The infection, known as toxoplasmosis, is caused by the parasite Toxoplasma gondii. Humans can become chronically infected by eating undercooked meat or unwashed vegetables or by handling cat litter, as the parasite is known to multiply in the gut of infected cats.

Toxoplasmosis is often symptom-free, but can be dangerous in people with weak immune systems or during pregnancy, since the parasite may be passed to babies.

Some studies have linked the parasite to a higher chance of developing schizophrenia, and researchers believe because the T. gondii parasite lives in the brain, it could have an effect on emotions and behavior.

For the new report, Dr. Teodor Postolache from the University of Maryland School of Medicine in Baltimore and his colleagues used Danish medical registries to track 45,788 women who were originally included in a study that screened newborn babies for toxoplasmosis.

All of the infants were tested for antibodies against the parasite through a blood sample drawn five to 10 days after birth. Because the babies were still too young to make their own antibodies, any that showed up in their blood would have been passed from moms.

Just over one-quarter of the babies were positive for T. gondii antibodies, meaning their mothers likely had a chronic, underlying toxoplasmosis infection.

And over the next 11 to 14 years, infected women were about 50 percent more likely to cut, burn or otherwise hurt themselves, according to their medical records, and 80 percent more likely to attempt suicide.

In total, 488 women hurt themselves for the first time during the study - or eight out of every 10,000 annually - and 78 tried to kill themselves.

"That's not a very high risk, when you come down to it," said Dr. Louis Weiss, who studies toxoplasmosis at Albert Einstein College of Medicine in New York.

Still, he told Reuters Health the findings are "really quite interesting." Part of the study's strength, he added, comes from its size and how long it followed the Danish women.

"There probably is an effect of this parasite on human behavior, which has been suspected," based on studies of animals infected with toxoplasmosis, said Weiss, who wasn't involved in the new research.

CAUSES NOT CLEAR

Eighteen women in the study committed suicide, which was too small a number for the researchers to determine whether T. gondii put some women at higher risk, according to their findings published this week in the Archives of General Psychiatry.

Postolache and his colleagues note that some instances of self-harm might not have shown up in their records if women weren't seen at a mental health clinic after the incident.

Based on the study, they also can't say for sure whether toxoplasmosis infection caused women to hurt themselves or attempt suicide. It could be, for example, that women with underlying mental problems were more likely to pick up the parasite because they cooked their meat or washed their vegetables improperly.

But it's reasonable, Postolache added, that the parasite could directly affect the brain, such as by making cells produce more or less of certain neurotransmitters that control mood and behavior.

"It's (also) possible that the immune system containing Toxoplasma gondii does it at the cost of affecting brain function," he told Reuters Health.

Underlying infection could trigger inflammation, Weiss explained, which might subtly alter brain chemistry.

Postolache said more studies will be needed to further understand the link between toxoplasmosis and suicidal tendencies, including whether certain people with the infection are more prone to mood and behavior issues than others - because of the genetic factors, for example.

Both researchers emphasized that pregnant women should not avoid or get rid of their housecats based on the findings. Most of the parasites that cause infection, Weiss said, are passed by feral cats and end up in the environment.

"This is not a reason to be fearful of pussy cats," he said.

SOURCE: bit.ly/NsuK4O Archives of General Psychiatry, online July 2, 2012.

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