University of Pennsylvania researcher Greg Smith calculated that around a fifth of schizophrenia cases may be attributable to T. gondii infection.
The Centers for Disease Control and Prevention (CDC) estimate that around 60 million people in the US may be infected with T. gondii. Infection most commonly occurs through eating undercooked, contaminated meat, drinking contaminated water and coming into contact with cat feces that contain T. gondii.
Most people with T. gondii infection are unaware they have it; people with healthy immune systems are usually able to stop the parasite causing illness. But for those with weaker immune systems, such as older people, pregnant women and those with immune system disorders, the parasite can cause toxoplasmosis.
Toxoplasmosis a disease characterized by flu-like symptoms, including swollen lymph glands and muscle aches and pains. In severe cases, toxoplasmosis can cause damage to the eyes, brain and other organs.
Some studies, however, have linked T. gondii infection to mental health conditions. In 2012, for example, Medical News Today reported on a study linking T. gondi to increased risk of self-harm or suicide among new mothers.
More recently, studies have linked T. gondii infection to schizophrenia, and some have found that antipsychotic medication may even stop the parasite from replicating. But such research has been met with much criticism.
In this latest study, Gary Smith, of the School of Veterinary Medicine at the University of Pennsylvania, wanted to gain a better understanding of the link between T. gondii infection and schizophrenia.
Link between T. gondii and schizophrenia 'should be considered, not ridiculed'
Smith wanted to determine the proportion of schizophrenia cases that could be attributable to T.gondii infection. He did this by calculating the population attributable fraction (PAF) - a measure used by epidemiologists to understand the importance of a risk factor.
"In other words," explains Smith, "we ask, if you could stop infections with this parasite, how many [schizophrenia] cases could you prevent?"
Smith calculated the PAF fraction throughout an average lifetime to be 21.4%, meaning that a fifth of all schizophrenia cases over a lifetime could be prevented by stopping T. gondii infections from occurring. "That, to me, is significant," says Smith.
He notes that many countries have a much higher prevalence of T. gondii infections than the US, and such countries also have a higher prevalence of schizophrenia.
Schizophrenia is one of the leading causes of disability in the US, affecting more than 3.5 million people.
Smith believes that his findings indicate the importance of gaining a better understanding of the link between T. gondii infection and schizophrenia. He adds:
"By finding out how important a factor T. gondii infection is, this work might inform our attitude to researching the subject.
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."