July 2, 2012 -- A common parasite that can lurk in the cat litter box may cause undetected brain changes in women that make them more prone to suicide, according to an international study.

Scientists have long known that pregnant women infected with the toxoplasma gondii parasite -- spread through cat feces, undercooked meat or unwashed vegetables -- could risk still birth or brain damage if transmitted to an unborn infant.

But a new study of more than 45,000 women in Denmark shows changes in their own brains after being infected by the common parasite.

The study, authored by University of Maryland School of Medicine psychiatrist and suicide neuroimmunology expert Dr. Teodor T. Postolache, was published online today in the Archives of General Psychiatry.

The study 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. The relative risk was even higher for violent suicide attempts.

"We can't say with certainty that T. gondii caused the women to try to kill themselves, but we did find a predictive association between the infection and suicide attempts later in life that warrants additional studies," said Postolache, who is director of the university's Mood and Anxiety Program and is a senior consultant on suicide prevention.

"There is still a lot we don't know," he told ABCNews.com. "We need a larger cohort and need a better understanding of the vulnerabilities that certain people have to the parasite."

Suicide is a global public health problem. An estimated 10 million attempt suicide and 1 million are successful, according to Postlache's work.

More than 60 million men, women, and children in the United States carry the toxoplasma parasite, according to the Centers for Disease Control and Prevention, but very few have symptoms.

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.

About one-third of the world is exposed to T. gondii, and most never experience symptoms and therefore don't know they have been infected. When humans ingest the parasite, the organism spreads from the intestine to the muscles and the brain.

Previous research on rodents shows that the parasite can reside in multiple brain structures, including the amygdala and the prefrontal cortex, which are responsible for emotional and behavioral regulation.

Rat Study Showed Parasite Changes the Brain

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."