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Cat lovers, take note: a parasite that lurks in cat faeces could be linked with schizophrenia. Toxoplasma gondii (often called toxo) is estimated to infect around 30 per cent of the population. While it can trigger miscarriage, in most people it was thought to cause little more than a headache and sore throat.

Over the past few years, however, evidence has emerged that toxo can have some rather sinister effects on our behaviour: some people infected with the parasite have odd symptoms, such as hallucinations and a tendency to take more risks.

This isn’t as crazy as it sounds. In rodents, toxo’s natural hosts, the parasite gets into the brain and makes the animals less fearful, resulting in them taking more risks. This makes them more likely to be eaten by cats, thus completing the parasite’s life cycle. Toxo cysts have been found in the brains of people, too.

In a 2007 meta-analysis of 42 studies, psychiatrist Fuller Torrey at the Stanley Medical Research Institute in Chevy Chase, Maryland, found that people with schizophrenia are nearly three times as likely to have antibodies to toxo compared with those who don’t have the condition. Torrey also found that people taking schizophrenia medication have lower levels of antibodies than those not taking such drugs. He suggests that the drugs may reduce symptoms in part by harming the parasite. They are known to do so in the test-tube.

Torrey and others suspect that toxo affects the brain by somehow raising levels of the chemical signalling molecule dopamine. Excess dopamine has long been linked to schizophrenia, and it is also thought to increase risky behaviour.

Earlier this year, a team led by geneticist Glenn McConkey at the University of Leeds in the UK discovered what may prove to be the smoking gun. The group found that the parasite has two genes that encode an enzyme that makes dopamine, which is normally only present in animals that have a nervous system ().

While it may have other functions, the parasite could be manufacturing dopamine to directly manipulate its host’s nervous system, the team say. “This is a very important breakthrough,” says Joanne Webster, an epidemiologist at Imperial College London, who has long studied toxo.

“The toxo parasite could be making dopamine to directly manipulate its host’s nervous system”

Various groups are now trying to develop new schizophrenia drugs that work by eradicating the parasite. Existing medicines are of limited help against schizophrenia and can have nasty side effects, such as weight gain and facial spasms.

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Read more: Six diseases you never knew you could catch