Parasites are more than dormant feeders. Microscopic science continues to uncover the very active ways in which viruses and bacteria prey on their hosts, influencing them to behave in some very strange ways. Perhaps the most famous of these is the parasite Toxoplasma gondii, which cats defecate and then rodents pick up from scavenging on the ground. Once inside the rodent, the parasite alters the animal’s neurocircuitry such that it becomes sexually attracted to cats. Naturally it’s existence proves short-lived from this point forward as the rodent actively courts its main predator. Once in the belly of the cat, the parasite is free to reproduce once more — its bizarre life cycle complete and ready to begin again.

Science journalist and disease researcher Kathleen McAuliffe was shocked to learn that the parasite could — and does — also affect humans. In fact, approximately twenty percent of Americans are believed to be infected with Toxoplasma gondii, which potentially influences their behavior in adverse ways. McAuliffe reports that schizophrenia patients are likely to have two to three times the amount of virus antigens in their body. In addition, international studies have found that suicide rates increase proportionally to the virus’s infection rate. Of course any behavior changes caused by the virus are likely to be subtle beyond direct detection, but its effect on human behavior is real.

Another parasite McAuliffe profiles is the parasitic barnacle which can attach itself to a crab and inject a small clump of its cells into the crab. “Those cells then grow into a tangle of root like structures,” she says, “and these roots wrap around all the crab’s internal organs and eventually even sterilize the crab. Where the crab would normally grow a brood pouch on the bottom of its belly to incubate it’s young, the parasitic barnacle pushes out of the crab and grows a brood pouch of its own. And from that moment forward this crab lives and exists solely to feed the parasite and to take care of it’s young.”

It all goes to show the wonder of the microscopic world, and how our free will is so commonly influenced by factors widely out of our control. Beyond social, cultural, and historical factors, there are biological agents inside of us which affect how our brain functions, and so how we interact with the world around us.

Kathleen McAuliffe’s book is This Is Your Brain on Parasites: How Tiny Creatures Manipulate Our Behavior and Shape Society.

Kathleen McAuliffe:  We know that parasites can make us sick, of course, and siphon off our nutrients, but it's very surprising to hear that some of them, in fact there may be a large number, at least a hundred are known about at this point in time manipulate the behavior of their host in order to enhance their own transmission. And the best way to understand this phenomenon I think is with an example. And one is a cat parasite called Toxoplasma Gondii or just Toxoplasma for short. I first learned about this parasitic manipulation while just reading about scientific research and I came across a study that showed that rodents that are infected with this parasite, they can pick up the parasite from the ground. Cats defecate this parasite so rodents as they are scavenging around can pick up the parasite and it then invades their brain and it actually tinkers with the animal's neural circuits in such a fashion that it makes it attracted to the scent of cat urine.

And when I say attracted I mean sexually attracted. The rodents become sexually aroused by the scent of cat urine so they approach and needless to say they're not long for this world they soon end up in the belly of a cat and that's the only place where this parasite can sexually replicate. So that's its little trick. And it does many other things as well. For example, the same parasite goes to the testicles and jacks up production of the sex hormone testosterone. In females, by means nobody's figured out yet, it can increase the level of the sex hormone progesterone. And in both cases these changes make the rodent more embolden and cause the rodent to sort of lower its guard and to act in foolish ways around cats. So that's yet another example of other tricks it has for getting back into the belly of a cat.

This parasite can also infect us. One of the ways we can get it is changing a cat's litter box. And the current thinking in medicine is that the parasite it mainly poses a threat to a developing fetus and can harm the developing baby's nervous system or even cause blindness. And it's also well known to be a threat to people who are immuno-compromised, so for example, people who have received transplanted organs or being treated with chemotherapy. And it's still assumed that for most healthy people it poses no threat that once the parasite gets inside the brain that I just hunkers down inside neurons never again to cause any problems. But there's now several labs, both in Europe and the United States that are challenging that dogma. And they have uncovered a lot of evidence that for a small percentage of people the dormant infection may indeed have adverse consequences. Nobody yet has a handle on what percentage but about 20 percent of all Americans are infected with the parasite. So people's guess is that we're only talking about a small percentage of people who have these adverse responses.

