There's a single-celled parasite called Toxoplasma gondii, and it can turn a normally risk-averse mouse into a bold, cat-seeking rodent. Cats that devour such mice can then pass the parasite onto humans.
But once humans are infected, what happens to them? Do they become Toxoplasma gondii zombies, acting however the parasite deems fit?
The answer is complex; studies show that people who test positive for this condition are more likely to take certain kinds of risks than those who don't have toxoplasmosis, but it's not yet clear how this happens.
Related: Why do cats hate car rides?
Toxoplasma gondii can live in most mammals, but its life cycle traditionally involves living first inside a mouse or rat. That's likely because cats prey on rodents, giving the parasite a way to reach the cat gut — the only known place where the parasite can reproduce, as cat guts are rich in linoleic acid, an ingredient necessary for Toxoplasma gondii sex, a 2019 study posted on the preprint database bioRxiv found.
To aid this transition from vermin to feline host, Toxoplasma gondii has a pretty disturbing technique: brain manipulation. The parasite alters the behavior of rodents, making them less afraid of taking risks. It is also known to make mice attracted to the scent of cat poop. In other words, the parasite does something to the brain of mice and rats to make them more likely to dart out in front of a cat to then be caught, killed and eaten. Humans are not immune to Toxoplasma gondii — in fact, at least a third of the world's population is thought to have toxoplasmosis, the infection this parasite causes. Some humans get infected when they clean out their pet cat's litter box, but many of us simply eat undercooked meats or unwashed vegetables. So, what does this mean for the one in three of us who end up playing host to the parasite?
"When you first get infected, you might have mild flu-like symptoms but most of us don't even realize. If you're a pregnant women then it's more concerning because it can harm the child, but largely, infected people show almost no health issues or noticeable symptoms," said Markus Fitza, a professor of strategy and entrepreneurship at the Frankfurt School of Finance and Management in Germany, whose research has focused on how the parasite can affect decisions made in the business world.
But that's not where the story ends. "The fundamentals of the human brain are fairly similar to that of a mouse in the grand scheme of things," he told Live Science.
In short, just like an infected mouse is more adventurous, an infected human is also more prone to take risks — sometimes with pretty devastating consequences. At least two studies — one analyzing 370 people in Turkey and another looking at nearly 600 people in the Czech Republic — have shown that people who test positive for Toxoplasma gondii antibodies are more likely to have a car crash. There isn’t a lot of evidence to explain why and how latent infections of toxoplasmosis might affect people’s personalities, such as making them less afraid of risk. The researchers behind the car crash studies, however, wrote that the parasite leaves behind life-long cysts in the brain, which is thought to increase the production of dopamine (a chemical messenger in the brain that is known to affect people's risk and reward calculations), and that may have a role to play.
Related: Why do our brains have folds?
Toxoplasmosis is also linked to rage problems. People with the psychiatric disorder known as Intermittent Explosive Disorder, or IED, were twice as likely to have a toxoplasmosis diagnosis than healthy individuals with no psychiatric diagnosis, Live Science previously reported.
Interestingly though, a decreased fear of peril may also have its advantages. Fitza's own work has shown that entrepreneurs are more likely to be infected than the general population and that entrepreneurs with the parasite tend to earn, on average, $6,000 more per year than those without it. In one study, he and colleagues tested nearly 1,500 biology and business studies students at a major U.S. university. The results showed that business majors were 1.4 times more likely to test positive for the parasite than biology majors, and within business majors, those specializing in entrepreneurship were 1.7 times more likely to test positive over students in less risky business studies subspecialties.
Fitza also tested 197 professionals attending entrepreneurship workshops and events. He found that 124 of them had been infected by the parasite. Of those 124 professionals, 17 had started their own business compared with just four out of the 73 non-infected people who attended the same events. While his sample size was small, the findings have been backed up by subsequent, larger studies. The working theory is that Toxoplasma gondii is manipulating people's brains into making them less afraid of quitting their jobs and going it alone to start up their own company. "We can't say for sure this is what's happening," said Fitza. "But this is the argument we're making based on our studies."
However, scientists have yet to pin down a direct way that Toxoplasma gondii might control human minds, Live Science previously reported. In rodents, it's possible that the parasite secretes molecules that somehow rewire the brain; others think it may be due to brain cysts interfering with dopamine production; and another idea is that Toxoplasma gondii triggers inflammation in rodent brains, which may, in turn, change brain behavior and function, a 2019 study in the journal mBio found.
That said, humans with toxoplasmosis aren't mindless zombies doing the parasite's bidding. When in humans, there is no benefit to the parasite in making human behavior more risky; any effects are just a hangover from it manipulating the mouse brain, which is advantageous to the parasite. But given that the parasite might lead to some behavioral changes in humans, be mindful of that if you have cats and you're driving your car to a potential business opportunity.
Originally published on Live Science.
Live Science newsletter
Stay up to date on the latest science news by signing up for our Essentials newsletter.
Benjamin is a freelance science journalist with nearly a decade of experience, based in Australia. His writing has featured in Live Science, Scientific American, Discover Magazine, Associated Press, USA Today, Wired, Engadget, Chemical & Engineering News, among others. Benjamin has a bachelor's degree in biology from Imperial College, London, and a master's degree in science journalism from New York University along with an advanced certificate in science, health and environmental reporting.