A parasite conducts "mind control" on mice so that they're not afraid of cats, an animal that can host breeding grounds for the mini-monsters.
The cat gets an easy meal out of the deal, but picks up some parasites. The parasites themselves get to have a love session in the feline's intestines, a University of California, Berkeley press release reported.
"The idea that this parasite knows more about our brains than we do, and has the ability to exert desired change in complicated rodent behavior, is absolutely fascinating," UC graduate student Wendy Ingram, said. "Toxoplasma has done a phenomenal job of figuring out mammalian brains in order to enhance its transmission through a complicated life cycle."
Toxoplasmosis, the parasitic infection, can also cause "spontaneous abortion" in pregnant women and even kill humans suffering from immune problems. The parasite creates flu-like symptoms in the mice, along with making them fearless.
Once the "flu" goes away and the parasite is cleared from the rodent's body (unless they get eaten by a cat that is) the symptoms persist.
"Even when the parasite is cleared and it's no longer in the brains of the animals, some kind of permanent long-term behavior change has occurred, even though we don't know what the actual mechanism is," Ingram said.
The researchers believe the parasite could damage the part of the mouse's brain responsible for detecting the smell of cat urine, which could explain their nonchalant attitude around carnivorous felines.
The team also toyed with the idea that the Toxoplasma gondii parasite could interfere with nerves responsible for "memory and learning."
Ingram and colleagues started studying the effect of Toxoplasmosis on mice three years ago, when she tested if the rodents avoided bobcat urine (which they usually do) and rabbit urine (which they usually don't).
The research team also tried the experiment with mice that were infected with a type of toxoplasma parasite that cannot form chronic brain infections or cysts. These rodents' fearlessness also continued for at least four months.
She found that mice stop responding to the bobcat urine after a few weeks of infection, and the lack of reaction persists for at least four months.
"This would seem to refute - or at least make less likely - models in which the behavior effects are the result of direct physical action of parasites on specific parts of the brain," UC Berkeley professors of molecular and cell biology, Michael Eisen, said.
About a third of people are infected with the parasite globally. Most people have an immune system strong enough to fight off the infection's symptoms, and the live cysts sit dormant in their brains. In some cases Toxoplasmosis has been linked to a higher risk of schizophrenia or suicide. Pregnant women have also been advised to stay far away from cat droppings.
Eating undercooked pork is another common way the parasite is spread.
