A parasite commonly found in cat litter has been found to permanently alter the brains of mice, making them perpetually fearless of their natural predators, cats.
A new study published today in PLOS ONE Journal examined the behavior of mice after being infected with the toxoplasma gondii (T. gondii) parasite. The study found mice were significantly less afraid of the scent of a predator even when there was no sign of infection.
Although it's bad news for the mouse, for the parasite it means getting into the cat's digestive system, which is the only place it can reproduce.
The T. gondii parasite can be found in a number of mammal hosts and, according to the U.S. Centers for Disease Control, the parasite has been contracted by 60 million Americans. However, the majority of people infected will never exhibit any symptoms of a toxoplasmosis infection.
The parasite is especially dangerous for pregnant women, because it can cause spontaneous abortion, and for people with compromised immune systems.
The new study was led by graduate student Wendy Ingram at the University of California, Berkeley. Ingram tested the effects of all three strains of toxoplasma gondii by placing infected mice in a dark box with a petri dish of bobcat urine. Mice that had been infected with the parasite would fearlessly wander throughout the area, while those that were uninfected cowered at one of end of the box.
Surprisingly, even after there was no detectable sign of T. gondii or any infection, the previously infected mice still didn't appear to mind the smell of a predator in their immediate area.
"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.
Ingram said it is possible the parasite could directly alter neurons involved in memory or learning, or trigger or damage the smell center of the brain. She is particularly interested in the possibility that the effect could be related to a host response, similar to an auto-immune response in a human.
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"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."
Toxoplasmosis is considered one of the "neglected parasitic infections," a group of five parasitic diseases that have been targeted by CDC for public health action.
When humans ingest the parasite, the organism spreads from the intestine to the muscles and the brain. About one-third of the world is exposed to T. gondi, although few exhibit any symptoms of toxoplasmosis.
Although her research was specific to mice, Ingram said that, going forward, research should be done to see what other effects the parasite could have on humans.
"They seem like normal mice. It's a really subtle effect," said Ingram of the parasite's impact on mice. "Humans should be studied, and there are things we could look for."
Recent research has studied what long-term effects the parasite can have on the brain of its human hosts.
A 2012 study of 45,000 women in Denmark found that women infected with T. gondii were one and a half times more likely to attempt suicide than those who were not infected. As the level of antibodies in the blood rose, so did the suicide risk.
There was also a slightly higher risk for violent suicide death.
A 2011 study in the The American Journal of Psychiatry found those exposed to T. gondii had a 24 percent greater risk of developing schizophrenia.