There is mounting evidence linking the toxoplasma parasite to changes in mood or personality even though the infectious agent is widely believed to be completely harmless in more than 80 per cent of infected people.

A number of studies published in recent years have suggested that toxoplasma infection increases the chances of someone developing serious psychological disturbances, such as schizophrenia and bipolar disorder.

Other scientists have shown that toxoplasma is linked with more subtle psychological changes such as increased reaction times, which could explain why infected people appear to be more likely to have traffic accidents.

None of these studies have proven a direct cause and effect. However, they have been sufficiently robust for some experts to now call for bigger studies to investigate whether toxoplasma could trigger changes to the chemistry of the human brain, which might affect a person’s behaviour.

“In terms of diseases, all we can say is that there is a correlation between antibodies to this infectious agent and serious mental illnesses, specifically schizophrenia. But correlations do not equal causation,” said Fuller Torrey, a psychiatrist and director of the Stanley Medical Research Centre in Maryland.

“I would put it in the same category as the [now proven] theory of bacteria causing stomach ulcers, where only a correlation was initially shown. But until you can show that treatment of the infectious agent causes people to significantly improve, we don’t have a believable cause and effect,” Dr Torrey said.

One of the latest studies into the subject, published in July, showed that women with toxoplasma antibodies – which indicate past exposure to the parasite – are significantly more likely than non-infected women to show suicidal or self-harm behaviour, with the risk increasing as levels of antibodies rise.

The study, based on nearly 46,000 Danish women who were tested for toxoplasma antibodies while pregnant between 1992 and 1995, also found that those who had been infected at some point in their lives were twice as likely as non-infected women to commit suicide.

Another study by American scientists, published in August, found that among 7,440 mental health patients there was a significant link between toxoplasma infection and a type of bipolar disorder where patients suffer symptoms of both manic and major depression.

However, the scientists failed to find a link with other types of depression, suggesting that if there was a causal effect, it was subtle and complex.

Smaller psychological studies carried out by Jaroslav Flegr at Charles University in Prague have found gender differences between men and women infected with toxoplasma. Men were more likely than non-infected men to be expedient, suspicious, jealous and dogmatic, whereas women were more warm-hearted, outgoing, easy-going, and more willing to follow rules compared with uninfected women.

Dr Flegr has also shown that infected people have slower reaction times compared to uninfected individuals using standard computerised tests. They are also more than twice as likely to be involved in traffic accidents, a finding that has been independently replicated by scientists in Turkey.

“If it is true, then latent toxoplasmosis is the second most important protozoan killer, just after the malaria,” Dr Flegr said.

As regards more serious psychiatric illnesses, Dr Torrey said that two separate reviews into toxoplasma and schizophrenia have both found that people infected with the parasite are about 2.7 times more likely to develop the illness than if the person had not been infected.

Again he emphasised that this type of correlation does not show that one is causing the other. However, he believes the hypothesis is becoming increasingly plausible given that other scientists have now established a potential mechanism for how the single-celled parasite could be affecting human brain chemistry.

“We know that the organism can produce dopamine, and we know that schizophrenia has the symptoms of excess dopamine, but we don’t know how that comes about,” Dr Torrey said.

The fact that Toxoplasma gondii, a single-celled protozoan microbe, has two genes for manufacturing L-dopa, the precursor molecule to dopamine, strongly suggests that this could be the biological mechanism that might explain how this feline parasite could be influencing the behaviour of some people who are infected, he said.

“It’s still a theory. We’ve shown conclusively that it changes the behaviour of other animals and the work from Czechoslovakia would suggest that it has an effect on human personality,” Dr Torrey said.