"The ultimate experiment would be to have a bunch of people, and to infect some of them, and then measure their behavior before and after," Adamo says. "We're never ever going to be allowed to do that with humans." In a lab setting, Toxoplasma also binds to dopamine receptors, which are the brain's reward centers.
"It's quite possible and plausible that if you have an agent affecting dopamine, it could influence people's behaviors," Adamo says.
A disease that completely alters a human's behavior isn't unprecedented. A human infected by rabies will nearly always display a fear of water, just the way a dog would—so it's not beyond the realm of possibility that a parasite could influence a human's behavior in more subtle ways.
"In invertebrates, the effects are far more drastic. In mammals, the effects are smaller. I say that, but then you look at something like rabies," Adamo says. "And you see that things like this can cause huge changes in behavior."
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Jason Koebler is a science and technology reporter for U.S. News & World Report. You can follow him on Twitter or reach him at firstname.lastname@example.org.