Rodents really have it rough. When hit with a Toxoplasma gondii infection, mice don’t just end up with parasites in their brains. They also get weirdly reckless, venturing into scary new environments and brashly investigating the smells of animals they should fear, like cats, foxes and intrusive human researchers, according to a study published this week in Cell Reports.
The feline part of this story has long been known, and often spun into a narrative that’s perhaps made the parasites seem more devious than they really are, reports Kelly Servick for Science magazine. While Toxoplasma spends much of its time hanging out in a wide variety of warm-blooded animals, including rodents and humans, it can only complete the sexually reproductive stage of its life in wild or domestic cats. So when researchers saw infected rats developing a kind of fatal feline attraction, they theorized that Toxoplasma had found a way to hijack the rodent brain and strip it of the singular fear that made them harder to catch and eat—and ingeniously ensuring the parasites’ continued success.
But perhaps parasite and host aren’t so picky after all. When a team led by Dominique Soldati-Favre, a parasitologist at the University of Geneva, exposed Toxoplasma-infected mice to a smorgasbord of smells from bobcats, foxes, guinea pigs and other mice, the parasite-plagued rodents eagerly explored all scented chambers. Despite previous theories on cat-specific preferences, they showed no preference for felines—or even for predators (bobcat and fox) over non-predators (guinea pigs and mice).
The mice seemed to abound with bravery around living threats, too. Per Science News’ Laura Sanders, when an researcher stuck her hand into inside into the mouse enclosure, infected rodents rubbed up against it while their parasite-free counterparts huddled and hid. And while uninfected mice shirked a room containing a live, anesthetized rat—another species known to eat them—mice carrying Toxoplasma happily ventured in.
Placed into a scary labyrinth full of open tunnels, infected mice were also bolder than uninfected ones, cavorting around the maze while Toxoplasma-free rodents explored only cautiously.
“We realized it wasn’t just about having lost fear against the cat,” Soldati-Favre tells Servick. “Really, these mice are very open-minded, and they go everywhere.”
If confirmed, the findings widen the scope of Toxoplasma’s effects. Just because the parasite isn't as targetted as previously thought, doesn't mean it's no longer considered a master manipulator, says Laura Knoll, a parasitologist at the University of Wisconsin–Madison who wasn’t involved in the study, in an interview with Servick. If mice are running around with reckless abandon and disregard for their own safety, it makes them an easy target for predators of any kind. Even if the infected mouse gets gobbled up by something that isn't a cat, the Toxoplasma can still spread in the unlucky diner’s tissues; the parasite is just unable to sexually reproduce without a feline host.
That’s actually how a lot of humans end up infected with Toxoplasma infections: eating other animals that carry the parasite—usually pigs or sheep, not rodents. Millions of people worldwide are thought to harbor these infections, which have occasionally been linked to mental illnesses or unusual psychiatric behavior. These symptoms are nothing like what mice experience, but at the molecular level, infections in all afflicted animals may lead to some kind of inflammation in the brain.
Given the difficulties of conducting behavioral studies in lab animals, future studies could still change researchers’ understanding of Toxoplasma’s true effects. But as Soldati-Favre says in a statement, it sometimes makes sense to keep an open mind and “reappraise the dogma.” Or perhaps in this case, catma.