Parasites affect entire human cultures?

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Study: Cat Parasite Affects Human Culture
By Ker Than, LiveScience Staff Writer

posted: 03 August 2006 03:47 pm ET

A parasitic microbe commonly found in cats might have helped shape entire human cultures by manipulating the personalities of infected individuals, according to a new study.

Infection by a Toxoplasma gondii could make some individuals more prone to some forms of neuroticism and could lead to differences among cultures if enough people are infected, says Kevin Lafferty, a U.S. Geological Survey scientist at the University of California, Santa Barbara.

In a survey of different countries, Lafferty found that people living in those with higher rates of T. gondii infection scored higher on average for neuroticism, defined as an emotional or mental disorder characterized by high levels of anxiety, insecurity or depression.

His finding is detailed in the Aug. 2 issue of the journal for Proceedings of the Royal Society, Biology.

Manipulating behavior

T. gondii infects both wild and domestic cats, but it is carried by many warm-blooded mammals. One recent study showed that the parasite makes normally cautious rats outgoing and more prone to engage in reckless behavior, such as hanging around areas frequently marked by cat urine, making the rats easy targets.

Scientists estimate that the parasite has infected about 3 billion people, or about half of the human population. Studies by researchers in the Czech Republic have suggested T. gondii might have subtle but long-term effects on its human hosts. The parasite is thought to have different, and often opposite effects in men versus women, but both genders appear to develop a form of neuroticism called "guilt proneness."

Other studies have also found links between the parasite and schizophrenia. T. gondii infection is known to damage astrocytes, support cells in the brain that are also affected during schizophrenia. Pregnant women with high levels of antibodies to the parasite are also more likely to give birth to children who will develop the disorder.

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Bizarre Human Brain Parasite Precisely Alters Fear
By Charles Q. Choi, Special to LiveScience

posted: 02 April 2007 05:02 pm ET

Rats usually have an innate fear of cat urine. The fear extends to rodents that have never seen a feline and those generations removed from ever meeting a cat. After they get infected with the brain parasite Toxoplasma gondii, however, rats become attracted to cat pee, increasing the chance they'll become cat food.

This much researchers knew. But a new study shows the parasite, which also infects more half the world's human population, seems to target a rat's fear of cat urine with almost surgical precision, leaving other kinds of fear alone.

This discovery could shed light "on how fear is generated in the first place" and how people can potentially better manage phobias, researcher Ajai Vyas, a Stanford University neuroscientist, told LiveScience.

Hijacking the mind

T. gondii is a parasitic germ whose primary hosts are cats. However, it can be found in most warm-blooded animals, including an estimated 50 million people in the United States. One study suggests the parasite has altered human behavior enough to shape entire cultures.

In cats, the protozoan reproduces sexually, while it reproduces asexually in other animals.

The germ seems to especially like infesting the brain—"parasites hijacking the mind," Vyas said. Although the disease it causes in humans is rarely dangerous, it is the reason that pregnant women are sometimes told to avoid cat litter boxes (toxoplasmosis is risky for infants and others with compromised immune systems). Some scientists have suspected it might be linked to mental disorders such as schizophrenia and even neuroticism.

In 2000, scientists revealed T. gondii could modify the brains of rats to make them attracted to cat urine instead of afraid of it. Researchers suspect the germ does so to make it easier for it to jump into cats to begin the sexual part of its life cycle.

Vyas and his colleagues now show how specific this brain reprogramming is when it comes to rats, findings detailed online April 2 in the Proceedings of the National Academy of Sciences.

Just cat pee

Rats infected with the parasite became mildly attracted to bobcat pee. However, they remained as fearful of open spaces as normal rats. They reacted normally to sound cues that suggested mild electrical shocks were coming. Normally rats are somewhat reticent when it comes to eating food that smells unfamiliar. And the infected rats were, just like the normal rats, reticent when it came to food scented with the unfamiliar odor of coriander.

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