This is your brain on parasites

Parasites. They range from microscopic bacteria and viruses to 50-foot long tapeworms. They've been living on and in their host organisms for millions of years. They are rather disgusting to think about, but the world would be a drastically different place without them.

Writer Kathleen McAuliffe’s recent book, "This Is Your Brain On Parasites," describes some of the many ways these creatures manipulate their hosts’ behavior to ensure their survival and successful reproduction.

“The genesis of the book was an item I came across one day on the web, a brief article about a parasite that gets into the brains of rats,” McAuliffe says. “It actually can turn the rat’s innate fear of cats into an attraction — and when I say attraction, I mean it actually makes the rodent sexually attracted to the scent of cat odor.”

Needless to say, a rodent afflicted with this parasite, called toxoplasma, soon finds itself in the belly of a cat, which, conveniently, is the only place the parasite can sexually reproduce.

“This so blew me away that I thought, ‘Can there possibly be more examples of this?’ It just seemed so wild,” McAuliffe says. “I started asking around, and I found out that there are scores of manipulations every bit as impressive…It’s a very broad phenomenon.”

One such example is a parasite called a trematode that can get into an ant's brain and basically hijack its behavior, McAuliffe says.

“It instructs the ant to leave its colony at night, climb to the top of a blade of grass, lock on to it and just hang there overnight,” McAuliffe explains. “If nothing happens, it goes back down to the colony the next day. Then the ant returns the next evening. It will do that again and again until a sheep comes by and happens to eat the blade of grass to which the ant is attached. When that happens, the parasite gets into the sheep, specifically into its bile duct, which is exactly where it wants to be, because that's the only place it can reproduce.”

Humans aren’t immune from manipulation by parasites. A malarial parasite can cause subtle changes within the human body in order to perpetuate its dangerous mission.

“When it infects a person, it introduces an anticoagulant — sort of like a blood thinner. So when other insects feed on that person, they can suck more infected blood out of them,” McAuliffe says. “And then these little flying syringes go around and infect many more people.”

The parasite can also change the odor of its human host; it may even amplify our scent. That means more insects, including not-yet-infected ones, will be attracted to the body of an infected person. They will feed on this person and then spread the parasite to more people.

Their cleverness doesn’t end there: A study in Kenya found that insects with the parasite have a stronger preference for people whose malaria is at a transmissible stage, and less of an attraction to people whose malarial infection is not yet transmissible.

Evidence suggests that toxoplasma, the parasite that alters a rat’s behavior, can change human behavior, too. Humans typically get this bacteria from cats. After it gets into a cat’s gut, the cat begins shedding the parasite's eggs in its feces. People can accidentally be exposed to it when changing a cat litter box. Exposure can also sometimes happen while gardening or by not thoroughly cleaning garden vegetables.

“Twenty percent of Americans have the parasite in their brain, and a lot of research suggests that, in a small percentage of people, it can contribute to mental illnesses such as schizophrenia and bipolar disorder,” McAuliffe says.

Only one percent of the population has schizophrenia, she says, and the parasite is likely involved in 10 to 40 percent of those cases.

There is evidence, however, that the parasite increases recklessness in humans. Several independent studies have shown that people who have the parasite are more likely to be in car accidents. A longitudinal study done in Czechoslovakia, involving many thousands of participants, identified people who tested positive for toxoplasma, and then followed them over a five- to 10-year period. The study confirmed that, indeed, people who had the parasite were nearly three times more likely to be involved in car crashes, especially when they were the party at fault.

McAuliffe says her favorite parasite is a parasitic barnacle. Suspend any notions you might have about how barnacles look or behave, she warns.

“This parasitic barnacle basically injects a clump of its cells into a crab and overtakes its whole body. It’s the closest real-life entity to the nightmarish image of a bodysnatcher,” McAuliffe says. “The parasite spreads its tissue to the insides the crab, and even sterilizes the crab. From that moment on, the crab lives and exists solely to nourish the parasite. When the parasite’s babies are ready to be born, the crab will even go into deeper water and release them. I call them robo-crabs.”

Again, a reassuring note: people don’t eat these crabs. Crabbers easily identify crabs that have the parasite and throw them back in the water.

So, why on earth do parasites exist? For one, parasites have a profound impact on the food chain, McAuliffe says. In fact, a lot of food chains would collapse without them. Biologists say the world would look very different without these parasites, because they are so incredibly common.

“Toxoplasma, for example, has infiltrated basically every warm-blooded animal on Earth. It's in birds and all mammals,” McAuliffe says. “It has even invaded the marine ecosystem, so it's in dolphins, whales, even walruses on ice sheets. So, these parasites have a profound impact on ecology, and the world wouldn't be the same without them. Yes, they cause horrible diseases, but nature would not work the same without them. In fact, a parasitic existence is the most common on the planet.”

This article is based on an interview that aired on PRI’s Living on Earth with Steve Curwood.