It isn’t just unsuspecting insects that can fall under the spell of a behavior-changing parasite. Mice that are infected with Toxoplasma gondii are often said to lose their fear of cats. If they’re less scared of cats, they’re more likely to get eaten by one, which suits T. gondii, since the intestines of domestic cats are where the species tend to reproduce. The perfect crime, right?
While few could deny the success of T. gondii – it’s thought to have infected billions of humans across the world – a new study suggests the parasite may not be quite as “efficient” as this "feline fear theory" indicates.
It turns out, the T. gondii actually makes rodents generally less scared of all species, not just cats. The mechanism might also be a lot less "smart" than it initially seems, likely to be the result of general inflammation in the brain, rather than the parasite playing around with specific neurons like an evil mastermind.
Reported in the journal Cell Reports, researchers from the University of Geneva in Switzerland devised an experiment that allowed mice to wonder in between chambers that contained the scent of different species. These different chambers included the scents of predators, such as bobcats and foxes, as well as harmless non-predators, such as guinea pigs and uninfected mice. Along with seeing how much time they spent in each chamber, a separate experiment in the study also kept a close eye on their behavior for signs of anxiety and fear while sniffing around.
Although they noted that the control mice (not infected with the parasite) froze in fear after smelling the predators and avoided the scent, the infected mice were happy to approach the smell, just as expected. Crucially, however, the infected mice were no more attracted to the cat smell than any other and were simply just more keen to explore all of the chambers.
The researchers dug a little deeper into this by analyzing the brains of infected mice. They found the brains of mice after infection contained a number of parasite-filled tissue cysts, especially in the cerebral cortex and brain regions involved in processing visual information. However, cysts were also found across the brain and their distribution varied widely across mice, suggesting a random infection.
"Taken together, the findings point toward behavioural manipulation mediated by neuronal inflammation rather than direct interference of the parasite itself with specific neuronal populations," co-senior study author Ivan Rodriguez, from the University of Geneva, said in a statement. "It is not a simple on/off system. In the future, the level of chronic infection should therefore always be taken into account when studying effects of T. gondii on its host.”
While a number of recent studies have thrown doubt on whether this cat-borne parasite can truly affect human behavior, some research has linked infection to increased suicide rates and schizophrenia in people. It has also been associated with a more general sense of fearlessness in humans, although – once again – some of those claims are pretty controversial. For example, one study from 2002 found that infected people have a 2.65 times higher risk of getting into a road traffic accident.