A mind-altering parasite causes mice to become permanently fearless around cats, according to a new PLoS ONE study that further suggests this creepy parasite could be present in many human brains.

The parasitic protozoan, Toxoplasma gondii, has a near foolproof life cycle. It infects mice, which are then eaten by hungry felines (mostly feral), which can then serve as a reservoir for the parasite, since it sexually reproduces in a cat’s intestinal tract.

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But the parasite doesn’t just sit in the gut. It can also go to the brain, where it may cause the individual to exhibit behavioral changes — such as overconfidence in mice.

“Even when the parasite is no longer in the brains of the animals, some kind of permanent long-term behavior change has occurred, even though we don’t know what the actual mechanism is,” UC Berkeley graduate student Wendy Ingram, who worked on the study, said in a press release.

Ingram and colleagues Michael Eisen and Ellen Robey tested mice by seeing whether they avoided bobcat urine, which is normal behavior, versus rabbit urine, to which mice don’t react. The researchers found that the three most common strains of T. gondii make mice less fearful of cats for at least four months.

The effect likely is then permanent. (And one wonders how long such a mouse would be in the land of the living anyway, given how easy it would be for cats and other animals to catch them.)

It should be noted that the scientists used a genetically altered strain of T. gondii that cannot form cysts and so is unable to cause chronic infections in the brain. The team is now looking at how the mouse immune system attacks the parasite to see whether the host’s response to the infection is the culprit.

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The take home message, though, is that if you have pets — especially cats– keep them indoors whenever possible. There are many reasons for doing this, but preventing them from getting a potentially brain-altering parasite is a pretty compelling one.

Also wash your hands after tidying up an indoor cat’s litter-box. Another route of transmission is eating underdone pork, since pigs can also become infected with this parasite, so experts advise cooking pork to at least 160 degrees F.

Ingram said other studies have found that one-third of the world’s human population has been infected by T. gondii and probably have dormant cysts in their brains.

She added that, when kept in check by the body’s immune system, these cysts sometimes revive in immune-compromised people, leading to death. Some preliminary studies suggest that chronic infection may be linked to schizophrenia or suicidal behavior.

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We obviously haven’t heard the last about this bizarre parasite. As Ingram said, “Toxoplasma gondii has done a stunning job of adapting to mammalian brains in order to enhance its transmission through a complicated life cycle.”

Photo: Wendy Ingram