Beware the body snatchers: parasites control the brains of rats, turning the rodents into daredevils. Can these parasites take over humans too?

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Any rat in its right mind would flee from a hungry cat. But not so with a rat infected by a mind-controlling parasite called Toxoplasma gondii. T. gondii are tiny organisms that can make their way inside a rat's brain without the animal ever knowing it. Rodents infected by T. gondii aren't afraid of cat bullies. In fact, they are attracted to the furry felines. As you might imagine, the result can be deadly for the rat.

Scientists are unnerved by T. gondii's brainwashing powers. Why? It turns out that these same parasites also infect humans. Researchers wonder: If T. gondii can mold a rodent into a cat lover, what effect might the parasite have on a human it inhabits?

PICKY PARASITE

T. gondii can't live on its own. Like other parasites, it needs an organism, or host, to call home. The host provides microscopic T. gondii with food and gives them a cozy place in which to hang out and grow.

The parasite has a two-stage life cycle, preferring different hosts at each stage (see Nuts & Bolts, below). It begins its life as an egg in the soil.

The egg is able to hatch and thrive inside warm-blooded hosts like rats, birds, or even humans. But the parasite soon becomes pickier about where it lives. A mature T. gondii must reside inside the gut of a feline--be it a housecat, bobcat, puma, or any other type. Only there can it become an adult and replicate, ensuring the survival of its species.

Although T. gondii eggs can hatch inside any warm-blooded animal, rats and mice are the ideal hosts. That's because they often fall prey to felines. By living its early life inside a rodent, the parasite ups its chances of landing in a cat's belly. To be extra sure that a cat eats its host, T. gondii hijacks the rat's brain.

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BITSY BRAINWASHER

Once a rat ingests T. gondii, the parasite makes its way straight to the animal's brain. There, the tiny organism manipulates its host. Scientists believe one way the parasite does so is by interfering with the rat's amygdala, the part of the brain that is responsible for learned behavior.

Rats generally are conditioned to have a fear of cats, "but the parasite seems to over-ride this [fear] into a suicidal feline attraction," says Joanne Webster, a parasitologist with the Imperial College Faculty of Medicine in London, United Kingdom. As a result, infected rats actually seek out places where cats are lurking. This makes it more likely that the rats will be preyed upon, says Webster.

Once a cat has eaten the rat, the sneaky parasite settles into the feline's intestine and begins to reproduce. Eventually, the cat sheds the eggs in its droppings. If an unlucky rodent scampers among the eggladen feces and ingests the eggs, the entire life cycle of T. gondii will play out again. If, on the other hand, another species-one less likely to be eaten by a cat--were to become infected, the cycle would reach a dead-end.

INVADING HUMANS

It is not uncommon for animals other than rodents to become infected with T. gondii. Many humans have the parasite, says Webster.

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How do people fall victim to T. gondii? Suppose a grazing cow slurped up the eggs left behind in cat droppings in the grass, and then a person chowed down on the meat of the infected cow. He or she could also become infected.

As many as 20 percent to 40 percent of people in the U.S. and U.K. are infected with T. gondii. In France, that number may be as high as 80 percent, says Webster. Infection rates are much higher in countries like France, where people prefer to eat meats that are on the rare side. Cooking meats at high temperatures helps kill the parasites.

ALTERING PEOPLE

If T. gondii gives rats a death wish, what does it do to humans? Studies have found that in serious cases, infected people show subtle changes in how they act. Some people have slowed reaction times and, in extreme cases, individuals may show signs of schizophrenia, a psychiatric condition that can cause hallucinations and harm the thinking process.

A group of scientists in the Czech Republic recently discovered yet another possible consequence of these parasites. They studied a group of women who had recently given birth and found that those women who were infected with T.gondii were more likely to have had boys rather than girls. Of 1,800 women in the study who carried the parasite, a whopping 72 per cent of them gave birth to baby boys. Typically, of all the babies born around the world, just 51 percent are boys.

