‘Mind-altering’ Parasite Causes Major Changes in Mice Brains

Has the Myth of the Mind Altering Parasite been Shattered?

Jackson Ryan / CNET

There is a good chance that you are infected with a mind-altering parasite. And if not you … then maybe your cat. The parasite, known as Toxoplasma gondii, is one of the most prolific proliferators on the planet, capable of infecting virtually any warm-blooded animal. Scientists believe it may have infected more than 3 billion humans.

But that’s OK. There’s no real reason to freak out – it’s most safe.

Toxoplasma is a remarkable single-celled organism in the same category as the malaria parasite and has long captivated the imagination of researchers and the public due to its ability to change behavior in rodents and humans. Studies have shown that rodents infected with Toxoplasma are more intrepid, and some researchers have linked infection in humans with impulsivity and mental health issues like schizophrenia.

In most warm-blooded mammals, the parasite is basically inert and forms protective cysts to survive, including in brain tissue. Cysts stay in the body for life, sleeping in hosts – unless they have a weakened immune system, in which case, Toxoplasma can be bad news.

In the guts of a cat, it’s a different story. This is where the parasite goes to work. The life goal of Toxoplasma is singular: to enter a cat, to mature and to reproduce. Once the Toxoplasma “children” are born, the cat excretes them. And the cycle begins again.

A diagram from the Center for Disease Control and Prevention of the Toxoplasma lifecycle.


To enter the cat, the standing theory suggests that Toxoplasma modifies rodent behavior using a form of “mind control”. Instead of being repelled by cat scents or hiding, rodents are more likely to confront a feline. Toxoplasma appears to decrease anxiety and make rodents more courageous. This increases their chances of being eaten, giving Toxoplasma the best chance to sneak inside its preferred host.

Rodents are furry Trojans, overflowing with a battalion of parasites waiting to get inside a cat.

however, intense scientific debate still surrounds Toxoplasma and the extent of its mind-modifying abilities. Studies have shown that the organism affects the behavior of mice, but how the parasite achieves this remains unknown. Is it an adaptive mechanism stimulated by evolution? Does he directly manipulate the host’s brain, bending it to his will? Or does the infection cause a disruption of the host’s immune response, causing inflammation in the brain that leads to behavioral changes?

New search, published in the journal Cell Reports on Tuesday, tries to answer these questions. The researchers looked at behavioral changes in mice following Toxoplasma infection, subjecting the rodents to a series of behavioral tests to determine if they were less afraid of predator smells and open spaces. Next, they dug into the brain, focusing their attention on parasitic cysts and genetic markers of inflammation, to determine if any tangible physiological changes were contributing to the behavior of the mice.

“There has been a lot of controversy around whether or not general anxiety is reduced,” explains Ph.D. student Madlaina Boillat, neurogeneticist at the University of Geneva and co-first author of the ‘article. “The studies reported no change in anxiety upon infection, increased anxiety or reduced anxiety, depending on variables such as the behavioral test used and the level of infection, which is highly strain dependent. mouse and parasite strain. “

To scan

In a behavioral test, mice were placed in an X-shaped “maze”. Two arms of the X were lined with large walls and two arms were fully open. Uninfected mice usually stick to the arms containing the border walls for added security, but mice infected with the parasite spent much more time exploring with open arms. Another test, says Boillat, showed that infected individuals were ready to smell foxes and guinea pigs just as easily.

This new finding is particularly important because it demonstrates that the parasite does not selectively manipulate the behavior of the host to be less fearful of cats, as the so-called “fatal attraction” theory suggests. Rodents do not scan specifically for cats. Instead, it looks like the parasite is likely tinkering with more generalized anxiety pathways in the brain and causing increased curiosity.

The physiological response is revealing.


A Toxoplasma gondii cyst embedded in brain tissue.


“Our RNA Sequencing Experience Shows Major Changes Are Taking Place In The Brain [of] infected mice, “says Boillat.” Heavily infected mice show signs of immunopathology, such as neuronal death and excitotoxicity. “

The severity of these symptoms was related to the number of ball-shaped cysts formed and the amount of inflammation observed in the brains of the mice. It then follows that Toxoplasma does not directly disturb brain cells, somehow pulling the strings of a rodent’s gray matter for its own benefit, but rather causes inflammation. This follows previous research that demonstrated a similar phenomenon: When cysts and inflammation increased, the activity level of infected mice also increased.

Rather than the (admittedly very cool) idea that the parasite is performing some sort of mind control, it seems likely that the immune response is causing unusual – and potentially fatal – behaviors. Such a discovery could have far-reaching implications if it persists in humans as well.

“We know that inflammation of the brain, whether induced by Toxoplasma gondii or other pathogens, can trigger mental illnesses in humans,” notes Boillat. However, she suggests that we should be “very careful” in interpreting the results. The team observed strong correlations between infection, gene expression in the brain, and behavior, but they cannot provide any cause. explanations.

In addition, humans and mice respond differently to infection with the parasite, which makes it difficult to make direct comparisons between species.

The team’s next challenge is to understand how inflammation in the brain can lead to behavioral changes in mice. Is it because brain cells are somehow overstimulated, or are molecular signals going haywire? Does the presence of the parasite affect the way the brain responds to stimuli?

Answering these questions will further shed light on Toxoplasma’s purported mind-control techniques, and by the way, help me view the neighborhood tabby with less dread.

Originally posted at 8:15 a.m. PT.

About Eric Buss

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