September 19, 2013
Mice Aren’t Scaredy-Cats When Infected By Toxoplasma
Brett Smith for redOrbit.com - Your Universe Online
Known to cause spontaneous abortions in pregnant women or death in immune-compromised patients, the parasite Toxoplasma gondii has an even stranger effect in mice – it takes away their fear of cats. The fearless mice are a boon to both cats, which get an easy meal, and the parasite, which gets access to the cat's intestinal track – the only place the parasite can sexually reproduce.
"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," said study author Wendy Ingram, a graduate researcher at UC Berkeley.
Ingram said the parasite could affect the mouse brain so that the scent of cat urine can't be detected. The parasite may also be affecting neurons involved in memory and learning or it could trigger a response from its host that affects the natural fear response.
The Berkeley team began by seeing if parasite-hosting mice avoided bobcat urine, a normal response in uninfected mice, versus rabbit urine, which mice typically ignore. While mice typically lose their fear of bobcat urine for a few weeks after initial infection, the researchers found that the three most common strains of Toxoplasma gondii eliminate the fear response to cats for at least four months.
Using a genetically modified strain of Toxoplasma that was unable to cause chronic infections in the mouse brain, the team showed that the behavioral effect still took hold and lasted for four months after the mice eliminated the microbe from their bodies.
"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," study author and Berkeley professor Michael Eisen wrote in a blog post about the research.
"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," Ingram said. "Toxoplasma has done a phenomenal job of figuring out mammalian brains in order to enhance its transmission through a complicated life cycle."
The team said they are currently looking at how the mouse immune system responds to the parasite to determine if the response or the infection is causing the behavioral change.
About one-third of people around the world are thought to be infected with Toxoplasma and have dormant cysts in their brains being kept in check by their body's immune system. These cysts may spring to life in immune-compromised people, resulting in death. Some preliminary studies indicate that chronic infection may be associated with schizophrenia or attempted suicide.
Because the parasite can be passed via cat feces and infect a developing fetus, pregnant women are warned to avoid kitty litter. Another source of spread is undercooked pork, Ingram noted.