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Women who are infected with the common cat parasite Toxoplasma gondii may be more vulnerable to suicide, a new study finds, adding to the evidence that T. gondii or Toxo, as the bug is known, may cause subtle changes in the human brain that lead to personality changes and even mental illness.

The parasite is excreted in cat feces — which is why pregnant women are advised not to change the litter box — but it also spreads through undercooked meat and unwashed vegetables. Pregnant women who become infected with T. gondii can pass it onto their fetus, possibly causing brain damage or stillbirth. Now the new study finds that expectant mothers who have the infection, called toxoplasmosis, may themselves be at higher risk of suicide.

The finding comes from a study of 45,788 Danish women who gave birth between May 15, 1992, and January 15, 1995. University of Maryland School of Medicine researchers tested the women’s babies for T. gondii antibodies, which the infants could only have acquired from their mothers, and compared infection rates to the women’s suicide rates logged in the Danish health registry. The team also cross-checked the mental health registry to find out if any of the women had been previously diagnosed with mental illness.

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They found that women who were infected with T. gondii were one-and-a-half times more likely to attempt suicide than uninfected women. The higher the levels of T. gondii antibodies found, the higher the suicide risk. They were also more likely to try to commit suicide violently, with a gun, sharp object or by jumping. When the researchers took into account women’s previous mental illness, they found that those who had toxoplasmosis were more likely to attempt suicide than those who had been mentally ill.

“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,” lead study author Dr. Teodor T. Postolache, an associate professor of psychiatry and director of the Mood and Anxiety Program at the University of Maryland School of Medicine, said in a statement.

The findings fall in line with previous studies on T. gondii infection in humans. (In animals also, the parasite has been shown to subvert brain chemistry and manipulate behavior, sometimes dangerously.) A Czech scientist, Jaroslav Flegr, has studied T. gondii‘s effect on human personality and mental illness for decades, as detailed in a lengthy article in The Atlantic in March. The bug resides in about one-third of the world’s population (in the U.S., 10% to 20% are infected), but it usually doesn’t cause any noticeable effects — healthy people fight off the flu-like symptoms of an initial infection, after which the parasite lies dormant in the brain. “[O]r at least that’s the standard medical wisdom,” wrote Kathleen McAuliffe in The Atlantic:

If Flegr is right, the “latent” parasite may be quietly tweaking the connections between our neurons, changing our response to frightening situations, our trust in others, how outgoing we are, and even our preference for certain scents. And that’s not all. He also believes that the organism contributes to car crashes, suicides, and mental disorders such as schizophrenia. When you add up all the different ways it can harm us, says Flegr, “Toxoplasma might even kill as many people as malaria, or at least a million people a year.”

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Still, Flegr acknowledged that the effects of the parasite on personality were “very subtle” and that the “vast majority” of people wouldn’t even know they were infected. As for whether T. gondii infection could be used to predict self-harm or the odds of a car crash, Stanford neuroscientist Robert Sapolsky told McAuliffe: “[I]’m not too worried, in that the effects on humans are not gigantic. If you want to reduce serious car accidents, and you had to choose between curing people of Toxo infections versus getting people not to drive drunk or while texting, go for the latter in terms of impact.”

In the new study, researchers couldn’t establish that T. gondii infection caused increased risk of suicide, only that it was associated. And they’re not sure exactly why the link exists. “Is the suicide attempt a direct effect of the parasite on the function of the brain or an exaggerated immune response induced by the parasite affecting the brain? We do not know. In fact, we have not excluded reverse causality as there might be risk factors for suicidal behavior that also make people more susceptible to infection with T. gondii,” said Postolache in the statement.

The authors call for further studies focusing on the biological mechanisms of the parasite and how it may affect people’s suicide risk and other personality factors. If the findings hold up, perhaps T. gondii infection could be used to help prevent some of the 10 million suicide attempts that occur each year. “If we can identify a causal relationship, we may be able to predict those at increased risk for attempting suicide and find ways to intervene and offer treatment,” Postolache said.

In the meantime, people should cook their meat through, wash their vegetables and give their kitchen knives a good scrubbing to avoid spreading or contracting the parasite.

The study was published online in the Archives of General Psychiatry.

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