A comprehensive meta-analysis of 17 studies spanning 44 years in 11 countries provides strong support for growing evidence that sharing a room with cats during childhood is associated with a significantly increased risk of schizophrenia. Scientists from the Queensland Mental Health Research Center analyzed the results of these international studies and, after adjusting for comorbidities, they still found that the estimated risk of being in the company of felines during childhood was 2.24 times greater than the estimated risk of not having a cat in the home.
While there is no universal age of exposure to cats that is most concerning, there is consensus that the window of risk exists throughout childhood. A Finnish study showed that mental disorders were associated with exposure to children younger than 7 years old, while a British study showed an association with exposure to children aged 4 to 10 years.
But before you pack your bags and march out the door, more work needs to be done to understand this link and the many factors that may influence a later diagnosis of schizophrenia-related disorders, researchers say.
However, scientists already know the cause of this risk factor: the domestic cat parasite Toxoplasma gondii. Although this parasite can also be spread through undercooked meat and contaminated water, it is most often spread through eggs in cat feces. It has long been associated with changes in central nervous system (CNS) and brain function.
The parasite can hide in a human host for life, and healthy people are unlikely to experience any symptoms because the immune system keeps the bugs at bay. In fact, it is estimated that as many as 15% of Americans have been infected with Toxoplasma gondii.
Pregnant women and more vulnerable people are at greater risk, and children may also be part of this group. Toxoplasma gondii infection is the leading cause of neonatal blindness worldwide and a leading cause of sequelae such as seizures and vision loss. More recently, the parasite has been linked to frailty in later life and bizarrely risky sexual behaviors.
Although it is tempting to think of cats as bringing this complex infectious disease into human homes, cats are simply hosts for this opportunistic single-celled protozoa. Toxoplasma gondii finds fertile ground in the intestines of domestic cats, and scientists believe this is because cats lack an enzyme called delta-6-desaturase. In other mammals, this enzyme creates a biochemical barrier to the parasite's reproductive targets.
In most cases, cats are not bothered by parasites or show signs of disease, but they shed millions of egg cells (eggs) in their feces, which end up on their paws and fur, and then come into contact with human hands. Although the parasite cannot complete its life cycle in the human host, its tiny form allows it to evade the immune system and sneak across the blood-brain barrier to wreak havoc in the central nervous system and brain.
While there is still much work to be done to understand exactly why some children are more likely to develop specific mental illnesses as adults, researchers recommend that children who own cats take note of the importance of good hygiene.
The researchers said: "Our review provides support for a link between cat ownership and schizophrenia-related disorders. The field of research needs to generate new candidate environmental risk factors, especially those that are potentially modifiable. In this context, more high-quality studies based on large, representative samples are needed to better understand the role of cat ownership as a candidate risk modifier for psychiatric disorders."
The research was published in the journal Schizophrenia Bulletin.