Since us humans are such good pals with cats, a high proportion of the population (especially in cat-loving cultures) are infected with T. gondii. Olha Tsiplya/Shutterstock

Your cat is friends with, or perhaps unwittingly an accomplice of, a very strange parasite that’s long been accused of having a peculiar effect on human behavior, known as Toxoplasma gondii. A number of previous studies have linked the parasite to everything from risky behavior to severe psychiatric problems, while other research has found that this "mind-controlling" ability is perhaps a little over-stated.

A new study – the largest study of its kind to date – has weighed in on the debate and concluded that the notorious parasite has ties to schizophrenia, although the nature of the link is unknown.

As reported in the journal Brain, Behavior, and Immunity, researchers looked at blood samples from over 11,500 participants who took part in the Danish Blood Donor Study and searched for specific antibodies against T. gondii and cytomegalovirus (CMV), a virus that infects the majority of humans. Signs of the parasite were detected in 25.9 percent of the blood samples. They also noted that evidence of the parasite was found significantly more often in the blood of people who had been diagnosed with schizophrenia.

The study authors concluded: “We found that individuals with a T. gondii infection had increased odds of being diagnosed with schizophrenia disorders compared to those without infection,” adding that the “level of association exceeds both the genetic and most other environmental risk factors for schizophrenia to date, supporting the close relation between T. gondii infection and schizophrenia.”

Image of three tachyzoites, a developmental stage in the Toxoplasma gondii parasite life cycle. Leandro Lemgruber/University of Glasgow/Wellcome Collection (CC)

Transmitted via cat poop, T. gondii is also known for its ability to affect the behavior of mice, although it can affect the brains of most warm-blooded animals. Mice usually stay well away from the smell of cat pee, understandably so, because it's a clear indication they’re in the territory of their top predator. However, mice infected with the parasite appear to actively seek out cat pee. In turn, the mouse acts as a vehicle for T. gondii to come into contact with a cat, one of the only known animal hosts in which the parasite can sexually reproduce. Fiendishly smart, eh?

Since us humans are such good pals with cats, a high proportion of the population (especially in cat-loving cultures) is also infected with T. gondii as well. It’s unclear how or why the parasite affects the human brain and our behavior, although some research suggests it could be due to changes in dopamine levels. It’s another common trope that T. gondii is linked to an increased risk of suicide and traffic accidents, perhaps because the parasite can spark risky behavior. However, this study didn’t find a strong association between these factors and the parasite.

Still, that point remains relatively speculative because this study did not look for an underlying mechanism that might explain the elusive link between T. gondii and mental health problems. Nevertheless, although the case is far from settled, this study adds to the growing pile of evidence that hints at some strange link between T. gondii and mental health.