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It is widely accepted that people with a family history of schizophrenia are more susceptible to the mental health condition.

But the theory that people can develop it after being infected with Toxoplasma gondi – a parasite transmitted by soil, undercooked meat and cat faeces, is controversial.

Now, a new study suggests that one fifth of cases of schizophrenia may involve the parasite.

A new study suggests that a fifth of cases of schizophrenia may involve the parasite Toxoplasma gondi, which is transmitted by soil, undercooked meat and cat faeces. A tissue smear containing the parasite is shown

Gary Smith, professor of population biology and epidemiology at the University of Pennsylvania's School of Veterinary Medicine is behind the research.

'Infection with Toxoplasma is very common, so, even if only a small percentage of people suffer adverse consequences, we could be talking about problems that affect thousands and thousands of people,' he said.

Just over one third of the US population is infected with T. gondii and the vast majority of people aren't aware of it, according to the study published in the journal Preventive Veterinary Medicine.

The parasite can be especially dangerous to certain groups of people. For example, if a woman becomes infected for the first time during pregnancy, her foetus can die or suffer serious developmental problems.

The theory that Toxoplasma gondi causes schizophrenia (illustrated with stock image) isn't new, but now the health risk has been quantified

People with HIV or other diseases that weaken the immune system are susceptible to a complication of T. gondii infection called toxoplasmic encephalitis, which can be deadly.

While it has long been believed that most healthy people suffer no adverse effects from a T. gondii infection, recent studies have found evidence of worrying impacts, including an association with schizophrenia, because the parasite is found in in the brain as well as in muscles.

Some antipsychotic drugs can stop the parasite from reproducing, but studies in mice, rats and people have also shown that infection with T. gondii triggers changes in behaviour and personality.

To investigate this connection, Professor Smith sought to calculate the population attributable fraction (Paf) which experts use to determine how important a risk factor might be.

In this case, the Paf is 'the proportion of schizophrenia diagnoses that would not occur in a population if T. gondii infections were not present,' he explained.

By taking into account the fact that the proportion of people that are infected with T. gondii increases with age, he worked out the average Paf during the average lifetime to be 21.4 per cent.

'In other words, we ask, if you could stop infections with this parasite, how many cases could you prevent?" Professor Smith said.

'Over a lifetime, we found that you could prevent one-fifth of all cases. That, to me, is significant.'

He noted that in some countries, the prevalence of T. gondii infection is much higher than in the US and these countries also have a higher incidence of schizophrenia.

'By finding out how important a factor T. gondii infection is, this work might inform our attitude to researching the subject,' Professor Smith said.

'Instead of ridiculing the idea of a connection between T. gondii and schizophrenia because it seems so extraordinary, we can sit down and consider the evidence. Perhaps then we might be persuaded to look for more ways to reduce the number of people infected with Toxoplasma.'

'Infection with Toxoplasma is very common, so, even if only a small percentage of people suffer adverse consequences, we could be talking about problems that affect thousands and thousands of people.'