Before you rid yourself of your typhoid tabby, however, bear in mind that, like all studies that retrospectively examine casual associations, there are a lot of caveats that need to accompany those claims.
The study itself has limitations. It's based on a 1982 questionnaire of 2,125 people who had a family member with schizophrenia, just over half of whom said that they owned a cat in childhood. According to the paper, 37 per cent of American families have a cat. (Other studies suggest that number could be as high as 50 per cent.)
Based on these data, you can draw a few different conclusions:
1) That cat ownership "significantly increases" your risk of schizophrenia, as the researchers conclude;
2) That cat owners are more likely to fill in questionnaires, especially if they have children with schizophrenia;
3) That there is absolutely no link between having a cat and developing schizophrenia.
The research team was led by E. Fuller Torrey, the enfant terrible of mental-health research. Over the years, he has done wonders for patients by challenging conventional wisdom, particularly with his landmark study showing people with severe mental illness are about 11 times more likely to be victims of violence than perpetrators of violence.
But, in this instance, what we seem to have is a researcher digging for data to back a theory.
The reason this study is even worth talking about though is that it's rooted in a fascinating notion – that childhood infection with a pathogen, toxoplasma gondii, triggers neurochemical changes that lead to schizophrenia later in life.
Cat feces is nasty stuff, principally because it often carries T. gondii. That's why people with suppressed immune systems – pregnant women, people with HIV-AIDS, patients undergoing cancer treatment and so on – are cautioned to not have cats or, at the very least, to steer well clear of the litter box.
T. gondii infection can cause hallucinations, psychosis and impaired cognitive abilities. In fact, people with the infection behave in a remarkably similar way to those with untreated schizophrenia. But this does not mean infection causes schizophrenia – that's speculation.
The reality is that we don't know what causes mental illnesses such as depression, bipolar disorder and schizophrenia.
Like most chronic illnesses, there is rarely a simple cause-and-effect explanation. Rather, there is a complex interaction of genetics, environment and triggers.
In other words, you can be genetically predisposed to have a higher risk of developing an illness such as schizophrenia, but it is the environment in which you live and the choices you make that can flip genetic switches. Being exposed to T. gondii may be one of those triggers.
What's intriguing is that studies have consistently shown associations between infections early in life and severe mental illness later in life. One study, for example, showed that one-fifth of schizophrenia patients had a mother who suffered from influenza during pregnancy.
We know, too, that a small percentage of children develop obsessive-compulsive disorder after infection with strep. This syndrome, known as PANDAS – pediatric autoimmune neuropsychiatric disorders associated with streptococcal infections, remains controversial.
Mental illnesses are often described as being caused by a chemical imbalance in the brain. But the brain is more than a gooey bag of chemicals; it's a collection of neurotransmitters that are linked to other parts of the body, which are influenced by our genes, our environment and our actions.
Increasingly, researchers are asking what role infectious agents play in the development of neurological disorders, from schizophrenia to autism.
We need to tread carefully here, but there is no question that there is a link between the gut (a key part of our immune system) and the brain and, as we work to decode the microbiome as well as the genome, those links should become clearer.
It's not as simple as saying owning a cat will make you crazy, but asking provocative questions such as "Can owning a cat give you schizophrenia?" could open the door to a world of new possibilities for prevention, treatment and understanding the causes of mental illness.