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Invasion of the brain snatchers

House Ear Institute
The parasite known as Toxoplasma gondii gets its microscopic close-up.

Parasites may seem merely icky, but some of them have the Halloweenish capacity to take over your brain. Scientists have happened upon a number of neurological nuisances in the animal world, but the scariest of the lot is a tiny critter known as Toxoplasma gondii - which makes rodents, and perhaps even humans, go loco.

Stanford neurobiologist Robert Sapolsky provided a status report on the fabled Toxoplasma and other brain snatchers this week on the university's Palo Alto campus, as part of the Council for the Advancement of Science Writing's annual New Horizons in Science seminar.

Over the past few years, neuroscientists have used brain imaging and other high-tech tools to track exactly how the one-celled Toxo organism does its nefarious deed. The parasite can reproduce only in cat feces - but once the next generation has been spawned, how does it get into another feline host?

That's where zombie rodents play a role: When mice or rats consume the feces, as is their wont, the Toxo protozoans migrate to the brain - specifically, to the amygdala, which is the brain's switchboard for emotional response. There they form encapsulated cysts and proceed to manipulate the wiring of the rodent brain.

Studies have shown that the Toxo genome contains what appear to be mammalian versions of two genes that are involved in the production of dopamine, a neurotransmitter that is linked with the brain's reward system. "Toxo has evolved to take over the reward pathway," Sapolsky said.

He and other researchers are still working to nail down the exact mechanism, but Toxo apparently targets a neural pathway that instills mice and rats with a natural fear of ... cat urine. "It's lasering out this one thing of not liking the smell of cats," Sapolsky said.

In a long series of experiments, scientists have found that Toxo-infected rodents lose that fear of cat pee, while other fear mechanisms (for example, the fear of being in an exposed area) are unaffected. "What this damn Toxo knows how to do is make cat urine smell sexy to male rats," Sapolsky said.

As for the female rats, "I'll bet you it makes [cat urine] smell like babies," he added.

The result is predictable: The rats are drawn to the cats, the cats eat the rats, and the circle of life begins anew.

So far, this sounds like some sort of hallucinogenic "Tom and Jerry" cartoon - but if Toxoplasma gondii can affect rodent brains, it can affect human brains as well. One out of every five Americans are thought to carry the parasite, and the infection rate can be higher in other parts of the world. Toxo poses such a risk to the fetal nervous system that pregnant women are advised to avoid contact with cat litter boxes.

Although scientists are just starting to study Toxo's effects on adult humans, the initial evidence suggests that "Toxo does something to humans quite reminiscent to what it does to rats and mice," Sapolsky said. He's not talking about a sudden fondness for cat pee - rather, the organism is thought to mess with the way our brains handle rewards and impulse control.

One study, conducted in Turkey, showed that a Toxo-infected group of drivers were two to four times more likely to be involved in traffic accidents than the experiment's control group. Another study has claimed that high rates of infection could influence an entire culture - making people more naturally neurotic, for example. One researcher has charted what he says are Toxo-linked personality differences. Others have suggested a link between the infection and schizophrenia. (Here's a scary study on the subject.)

The bottom line is that more research is needed to pinpoint Toxo's effects on humans. "Three studies - that's the entire literature at this point," Sapolsky noted.

The tale of Toxoplasma gondii also may spark interest in figuring out how other infections affect our zombie brains. Some researchers speculate that there's a whole class of psychiatric disorders known as PANDAS that are linked to childhood infections. Among the illnesses are obsessive-compulsive disorder, hyperactivity and Tourette's Syndrome.

If you're still doubtful that brain invaders really exist, Sapolsky can cite a few more examples, including:

  • Hairworms that invade crickets, take over their nervous system and then order their buggy hosts to drown themselves so that the grown worm can take to the water.

  • Barnacles that latch onto male crabs and blast them with so much estrogen that they dig empty nests, made to order for the barnacle to lay its eggs inside. If the crab host happens to be a female, no problem: The barnacle merely wipes out the host's reproductive system, and then sets her digging.

  • A mind-controlling fluke known as Dicrocoelium dendriticum that makes its way through the innards of cows, and then snails, and then ants. Once the parasites are inside an ant's head, they turn the bug into a zombie. The ant repeatedly climbs up blades of grass, a type of behavior that doesn't seem to make sense ... until you realize that it makes the ant more vulnerable to being eaten by a grazing cow, where the life cycle begins again.

To learn more about how the amygdala and other parts of your brain work, check out our interactive "road map to the mind." To learn more about zombifying parasites, check out Carl Zimmer's book, "Parasite Rex." And if all this gets you in a Halloween mood, click through this ghoulish gallery of past postings: