It's a given: parasites are creepy. The idea of something invading our bodies and feeding off of us is disturbing in any number of ways. Apart from the general sense of violation, they are known to cause disease and dysfunction. Whether they're causing diarrhea in humans or gid in sheep, parasites make us sick. But could a parasite ever directly control the behavior of its host?
In fact, documented cases of parasites directing their victim's actions date back to the 1930s, and there is some fossil evidence that indicates that this behavior has been going on for eons. In the modern age, as observation techniques and technology have improved, an ever more bizarre list of example behaviors have been uncovered.
(Illustration: Patrick Kastner / The Columbus Dispatch)
Parasites that show evidence of being able to exert such influence have a multi-step life cycle. They often are only able to breed inside one kind of host, and must travel through one or more secondary hosts in order to reach their destination. If they develop into a new stage while in their host, it is called a “intermediate” host, but if they just hitch a ride with transforming they are said to be in “transport” host. The final host is called “definitive”.
Most evidence of parasitical control is found in transport or intermediate hosts whose behavior is modified to make it more likely that they will be consumed by the definitive host. For example, the “gid” disease in sheep mentioned earlier is caused by a tapeworm which infects the sheep and causes them to stumble around. This makes them prime targets forwolves, which are the definitive host for the tapeworm.
There's a slide show at ScientificAmerican.com with a few excellent photos of some of this parasitic behavior, including a wasp that stings cockroaches then rides them around like motorbikes before laying eggs inside them Alien style, and this great photo of some enflamed ant butt:
The parasite Myrmeconema neotropicum turns its ant victim's endparts bright red, then forces it to march away from the colony, and position itself in leafy vantage points where, mistaken for a berry, it will make a nice snack for the definitive host: a bird.
Here's a great excerpt from Parasites: Latching on to a Free Lunch by Paul Fleisher:
This worm's definitive host is a sheep, cow, or other grazing animal. A snail picks up the fluke's eggs from the animal's manure. The eggs hatch inside the snail. The worm larvae make their way to the snail's digestive system. The snail soon sheds the larvae inside little balls of slime. Each slime ball contains as many as five hundred larvae. An ant finds the slime and eats it. The ant in now infected.
Here's where the story becomes astounding. Fluke larvae make their way to the ant's tiny brain and take control. Somhow the larvae force the ant to act in a way it never would otherwise. In the evening, instead of returning to its nest, the ant climbs a blade of grass. It grabs the grass with its jaws and holds on. Why?
The flukes need to get into a sheep's stomach to complete their life cycle. When a sheep or some other grazing animal eats the ant along with a mouthful of grass, it gets a bellyful of worms. What if the ant isn't eaten? The next morning it crawls back down and goes about its business. But that evening, the worms take over and it climbs up another blade of grass to try again.
These last two examples are especially interesting because they involve a change in the appearance of the host, as well as behavior changes that only take place at night. For the sci-fi or folklore buff, this provides an explanation for countless stories about everything from zombies to werewolves. Is it possible that a parasite-infected human could have such strange behavior and physical changes to eventually inspire myths and legends that we still tell today? Of course, in order to believe that we would need evidence showing that human behavior can be affected by parasites. (You can probably see where I'm going with this one...)
Toxoplasma gondii is a parasite which affects warm-blooded mammals, including humans. Its definitive hosts are cats, with the intermediate host of mice and rats. The smaller animals pick up the parasite through cat feces, and then begin to be bolder and less likely to run away from cats. This unnatural behavior makes them more likely to be eaten by felines, and keeps the cycle going. The problem for humans is that we are also susceptible, and in fact Toxoplasma gondii is estimated to infect up to a third of the world's human population . Although the infection usually results in mild flu-like symptoms psychiatrists like Robert Yolken and E. Fuller Torrey have tied T. gondii to schizophrenia, and separate research from Kevin Lafferty has looked at broader personality changes among the infected. Interestingly, Lafferty has found that while both infected men and women were more likely to be guilt-prone and self-doubting, other symptoms such as changes in aggressiveness seemed to have a strong split between the sexes. And for the record, women show increased intelligence, while the men's IQ dropped. Lafferty's initial article is here, and is good reading.
So if we know that some parasites are capable of altering the mental processes of their victims, and we have clear record that such parasites can infect humans, how much of a stretch is it to imagine some new parasite that could reshape human behavior?
Pleasant dreams, kids.