Image: Toxoplasma gondii
House Ear Institute
The tiny parasite known as Toxoplasma gondii shows up in brilliant shades of stain.
By Senior writer
updated 8/17/2011 8:46:13 PM ET 2011-08-18T00:46:13

When the bizarre parasite known as Toxoplasma gondii infects rats, it turns the rodents fearless, reducing their natural aversion to the odor of cat urine. But despite this bravery, infected rats remain terrified of other scary stimuli.

Now, a new study hints at how T. gondii, or "toxo," makes this strangely specific fearlessness happen: In infected rats, the smell of cat urine activates sexual attraction pathways in the brain, spurring the animals to approach the odor rather than run away.

Although T. gondii can infect many mammals, including humans, this rodent mind control is likely an adaptation by the parasite to ensure it gets into the intestines of a cat, the only place it can reproduce sexually.

"Something is perturbing these pathways, and it looks like that something is toxo," said study researcher Patrick House, a neuroscientist at Stanford University. [Top 10 Diabolical and Disgusting Parasites]

Mind-altering infection
About 30 percent of people worldwide are infected with T. gondii, mostly through eating undercooked meat or contact with cat feces. In healthy humans, the parasite doesn't cause symptoms, though pregnant women are advised to stay away from cat litter boxes, because T. gondii can cross the placenta and kill a developing fetus.

Infected rodents, on the other hand, lose their fear of cats. The goal of the new study, published on Wednesday in the open-access journal PLoS ONE, was to tease apart whether the rodents are actually attracted to the urine or whether they're just less afraid of it, House told LiveScience.

House and his colleagues exposed infected and non-infected male rats to either the smell of a female cat in heat or the scent of bobcat urine. Shortly after the scent test, they anesthetized and killed the rats and then removed their brains to analyze them for signs of recent activity in regions responsible for approach-and-avoidance behavior.

In noninfected rats, the researchers found an expected pattern in the limbic system, the brain system responsible for emotion, fear and long-term memory processing. Urine triggered fear-related areas of the limbic system, while a female in heat triggered sexual attraction areas.

In contrast, the brains of the toxo-infected rats looked odd. In rats exposed to cat urine, the fear pathway still showed signs of activity, House said. But so did the sexual attraction area, suggesting that sexual drive may trump infected rats' fears.

"The animals still seem to be afraid, but we're getting activity in both," House said.

Parasite mysteries
How T. gondii pulls this off, "we don't know at all," House said. Intriguingly, the parasite does preferentially settle in the limbic areas of the brain near the fear and sexual attraction regions.

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Toxoplasma infection also raises brain levels of the neurotransmitter dopamine. Researchers have found that the parasite has a gene that codes for an enzyme crucial in dopamine production, suggesting that altering neurotransmitter levels might be a key way the T. gondii controls the brain.

High dopamine is a factor in schizophrenia, raising concerns about whether T. gondii might play a role in the genesis of the mental disorder. A 2006 study published in the journal Proceedings of the Royal Society B found that antipsychotic drugs, commonly used to treat schizophrenia, reverse the fearlessness effects of T. gondii in the brain.

Studies also show that people with schizophrenia are more likely to be infected by the parasite, House said, but there is no proof that the parasite causes the disorder. People with schizophrenia might have reduced hygiene or personal care, he said, meaning they could simply come into contact with raw meat and cat feces more than the general population.

House's next step is to drill deeper into T.gondii's behavior in the brain.

"These findings showed what is happening, but don't show how it's happening," he said. "So what I want to do is get in and figure out at the neuronal, cellular level, how is toxo changing neurons?"

You can follow LiveScience senior writer Stephanie Pappas on Twitter@sipappas. Follow LiveScience for the latest in science news and discoveries on Twitter @livescience and onFacebook.

© 2012 All rights reserved.

Explainer: Ten of nature's scariest animals

  • Stephen Chernin  /  AP file

    Humans seem to love a good scare, particularly on Halloween. Ghosts, zombies, vampires and other fictional characters are good for putting a chill down your spine. But if you're looking for something truly scary, let nature be your guide. Click on the "Next" label to see 10 of the animal world's scariest creatures.

  • Box jellyfish pack a deadly sting

    Anders Garm

    The box jellyfish is ghostly and squishy, with 24 eyes and a tangle of tentacles, each equipped with about 5,000 stinging cells. The creatures pack a special type of venom — the most deadly in the animal kingdom — that is activated by contact with certain chemicals found in fish, shellfish and humans. The venom can cause cardiac arrest, cripple the nervous system, and eat away skin. Several victims stung at sea die before they reach shore.