To reduce your risk of contracting T. gondii, scientists recommend cooking meat fully before eating it, and washing your hands after touching soil, sand, or kitty litter that could be contaminated by infected cat droppings.

PRE-READING PROMPT

* What is a parasite? How does a parasite survive?

DID YOU KNOW?

* Toxoplasma gondii can cause deadly brain infections ill endangered southern sea others. One way they become infected: As humans in parts of California flush parasite-infested cat litter down the toilet, some of the sewage runs into waters off California--where sea otters dwell. They become infected by the contaminated waters.

CRITICAL THINKING:

* How might some parasites benefit their hosts? Explain your answer. (Note: Have students review mutualism.)

CROSS-CURRICULAR CONNECTIONS:

ART: Have each student research a different Species of parasite besides T. gondii. Then, have him or her draw a life cycle diagram of the selected parasite.

RESOURCES

* Parasite Sleuth is a kid-friendly site that shows how some common parasites live and breed. Visit: http.//net.unl.edu/wenderwise/12parasi/kidactivity/index2.htm

* For detailed information on how parasites affect humans, visit this Web site: www.dpd.cdc.gov/dpdx/Default.htm

DIRECTIONS: Complete the following as you read the story on pages 8 to 10.

1. Look at the images on pages 8 to 10. Then, read the article's headline and the subheadline. Based on this information, what do you think this article is about? Why?

2. Below are three vocabulary words. Read the story to find the definition of these words.

Host:

Amygdala:

Schizophrenia:

3. The last sentence of the section titled "Bitsy Brainwasher" says: If a species--one less likely to be eaten by a cat--were to become infected with T. gondii, then the parasite's cycle would end. Explain why.

4. Study the life cycle diagram on p. 9. How would the spread of T. gondii change if cows were removed from the diagram? Why?

5. Use the information in the story to help you defend or dispute this statement: Americans are more at risk than others for contracting a T. gondii infection. (Hint: Defend means to explain why a statement is correct. Dispute means to explain why a statement is incorrect.)

6. Summarize the entire article in two sentences.

7. Come up with a question about the story that you don't know the answer to, but that you're interested to know.

ANSWERS

1. Answers will vary.

2. Host: A host is an organism in or on which a parasite lives Amygdala: The amygdala is the part of the brain that is responsible for learned behavior, such as perception of fear. Schizophrenia: Schizophrema is a psychiatric condition that can cause hallucinations and harm the thinking process.

3. T. gondii, must reside inside--the gut of a feline to become adults and replicate. If a species--one less likely to be eaten by a cat were to become infected, then it would be very difficult for the parasite to enter a cat's gut As a result, the parasite's life cycle would end.

4. If cows were removed from the diagram then humans would be less likely to contract T. gondii, That's because eating beef would not expose humans to any risk of T. gondii infection.

5. Dispute: People in France are more likely than Americans to be infected with T. gondii. The parasite infects as many as 20 to 40 percent of people in the U.S. and the U.K. In France, that number may be as high as 80 percent. Infection rates are much higher in countries like France where people prefer to eat meats that are on the rare side Cooking meats at high temperatures helps kill the parasites.

6. Answers will vary.

7. Answers will vary.

nuts&bolts

HOW TOXOPLASMA GONDII SPREADS

T. gondii are microscopic parasites that are found in cats, rats, and other warm-blooded hosts. Even human can be infected with T. gondii. Here's how:

OH, RATS: Rodent ingests egg-containing droppings. T. gondii makes it seek out cats.

DINNERTIME: Cat eats rodent infected with T. gondii. The parasite matures and replicates in cat's gut.

LITTER TRAINED: Cat goes to the bathroom in litter box. Cat's waste contains T. gondii eggs.

OUTHOUSE: Cat goes to the bathroom outside. Cat's waste contains T. gondii eggs.

GREEN ACRES: Infected cat droppings contaminate grass and soil. Cow grazes on grass; gets infected.

HUMAN TOO: Person cleans litter box and does not wash hands; or person eats meat from infected animal. Either way, person can get infected.

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