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.

Chances are you or someone you know has been infiltrated by the parasite, called Toxoplasma gondii. Researchers estimate that T. gondii is carried by 10 percent 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.

Despite its prevalence in humans, the protozoan is most famous for the strange effect it has on the brains of rats and mice.

LURING RATS TO THE CATS

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 ingenious, and as yet unexplained, mechanism for ensuring survival: It turns rodents into willing cat food.

When a rat or a mouse is infected, it suddenly flips from being petrified of cats to being attracted to them. Studies have shown that the cells in the rodent brain that regulate sexual arousal become active when mice and rats get a whiff of cat urine, suggesting the smell turns them on. As a result, they drop their guard, the cats eat them — and the parasite wins the day, reproducing at will.

But studies in humans have suggested that rats and mice aren’t the only animals to see 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 thousands of women in Denmark.

The study, published Monday in the 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 the period covered in the study, 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 women who were not infected.

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.

A SURPRISE IN THE RESULTS

While previous studies had looked at people who already had a history of mental disorders, the study of Danish women was able to control for that by using a large population with no such disposition.

“To our surprise, a history of mental illness did not appear to play a major role,” said Dr. Teodor Postolache of the University of Maryland School of Medicine in Baltimore, the study’s senior author.

Though the study focused on women, there’s no reason to believe the results would be any different in men, according to Dr. Robert Yolken, an infectious disease expert at Johns Hopkins University in Baltimore, who was not involved with the study.

“Other smaller studies have been carried out in both men and women, and they find the same thing,” he said.

Scientists still do not know how T. gondii prompts behavioral changes in the animals it infects, but they have some intriguing theories.

Experiments conducted in the laboratory of Glenn McConkey at the University of Leeds in England have demonstrated that when the parasite reaches the brain of a rodent, it produces a key chemical component of the neurotransmitter dopamine.

Dopamine is fundamental for regulation of mood, motivation and social behavior. Perhaps increased levels of dopamine play a central role in the behavior changes that follow infection, Yolken said.

Another possible explanation is that the parasite causes inflammation in the brain, which may have numerous effects on behavior, he added.

KEEP CAT, COOK MEAT

The authors of the new report emphasized that people should not react to the findings by evicting their cats.

Postolache said he didn’t think contact with cats was responsible for most cases of T. gondii infection in people. “The No. 1 source is probably undercooked meat,” he said.

Postolache, a psychiatrist, said cats may even help people who contemplate suicide.

“I have seen people at a high risk of suicide where a pet saves them. Pets are dependent, and they provide emotional support,” factors that tend to reduce suicide risk, he said.Dr. Teodor Postolache, senior author of the study, said he didn’t think contact with cats was responsible for most cases of T. gondii infection in people.