A 2011 study on rats infected by the parasite showed that their fear of cats disappeared. Instead, the parts of their brains associated with sexual arousal were activated. Researchers theorized that the mind-manipulating T. gondii ensures that the parasite will reach and reproduce in the gut of a cat, which it depends upon for its survival.
"The parasite does actually alter the brain of its host," Stanford University study co-author Patrick House told ABCNews.com last year. "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."
It was this and other research that led Postolache to investigate the relationship between the parasite and biological changes in the brain that might lead to suicide. He was also intrigued by studies on allergies and research that showed a connection between toxoplasmosis and schizophrenia.
"I was interested in the neuron aspects of suicide and intrigued by low-grade activation in patients who attempted suicide, as well as victims," he said. "Other studies had looked at the brain and suicide risk and impulsivity. The next question was, what could be the triggers that perpetuate this level of heightened activation in the brain?"
Postolache collaborated with Danish, German and Swedish researchers, using the Danish Cause of Death Register, which logs the causes of all deaths, including suicide. The Danish National Hospital Register was also a source of medical histories on those subjects.
They analyzed data from women who gave birth between 1992 and 1995 and whose babies were screened for T. gondii antibodies. It takes three months for antibodies to develop in babies, so when they were present, it meant their mothers had been infected.
The scientists then cross-checked the death registry to see if these women later killed themselves. They used psychiatric records to rule out women with histories of mental illness.
Postolache said there were limitations to the study and further research is needed, particularly with a larger subject group.
Dr. J. John Mann, a psychiatrist from Columbia University, said Postolache's research mirrors his work in the field of suicidal behavior.
"The relationship of the brain to the immune system is more complex than it may appear," said Mann. "The brain regulates the stress response system, which impacts the immune response."
Scientists already know that steroids like cortisone can affect the immune response. Some antibodies whose goal is to kill off cancer can also affect the brain. Oftentimes the first symptom of pancreatic cancer is depression, he said.
Research also shows that streptococcus bacteria can trigger obsessive-compulsive disorder (OCD) in some children. Sydenham's chorea, the loss of motor control that can occur after acute rheumatic fever, may also be an immune response affecting the brain, according to Mann.
Maryland researcher Postolache suspects that some individuals have a predisposition to these neurological changes.
He speculates that the parasite may disrupt neurological pathways in those who are vulnerable, so that projections of fear and depression from the amygdala are not tempered or controlled by the "braking" function of the prefrontal cortex.
But, Postolache warns that even if a direct cause were found, no antibiotics for T. gondii yet exist and it could be a decade before effective vaccines or other agents that might stop the neurological damage are developed.
Right now, the most effective weapon against T. gondii is education about handwashing, the proper cooking of food, and not using a knife exposed to raw meat on cooked meat.
He also cautions against trendy food production techniques that let animals roam free. "The risk of infection could go up," he said, "and increase the rate of toxoplasmosis."