While the smell of cat urine is normally a turn-off for rats, a group of Stanford University researchers found a certain group of rats was actually attracted to that same odor.
Cat urine is naturally a warning to rats to stay away from an area where their natural predators may be lurking. But study rats infected by the parasite Toxoplasma gondii ("toxo" for short) didn't show that fear and, instead, parts of their brains associated with sexual arousal were activated.
"Normally, we would expect toxoplasma to knock out the normal fear function in the brain, but in these rats the parasite also tapped into the sexual arousal pathway, which is strange," said Robert Sapolsky, a professor of neuroscience at Stanford University and a co-author of the study.
The belief is that the mind-manipulating parasite acts that way in order to ensure its reproduction. Toxo can only reproduce in the guts of cats, so if an infected rat wanders into cat territory, then there's a possibility the cat will eat the rat and toxo can multiply.
"The parasite does actually alter the brain of its host," said Patrick House, a doctoral student who is also a co-author of the study. "The fact that a parasite can get into an organism, target its brain, stay there without killing the host and alter the circuitry of the brain -- we've seen this is insects and fungi, but it's the first time we've seen it in a mammalian host."
Toxoplasma affects a rat's amygdala, the part of the brain associated with fear and anxiety.
"It atrophies some of the neurons along the pathway associated with fear," said Sapolsky. "What we don't understand is how it affects the fear response and then accesses the sexual arousal circuitry."
Toxo does infect humans. Humans contract the parasite by consuming contaminated food or water or by coming into contact with cat feces.
"It doesn't make people sick at all. It just infects them and the body holds it off, and it becomes a latent infection," said Dr. William Schaffner, chair of the Department of Preventive Medicine at Vanderbilt University Medical Center.
But occasionally, Schaffner added, people who are immunocompromised can become ill. Pregnant women who are infected can also pass toxo along in the womb, which can cause serious complications, including still birth and neurological problems.
Previous research has linked to the parasite to schizophrenia and depression, but little is known about how it causes changes in human behavior. Some experts, including Schaffner, are skeptical that toxoplasmosis has a link to mental illness at all.
But while Sapolsky believes there could be an association between human behavior and infection with toxoplasma, that relationship needs additional study before making any firm conclusions.
His future research, he said, will once again focus on rats. He hopes to learn more about how this mysterious parasite affects rats and whether it plays tricks on the human mind, as well.
"One of the more interesting questions," he said, "is: How many cases are there of parasites manipulating human behavior?"