Women infected with Toxoplasma gondii, a very common parasite, have an increased risk of attempting suicide, according to a new study published online in Archives of General Psychiatry. More than 60 million people in the United States may be infected with this parasite, which causes a disease called toxoplasmosis.
Most people infected with T. gondii never know it. But for children born to infected moms or people with weakened immune systems, it sometimes causes severe problems, including brain damage.
Dozens of studies have already linked the infection to a higher risk for certain mental disorders, particularly schizophrenia. But the new research is the strongest evidence to date of a tie to suicide.
The study, based at Aarhus University in Denmark, included data from more than 45,000 Danish women. These women had given birth between 1992 and 1995, and their babies were screened at the time for antibodies to T. gondii. When newborns have these antibodies, it’s a sign that their mothers had the infection and passed it along during pregnancy.
The researchers scoured Danish health registries to find out if any of the women went on to attempt suicide over the next 11 to 14 years. They found that women infected with T. gondii had a 50 percent higher risk of attempted suicide, compared to uninfected women. (There were too few completed suicides to statistically analyze the risk of dying that way.)
This type of study just looks for associations. It can’t prove cause and effect. So it’s not certain that the infection caused some women to harm themselves, but there’s reason to suspect that it could.
T. gondii infects many animals, but it thrives best in guts of cats. Humans may swallow the parasite after accidental contact with cat feces; for example, when cleaning a cat litter box or gardening in soil frequented by cats. Several writers have suggested that the infection might explain the popular stereotype of “crazy cat ladies.”
But cat lovers—and women in general—aren’t the only ones at risk. People may also get the infection by eating contaminated foods, such as unwashed fruits or vegetables or undercooked meats (especially pork, lamb, or venison).
Once inside a person’s intestine, the parasite can migrate to other parts of the body, particularly the brain, eyes, and muscles. There, it hides out in cysts, which usually contain the infection in a harmless, inactive state. But if the immune system is weakened by disease or certain medications, the infection can be reactivated, even years later. When the brain is affected, this sometimes leads to severe encephalitis—inflammation of the brain.
An infected mom can also pass the parasite along to her baby in the womb. Most infected newborns show no signs of disease at birth, but some develop problems—such as mental disability, eye infection, or hearing loss—later in life.
The exact effects of T. gondii inside the brain are still under study. However, the authors of the new study have suggested that it might trigger an exaggerated immune response in some people. Certain immune system chemicals that promote inflammation have previously been associated with suicidal behavior and depression.
There may also be more direct effects on the brain. Schizophrenia researcher E. Fuller Torrey and his colleagues have noted that T. gondii affects both neurons—cells that send, receive, and process information—and glia—cells that nourish and support neurons. It may also affect dopamine and other neurotransmitters—chemicals that allow neurons to communicate with one another.
The Centers for Disease Control and Prevention say good hygiene and food safety can reduce the chance of becoming infected with T. gondii. For cat owners, these steps include changing the litter box daily and feeding cats only commercial food or well-cooked table food. Pregnant women and those with weakened immune systems should have someone else change the litter, if possible.
Everyone should peel or wash fruits and veggies before eating them and cook meat to a safe temperature. It’s also important to wash cutting boards, dishes, utensils, counters, and hands after contact with unwashed produce or raw meat. These are precautions we should all be taking anyway—and now there’s one more good reason.
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