How A Parasite Can Rewire Your Brain And Change Your Behavior

Toxoplasma gondii under the microscope. CDC

"Mind-controlling parasites" might sound like the baddies from a straight-to-TV horror movie, but that's a fairly accurate description of Toxoplasma gondii. 

The single-celled organism is thought to be one of the most common parasites in the world, infecting up to 40 million people in the US, according to the CDC. While most infections are believed to be asymptomatic – and some research argues its effects are overstated – it has been linked to a wide variety of behavior changes in humans and other animals, including increased suicide rates, schizophrenia, and risk-taking behavior.

The parasite also infects most species of warm-blooded animals. In mice, for example, the parasitic infection is well known to cause a reduced fear of predators, such as cats. If the mouse is less scared, they’re more likely to get eaten and end up in the intestines of domestic cats, where the species tend to reproduce. 

But how can a microscopic organism have such a seemingly powerful grip on the brain? 

"Even though a lot of neuroscientists study Toxoplasma infection as a model for immune response in the brain, we want to understand what this parasite does to rewire the brain, leading to these dramatic shifts in behavior," Michael Fox, study author and professor at the Fralin Biomedical Research Institute, said in a statement.

Reporting in the journal GLIA, the neuroscientists detail how the parasite sparks a significant loss of inhibitory signaling in the brain. This is effectively like running a city's road system with some broken traffic lights. 

Previous research has shown that Toxoplasma infections can change the levels of a key enzyme in inhibitory neurons needed to generate their neurotransmitters. Off the back of that research, this recent study of the brains of mice infected with T. gondii noted a loss of inhibitory perisomatic synapses in the neocortex and hippocampus, as well as observing that the cell body of neurons had become ensheathed by other brain cells, preventing their ability to signal to each other. 

Since the study was carried out in mice, it’s unclear how these findings might apply to human brains. Nevertheless, the observations appear to line up with what we know about the parasitic infection and psychiatric disorders. 

"In neuropsychiatric disorders, similar patterns of inhibitory synapse loss have been reported, therefore these results could explain why some people develop these disorders post-infection," added Fox.

 

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