A common parasite spread by cats that is carried by two billion people may lead to schizophrenia, researchers fear.

Toxoplasma gondii (T. gondii), which can spread to people through contact with cat litter trays and eating uncooked meat, is usually harmless.

But now, the largest study of its kind, has found having the parasite could raise the risk of developing schizophrenia by 50 per cent.

Unusual links between T. gondii and mind-altering behaviour, such as risk-taking, depression and car accidents, already exist.

A parasite, Toxoplasma gondii (T. gondii), spread by cats, 'may lead to schizophrenia', according to a large study by Danish researchers

Scientists at Copenhagen University analysed data from more than 80,000 individuals in the Danish Blood Donor Study, of which 2,591 people had registered psychiatric conditions.

The researchers, led by Dr Kristoffer Sølvsten Burgdorf, scoured traces of immunoglobulin antibodies for the presence of T. gondii.

Billions of people worldwide - nearly one third of the population, according to the University of Chicago - are believed to have been infected by the parasite, including 60million in the US and 350,000 a year in the UK.

The parasite was detected in the blood of a quarter of the population studied by the team in Copenhagen.

They also found that 61 per cent of people carried cytomegalovirus (CMV), which has also shown similar evidence of cognitive impairment.

Patients infected with T. gondii were almost 50 per cent more likely to have been diagnosed with schizophrenia than those without, results showed.

The association was even stronger when accounting for 'temporality' - patients who had not already been diagnosed with schizophrenia.

According to the researchers, this 'corroborates that Toxoplasma has a positive effect on the rate of schizophrenia'.

WHAT IS TOXOPLASMA GONDII? Toxoplasma gondii is a parasite which causes an infection called toxoplasmosis. The parasite is common worldwide and 60 million people in the US are thought to be infected by it. Few people who are infected show any symptoms because a healthy immune system can fight it off, but it can cause flu-like effects. However, it can cause more serious complications in pregnant women and people with weak immune systems. Toxoplasmosis can be caught from pet cats or their litter trays, eating undercooked contaminated meat, or drinking water contaminated by the parasite. Most people's infections go away without treatment but medication is available for those who are more at risk of serious complications. Source: Centers for Disease Control and Prevention Advertisement

Dr Burgdorf and colleagues added that 'T. gondii infection might be a contributing causal factor for schizophrenia'.

T. gondii was not associated with any other psychiatric disorder, according to the study published in Brain, Behavior, and Immunity.

However, CMV was, including risk of neurotic and stress-related disorder, somatoform disorder, a form of mental illness that manifests as physical pain, and attempting or committing suicide.

Explaining the link, the authors wrote that the parasite may disrupt the action of an amino acid in the body called tryptophan.

This leads to the secretion of large amounts of metabolites such as kynurenic acid, previously found to be high in people with schizophrenia.

'Tryptophan is also the essential precursor of serotonin, which is involved in depressive disorders,' the authors wrote.

The researchers said their study did not control for socio-economic factors which can influence the probability of having a psychiatric disorder.

But their 'statistically significant' findings add to a growing body of evidence that the parasite has strange cognitive effects.

In humans, T. gondii can lead to toxoplasmosis, which causes flu-like symptoms which can go largely unrecognised, according to the NHS.

Pregnant women who ingest the parasite in early pregnancy can suffer a miscarriage or stillbirth, according to studies.