Cats: whether you think they are trusted, furry, independent companions capable of a little mischief or you consider them to be more akin to murdering psychopaths, there’s no denying that they are a ubiquitous feature of modern society. We all know someone that owns one, or several. In a potential blow to cat hoarders, new research on a parasite commonly carried by cats – Toxoplasma gondii – suggests that it can be linked to mental illness in owners, particularly schizophrenia.

T. gondii is, as parasites tend to be, fairly deceptive. It has been shown to alter the behavior of mice, causing them to become "suicidal." Mice have acute olfactory glands, meaning that they have a keen sense of smell. Whenever they can smell cat urine, they tend to avoid it, wary of the nearby threat. If infected with T. gondii, however, they are not averse to it; in many cases, they actively wander towards the smell. This, as you might expect, leaves them exposed to cat predation. After a quick pounce and kill, the mouse – along with its piggybacking parasites – are ingested by the cat.

Cats are only the hosts wherein the parasite is able to sexually reproduce. After the parasite produces new eggs within the cat, they are expelled as it defecates. These eggs are able to last for many months in a variety of dry and temperate climates, ready to infect another warm-blooded host.

This can, unfortunately, include humans. Although the parasite cannot reproduce within a human host, it can cause toxoplasmosis. Apart from being acutely dangerous to those infected during pregnancy or those with a weakened immune system, there has been little evidence indicating a negative effect on healthy, infected adult humans. This is despite the fact that in the U.S. alone over 60 million people may be infected, according to the Centers for Disease Control and Prevention (CDC).

This study, published in the journal Schizophrenia Research, examines whether the ownership of a cat during childhood is more common in families with members who went on to develop mental health disorders later on in life. The researchers state that two earlier studies came to this conclusion and, using an extensive survey, attempted to replicate the finding. They were successful, reinforcing the link between early cat ownership and later-life schizophrenia and other serious mental illnesses.

It is important to note that this study merely highlights an association, rather than saying T. gondii is responsible. This is strong circumstantial evidence, not definitive proof of cause, and no studies have yet looked into potential underlying mechanisms. However, a second study published in the journal Acta Psychiatrica Scandinavica actually looked at this relationship more directly. After scrutinizing 50 peer-reviewed studies on the subject, the researchers here showed that an individual infected with T. gondii was almost twice as likely to develop schizophrenia in later life.

Antipsychotic drugs, commonly used to treat patients with schizophrenia, also tend to work in negating the effects of toxoplasmosis in both rats and humans, implying that there is indeed a link between the parasite and mental health, but at present, it appears to remain a distinctly uncertain one.

[H/T: Time Magazine]