What Do You and Your Cat Have in Common?


What do you and your cat have in common? Well, one possibility is a unicellular organism called Toxoplasma gondii. There are roughly 7 billion people on the planet and in a recent Scientific Mind article, Gustavo Arrizabalaga and Bill Sullivan suggest as many as 3 billion of us are carriers for Toxoplasma. It’s estimated that just in the U.S., one in five people are infected. In less industrialized countries, estimates are as high as 95%.

Up until recently, the protozoan known as Toxoplasma was considered largely harmless,  just one of the millions of parasites we humans play host to every day. But recent research by Charles University parasitologist Jaroslav Flegr and others indicates there is reason for concern.

Flegr’s work suggests that if your first experience with Toxoplasma occurs while you’re pregnant, the parasite can reach the fetus, moving through tissues and organs, spreading from cell to cell. Possible results include birth defects or miscarriage. 

Toxoplasma also is able to cross the blood-brain barrier in rats, where it sets up camp in the neural circuitry and begins to alter the brain, and with it, that rat’s entire demeanor.

What does Toxoplasma make these rats do? A single-cell organism is not sophisticated enough to have a master plan, but over time, it has evolved some freaky tendencies that bring Toxoplasma closer to its ideal host: cats. An infected rat doesn’t just show no fear of its natural predators, it is actively attracted to the smell of cat urine. The result? Bad news for rodents, and great news for Toxoplasma.

A parasite that encourages risky behavior in rats might not make headlines. But what about humans? Can Toxoplasma cross our blood-brain barrier as well? In a ten year experiment involving 2500 people, Flegr’s team found correlation between Toxoplasma infection and certain personality changes.

In Gary Wenk’s new book Your Brain on Food, he reports that infected women showed tendencies of becoming more “trusting and warmhearted, less prone to jealousy or suspicions.” This alone may not seem like cause to ring alarm bells, but it seems to line up with a general lack of self-preservation: infected women also had a noticeably higher suicide rate.

Infected men, on the other hand, showed more aggression, suspicion, and jealousy. Given the long history of human interaction with cats, it’s not beyond imagining that this tiny protozoa might have shaped our long history of warfare.

A brain parasite that can rewrite its host’s personality sounds more like a hacky sci fi plot point than fact. But although some of the more aggressive findings still need additional research to establish solid cause and effect, it’s enough to raise some questions. Perhaps chief among them, “Oh my god, is this thing inside me?” 

A simple blood test can determine whether or not you’re a carrier. In the meantime, keep in mind that Toxoplasma is passed through contact with an infected cat’s feces. Even if you don’t have pets, you can pick it up in your own backyard from stray cats, or on any food or vegetable that might have been exposed to them.


The Toxoplasma club is 3 billion members strong. Hopefully you’re not one of them.


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