A new study based on data of women screened for infection of Toxoplasma gondii -- the parasite primarily spread by cats -- by measuring the parasite's presence in their offspring revealed an additional danger posed by the freeloader, which is known to affect human behavior: The mothers infected with T. gondii were found to have a higher risk of attempting suicide than uninfected mothers, researchers said Monday.

Estimates for the rate of infection range from a little less than a quarter to a third of the U.S. population. Most people will feel no ill effects from the parasite, which lurks in brain and muscle cells, but pregnant women and people with weak immune systems do face risks.

In a paper published in the journal Archives of General Psychiatry, the researchers looked at medical records for nearly 46,000 women in Denmark who gave birth between May 1992 and January 1995 and had volunteered their children to be screened for T. gondii. Since babies don't produce antibodies to the parasite until they're three, the researchers could count a positive screen on the infant as evidence of T. gondii infection in the mother.

The team followed the women's medical progress up until 2006. They found that women infected with T. gondii were one and a half times more likely to attempt suicide compared to uninfected women, irrespective of a previous mental illness diagnosis.

"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," senior author and University of Maryland associate professor of psychiatry Teodor T. Postolache said in a statement on Monday.

Still, the small number of fatalities from suicide -- 10 in uninfected women and 8 in T. gondii-positive mothers -- is too small to pass statistical rigor, and merits further study, the researchers said.

One of the strengths of the study is that, thanks to the extensive medical records maintained in Denmark, the researchers could adjust the data for prior history of mental illness in the subjects and also their parents.

However, the study isn't perfect: what's missing are childless women, and men, and any suicide attempts  that went unrecorded.

T. gondii infection is likely not a random event and it is conceivable that the results could be alternatively explained by people with psychiatric disturbances having a higher risk of becoming T. gondii infected prior to contact with the health system, Postolache said.

Previous studies have shown a link between T. gondii and behavioral changes in humans, including schizophrenia.

A 2007 study from a Czech researcher published in the Schizophrenia Bulletin examined how adults infected with the parasite performed on personality questionnaires or on behavioral tests as compared to uninfected people.

"Men infected with the parasite scored lower on 'superego strength' - rule consciousness and behaviors that oppose the self-gratification of the id - and higher on vigilance. In short, they were more jealous and more likely to disregard rules," study author Jaroslav Flegr wrote.

Infected women, on the other hand, were warmer and scored higher on superego strength than uninfected women. Both infected men and infected women also scored higher on apprehension metrics.

In 2009, researchers from the University of Leeds described a possible method by which T. gondii could trigger schizophrenia and other mental disorders by affecting how the brain produces dopamine. When the parasite infects the brain, it forms a cyst and produces an enzyme called tyrosine hydroxylase, a building block of dopamine, the authors said in the journal PLoS ONE.

"It's highly unlikely that we will find one definitive trigger for schizophrenia as there are many factors involved, but our studies will provide a clue to how toxoplasmosis infection - which is more common than you might think - can impact on the development of the condition in some individuals," lead researcher Glenn McConkey said in 2009.

SOURCE: Pedersen et al. Toxoplasma gondii Infection and Self-directed Violence in Mothers. Arch Gen Psych, published online 2 July 2012.