasites control the behaviour of their hosts, you
may wonder how much of human behaviour is actually parasites doing the talking.
Since more than half of the species on Earth are parasites, it could be more
than we think.
Source: TED Ed – By Jaap de Roode