When people think of parasites, they usually think about leeches, lampreys, and squirmy things that latch on and feed. But it’s only in the movies where those creepy things could actually control the mind of the host, right? Wrong. The truth is, in the natural world, there’s not only an amazing diversity of parasites, but there are actual parasites that can control the behavior of their hosts, even to the point of manipulating them into suicide. Here are just a few examples.
A species of fungus belonging to the genus Cordyceps has the ability to enter the bodies of ants, as spores, and then slowly grow mycelia (fine fungal filaments) throughout the ant’s body, absorbing its soft tissues, while avoiding its vital organs. When the fungus is ready to reproduce, it then grows into the ant’s brain, where it produces chemicals that alter the host’s brain chemistry. This then causes the ant to climb up a blade of grass or a tall plant and then clamp its mandibles down into it. There, the ant rests, while the fungus begins to devour its brain, thereby killing the host. Once finished, the fungus sprouts out of the ant’s carcass through gaps in its exoskeleton. The sprouted fungus then releases its spores into the area nearby, where it can infect other ants who happen to pass by.
While some mind influencing parasites eventually kill their hosts by devouring them or leeching off of them to the point of death, others sometimes kill their hosts by forcing them to commit outright suicide. One such parasite is a nematomorph hairworm by the name of Spinochordodes tellinii, which is a worm that actually invades the bodies of various insects, including grasshoppers. When this parasite finds a way into a host, it spends most of its time feeding on the host’s insides. Upon growing to a certain length, the parasite then causes the host to committ suicide by “compelling” it to jump into a nearby body of water to drown itself. After this act is done, the worm’s adult form emerges from the carcass of the unfortunate host and becomes a free living aquatic creature that can reproduce in water.
And speaking of aquatic creatures, a particular barnacle, by the name of Sacculina carcini, is a unique example of why marine creatures take no special exception when it comes to behavior altering parasites. Starting off as a free-floating larva, the female Succulina finds its way to a crab and, once there, searches for a gap in one of the crab’s joints. Upon finding a vulnerable chink in its armor, the parasite then jabs a long hollow “dagger” into the joint and proceeds to “inject” itself into the crab’s body. Once done, the hollow husk of the parasite falls off the crab, while its gel-like insides (which resembles somewhat of a slug) remain inside of the host’s body. Over time, the parasite settles within the crab and begins to grow fleshy “roots” all throughout the crab’s body. These roots help to absorb nutrients from the crab’s blood. After growing to a certain extent, the Sacculina begins to grow “out” of the creature’s body, appearing as a knobby protrusion in the crab’s underside. As a result of becoming fully “merged” with its uninvited guest, the crab’s behavior and body begins to alter in a manner to nurture the parasite within itself. For instance, the crab will stop molting, growing, and mating in order to provide more energy for the parasite. On top of that it will begin to direct its “affection” towards the parasite, including regularly grooming the parasite’s knob by scraping away algae and fungi that might gather on its surface. From there until the end of the crab’s life, the crab will become nothing more than a subservient vehicle for its parasite master-never mating, never growing, and always in search for food that it can never fully have for itself.
For the most part, nearly all parasitic organisms with the ability to alter the behaviour of their host associate only with lower life forms. However, that’s not always the case. Perhaps, the most disturbing mind-altering parasite of them all is the single celled organism by the name of Toxoplasma gondii, which is a protozoa that favors to inhabit cats, but also has the ability to inhabit other animals, including humans. In fact, it was calculated that up to 50% of all humans have been infected by it at some point in their lives. But the question remains–how does this organism affect the human mind? For the majority of those infected, very few have shown drastic symptoms. However, for the minority who have, some of these symptoms range from hallucinations all the way to schizophrenia. In addition to this, a study done by parasitologist Jeroslav Flegr of the Charles University in Prague found that men infected by Toxoplasma exhibited signs of being more jealous and suspicious, while infected women exhibited signs of being more “warm-hearted” and outgoing. Can all this be a direct result of the parasite working sinisterly in the minds of unwitting hosts? Perhaps. Will further research be needed to determine the direct connection of these creatures with human behavior? Definitely. Either way, whether it’s through snails, ants, or crabs, nature’s creatures hold many surprises, both amazing and frightening as well.