General Knowledge

How parasites change their host’s behavior

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 behavior, 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 behavior, 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 parasitizes 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 behavior-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 feces, 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 behavior. In most cases, we don’t completely understand how these parasites manage their feats of behavior 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 behavior by putting the immune system into overdrive. But whatever the method, when you think about how effectively these parasites control the behavior of their hosts, you may wonder how much of human behavior is actually parasites doing the talking. Since more than half of the species on Earth are parasites, it could be more than we think.

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