OK, so parasites are creepy, disgusting, and often debilitating (but sometimes useful!). They sneak into a host’s body through food, drinking and bathing water, air and soil, or an insect bite. Less well-known, however, is their ability to alter a host’s behavior. I’m not referring here to host behavior that is a direct response to symptoms of parasite infection, such as scratching an itch, or making a hurried dash for the bathroom, or going to a doctor.
There are much more perfidious ways that parasites affect host behavior, such that the host ceases to do its normal things and, instead, behaves erratically in ways that altogether favor the parasite, usually improving its transmission to the next host. Very, very sneaky! Here are some examples.
Parasitic fungi called Cordyceps use insects as a means of promoting spore dispersal to new hosts. There are many kinds of Cordyceps, each with its own kind of host insect. An infected ant, for example, climbs up a plant stem. Somehow, the fungus alters the ant’s brain chemistry and the ant gives up all normal ant-ly behavior and just hooks itself to the stem, never to move again. The fungus gradually consumes the ant’s innards and eventually sends out a shoot that releases the spores to any passing wisp of moving air. By forcing the ant to climb upward and stay there, the fungus has improved the chances of finding a small breeze to carry its spores away. A spore that disperses successfully and lands on another ant dooms that ant to repeat the whole process.
Flukes are a kind of flatworm, and they too can create behavior modification in their hosts. For example, one species of fluke infests certain snails and castrates them. The snails go on eating, but they can’t reproduce, and the flukes multiply asexually inside the snail. The asexual offspring leave the snail and swim around looking for a fish. If they find one, such as a killifish, they hook onto the gills, move into the blood vessels until they find a nerve, and follow the nerve to the fish’s brain. They carpet the surface of the brain and sit there until the fish is eaten by a bird.
Infected fish behave differently than normal fish — they swim jerkily, too close to the surface, and turn to expose their shiny sides to the light. They become so conspicuous that foraging herons or shorebirds are way more likely to capture an infected fish than an uninfected one. Once inside the fish-eating bird, the flukes feed in the gut and reproduce sexually; the eggs pass out in the bird’s scat and get picked up by wandering snails. By entering a bird, the flukes can then be carried to new ponds and estuaries — the whole complex life cycle is geared to achieve successful dispersal.
Or how about the lancet fluke inside an ant; it makes the ant run up a blade of grass each night until the grass is eaten by an herbivore, where the fluke can reproduce. Or the horsehair worm that makes an infected cricket jump into water where the worm can reproduce. Or the parasitic wasp that makes a spider spin a novel kind of web — a protective awning under which the wasp larva will hang its cocoon. Or the thorny-headed worm that makes an infected pillbug stay out in the open when a bird can grab it; the worm can reproduce inside the bird. Or the several kinds of microscopic parasites that clog up the biting apparatus of mosquitoes or flies, such that they have to make many bites in order to get a good blood meal, thus spreading the little parasite to many new hosts. Then there’s the virus that seems to make male rats more aggressive, spreading the virus with every ratty bite, and a micro-parasite that makes rats unafraid of cats, which then pick up the parasite from the rats.
Those are just a few examples of behavior modification induced by parasites. Long before psychologists concocted the term, parasites were already very good at it. There are now so many examples from the rest of the animal kingdom, that one really must consider the possibility that parasites might be able to modify human behavior and personalities as well. Indeed, research is beginning to demonstrate that this is so!
There is a vast number of parasites in the world and virtually every species, even some of the parasites themselves, harbor multitudes of these hangers-on. There is still much to be learned, even about basic life cycles of many known parasites and their multifarious ways. Undoubtedly, many parasitic species remain to be discovered, along with their weird and wonderful habits. There is also the intriguing possibility that we could learn from parasitic effects on human behavior and thereby discover new treatments for certain afflictions such as depression or hyper-aggressiveness.
The word “parasite” comes from the Greek words, meaning “beside the food.” The ancient Greeks reportedly used it to refer to people who served food at temple feasts. Somewhere along the line, the word left that meaning and was (and still is) used to refer to people who curry favor with others in order to receive food or other benefits. After a very long time, the word was applied to animals and other organisms that live off of others — it took centuries for folks to figure out that there were such things as animal and microbial parasites, and that these entities were actually organisms that had their own lives and life cycles. Now it seems that parasites are an integral part of the lives of any other organisms, and we are just beginning to explore all the weird and wonderful interactions among them.
• Mary F. Willson is a retired professor of ecology.