  • Black mamba: Speedy snake with lethal venom

    Eric Marquette

    Even people who are fearless around snakes should be careful in the presence of a black mamba. Native to sub-Saharan Africa, the territorial and aggressive snakes grow up to 14 feet long and travel at speeds of more than 12 miles per hour. Fortunately, black mambas are shy and usually race away when humans approach. But when threatened, they attack with repeated strikes to deliver lethal venom. Only a quickly administered antidote saves victims from death. The black mamba gets its name from the color of the inside of its mouth, seen here.

  • Saltwater crocodiles eat people

    Greg Wood  /  AFP - Getty Images

    Saltwater crocodiles are aggressive and territorial. And unlike their North American alligator cousins, they regularly eat people. They live in saltwater estuaries, and freshwater rivers and swamps, ranging from Australia north to Southeast Asia. The biggest males weigh in excess of 2,200 pounds and measure 20 feet from toothy snout to scaly tail, making them the world's largest reptiles. Though they mostly dine on smaller prey such as fish and shorebirds, adults will occasionally tackle larger animals, including careless people. Tourists lure this croc out of the water with a chunk of meat.

  • Polar bears: a poster child best left alone


    Polar bears are a poster child for groups campaigning to save the world from the ravages of global warming. But the cute and cuddly image that polar bears project are an icy disguise. Polar bears are the world's largest land predators; some males top the scales at 1,500 pounds and wield 2-inch-long claws. They normally kill seals and the occasional walrus twice their size. Human attacks are extremely rare but potentially fatal.

  • Wolves are revered and feared

    AP file

    Revered by some, feared by others, the gray wolf is the world's biggest, most powerful dog. Strong jaws and sharp teeth help them rip into their prey. Wolves primarily hunt deer, moose and bison, but when wild supplies are tight, domestic cattle and sheep are easy targets — hence the wolf's uneasy relationship with humans. In the 20th century, the predators were nearly hunted to extinction. Continued fear of the wolf continues to muddy their road to recovery.

  • Lions take a mean bite

    Daniel Munoz  /  Reuters

    In 1898, a pair of lions reportedly ate 135 people working on a railroad in Kenya. Though lions continue to make the occasional human meal, most of the slaughtering now goes the other way. Today, the cats are vulnerable to extinction, due to their human predators. Conservationists are racing to keep the survivors alive. In this image, two lions at a Sydney zoo gaze at each other before a non-human meal.

  • Shark attacks: Very feared, very rare

    Image: Casa Rinconada

    Maybe the 1975 film "Jaws" is to blame, but whatever the cause, many humans are terrified of the great white shark, the largest predatory fish in the ocean. The fear is understandable: These giants have a mouth full of razor-sharp teeth, they can reach 20 feet in length, and they top the scales at 5,000 pounds. Despite all that, scientists insist the predators pose little risk to humans. They say the chances of winning the lottery are higher than the chance of being attacked by a great white. Moreover, most attacks are not fatal. In this image, a great white swims past a cage holding tourists.

  • Scariest spider is a recluse

    Image: Chankillo
    Justin Sullivan  /  Getty Images file

    Tarantulas give lots of people the creeps, but scientists say most of the big and hairy spiders are harmless to humans. The real spider to fear is the brown recluse, a six-eyed spider with a violin-shaped head and a venomous bite that can lead to necrosis — the death of skin tissue. Fortunately, as their name suggests, the spiders are reclusive and seldom aggressive, only biting when threatened. Most bites have little or no effect on their human victims, but some are nasty and even fatal.

  • Mosquitoes kill millions every year


    The world's deadliest animal? The mosquito. In some parts of the world, these pesky, bloodsucking insects spread diseases such as malaria, dengue fever and West Nile Virus that kill nearly 3 million people a year. Many of the malaria deaths occur in sub-Saharan Africa, where scientists and international aid organizations are busy developing strategies to stop the disease's lethal spread.

  • Vampire bats feed on blood

    Ho  /  AFP - Getty Images

    Finally, what list of scary animals would be complete without the vampire bat? The thumb-sized flying mammals with eight-inch wingspans feed exclusively on blood in the dark of night. Their prime targets are cattle and horses, but they are known to attack humans, too. Heat sensors in the bat's nose help it find flowing blood. They bite through the skin with razor-sharp teeth and lap up what oozes out.


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