By Jon Bardin Los Angeles Times
LOS ANGELES — A wily parasite well-known for influencing the behavior of its animal hosts appears to play a troubling role in humans, increasing the risk of suicide among women who are infected, new research shows.
Researchers estimate that Toxoplasma gondii is carried by 10 to 20 percent of Americans, who can get it by changing litter used by infected cats or eating undercooked meat from an animal carrying the bug, with undercooked meat believed to be the most common source.

Despite its prevalence in humans, the protozoan is most famous for the strange effect it has on the brains of rats and mice.
The parasite’s optimal host is the cat — it can fully complete its reproductive cycle only in the feline intestinal tract. So T. gondii has developed an as yet unexplained, mechanism for ensuring survival: It turns rodents into willing cat food.
When a rat or a mouse is infected, it becomes attracted to cats, studies have shown. But studies in humans have suggested that rats and mice are not the only animals to undergo worrying behavioral changes in response to T. gondii infection.

The parasite has been linked to an increased risk of schizophrenia and bipolar disorder in humans. A few small studies have also suggested a relationship between suicide attempts and infection with T. gondii.
A new study seems to confirm the link by examining infection rates and suicide attempts in women in Denmark.
The study, published Monday in Archives of General Psychiatry, takes advantage of Denmark’s relentless tracking of its population’s medical records. Those records allowed researchers to analyze T. gondii levels and the incidence of suicide attempts in more than 45,000 women who were tracked for more than 10 years.

Over that period, 1 percent of the women tried to take their own lives. But the risk wasn’t the same for everyone. Women with T. gondii infections were 53 percent more likely to attempt suicide than the others.
Moreover, the researchers found a dose-response relationship, with the women carrying the highest levels of T. gondii in their bloodstreams having a 90 percent increased rate of attempted suicides compared with women who were not infected.

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