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Study: A common cat parasite could be making humans mentally ill

By Katherine Ellen Foley

Cats are cute and cuddly. Some hate Mondays and others make YouTube stars. Many of them also carry the parasite Toxoplasma gondii, which spreads to humans and, as research suggests, can cause serious harm.

According to the Centers for Disease Control and Prevention (CDC), as many as 60 million people in the US may have T. gondii, and as many as one-third of people worldwide. For most of them, it’s not really a problem.

For some people, though, the condition caused by the parasite, called toxoplasmosis, can have severe consequences. In pregnant women, it can lead to miscarriages or problems with fetal development. A growing body of research links the parasite with serious mental illness, such as schizophrenia and bipolar disorder. Though it reproduces in cat digestive systems, T. gondii is commonly spread through contaminated food and water, particularly raw or undercooked meat. The connection to cat ownership is at best unclear.

A recent study (paywall), however, suggests that the families of people with serious mental illness were more likely to have pet cats during the person’s childhood. Researchers from the Stanley Medical Research Institute analyzed more than 2,000 surveys conducted by the National Alliance for the Mentally Ill from 1982—before research had connected cat ownership to mental health problems later in life. They found that approximately 50% of families with children that developed serious mental illness had cats in their homes while the child was young, versus around 42% in the control group. Two similar surveys the researchers analyzed generated similar results.

The findings don’t connect toxoplasmosis itself to mental illness—just cat ownership. On that front, there is new research too. A recent meta-analysis of 50 studies (paywall) concluded that the parasite could be linked to a number of psychiatric conditions. Previous research also connects infections with changes in behavior in otherwise healthy individuals.

This research can’t determine whether T. gondii directly causes mental illness or behavioral changes. So far, scientists have gathered this information through survey data, often after someone has been diagnosed with a mental health condition. It may also be that people who develop mental illness later in life also happen to become infected with T. gondii. The CDC doesn’t think you need to give up your cats.

Given the number of people infected with the parasite and the risks involved, it is clear that we need more research to investigate the association.