But among other things it's a link to mental illness. So people with schizophrenia, for example, they are two to three times more likely to have antigens against the parasite. It's also been linked to manic depression and it's been linked to suicide. There was actually a study done in 22 nations in Europe and the researchers found that suicide increased in direct proportion to the prevalence of the parasite in each country. And it's been a link to dangerous driving. Several studies in a few different countries have shown that people who test positive for the parasite are more likely to be in car accidents. So one theory, nobody knows for sure why what the reason for this association is, but one theory is that just as rodents lower their guard behave in a cocky way maybe people behind the wheel of a car are less vigilant. Or there's also research that shows that infected people have slightly slower reaction times so that's perhaps another factor that may influence their driving. I should emphasize these are all correlational studies, but as scientists have learned more about what this parasite does to the rodent brain it does make them think it's plausible that the dormant infection is indeed causing trouble for some individuals.

There are many amazing examples of parasites in nature that are manipulating behavior. One of my favorite examples is a parasitic barnacle. Suspend any preconceived notions you have about barnacles because this of barnacle is very iconoclastic; it doesn't have a shell; it doesn't attach to the sides of peers or to rocks. It's free-living during one phase of its lifecycle, which at that point it can alight on a crab and inject a small clump of its cells into the crab. And those cells then grow into a tangle of root like structures and these roots wrap around all the crab's internal organs and eventually even sterilize the crab. And where the crab would normally grow a brood pouch on the bottom of its belly to incubate it's young, the parasitic barnacle pushes out of the crab and grows a brood pouch of its own. And from that moment forward this crab lives and exists solely to feed the parasite and to take care of it's young. It waffs oxygen rich water around the brood pouch to keep the parasite's offspring well oxygenated. And then when its young are ready to be born the crab then goes into deeper water and bobs up and down and releases the parasite's babies into the currents were they then go off into the world only to commandeer the minds and bodies of more crabs. I mean to me this is the closest real life thing to a body snatcher, science fiction's idea of a body snatcher. I even call them robocrabs because they are basically like an amphibious robot controlled by this parasitic barnacle.

There is many other great examples in nature. Another one that I think is very impressive is a parasitic wasp and it will grab hold of an Orb spider and attach its egg to the abdomen of the spider. And then as this egg hatches into a larva the larva secretes all kinds of chemicals that effectively instruct the spider to build it a nursery. So the spider abandons its normal weaving pattern, it stops making that sort of classic circular motif and instead creates a sort of hammock like structure for the parasitic wasp's larva and the larva actually attach to the center of this net. And the spider even weaves a sort of decorative motif around the parasite to protect it from its own enemies. It's something even think fundamental as how a spider weaves can be radically changed by these parasitic wasps. And there's at least a dozen of them and they all induce completely different kinds of webs. So that's one example of I think a really impressive example of a parasitic manipulation.

There is a parasitic worm that gets into killifish, it's a very common fish found in California esrines, and it invades the fish's brain and the fish as a result rise to the surface and flip over on their sides and basically waive their fins at predatory birds that are circling overhead, which that needless to say swoop down and eat the fish. And the parasitized fish are much, much more likely to be caught by birds, at least four times more likely to be caught. And what the parasitic worm appears to do is it acts on parts of the brain that are controlled by serotonin, the neurotransmitter serotonin. And the infected fish seem normal and healthy in every single way except they're much more mellow. They're like a fish on Prozac. So that's how that manipulation works.

Another manipulation that actually makes me shudder when I even think about it, it's a fungus. And what happens is ants as they're on the forest floor they may pick up a spore of this fungus from the ground and there is nothing domicile about this fungus because as soon as it attaches to the ant it burrows into the ant and it starts rapidly growing. And as it's doing this it instructs the ant at exactly solar noon to go to the nearest sapling to climb exactly one foot above the ground to go to the northwestern a part of the plant, lock onto the main vein of the leaf and then this fungus, and this is the part of the creeps me out, like sprouts from the ant's head and it grows this long stock. And at the tip of the stock is this fruiting body that then explodes and rains spores down on more ants that are walking along the forest floor below.