The possible relationship between exposure to the protozoan parasite Toxoplasma gondii and mental disorders such as schizophrenia has been analyzed in various studies in recent years. In some cases it has been suggested that people who live with cats (which may be carriers of this intracellular parasite) may have a greater risk of this type of mental health problems.
A team of experts from the Queensland Mental Health Research Center (Australia) has reviewed 17 studies published on this subject over the last 44 years, with data from 11 countries, and now presents its conclusions in an article published in the specialized Schizophrenia Bulletin .
"We found an association between owning a cat or living with this type of domestic animal and greater chances of developing disorders related to schizophrenia," summarizes the team of co-authors of the study led by psychiatrist John McGrath, also an expert at the Queensland Brain Institute, from the University of Queensland, St Lucia (Australia).
This idea that cat ownership could be linked to the risk of schizophrenia was initially proposed in a 1995 study, with exposure to a parasite called Toxoplasma gondii suggested as the cause. But the research so far has yielded mixed conclusions: there are hints but not all results confirm the relationship.
Studies have found that being around cats during childhood can make a person more likely to develop schizophrenia; However, not all studies have found a significant statistical association.
Some also link exposure to cats with higher scores on scales measuring traits related to schizophrenia (which affects a person's thoughts, feelings, and behaviors) and psychotic-like experiences, but again, other studies do not show this connection.
To be healthy, McGrath and his team say they point out that given the contrast in results and to obtain a clearer picture, it would be necessary to conduct a comprehensive review and analysis of all the research on these topics.
T. gondii is a generally harmless parasite that can be transmitted through undercooked meat or contaminated water.
A bite from an infected cat or feces from an infected cat can also transmit T. gondii. It is estimated that about 40 million people in the US may be infected, usually without any symptoms. Meanwhile, researchers continue to find more strange effects that infections can have.
Once inside our body, T. gondii can infiltrate the central nervous system and influence neurotransmitters. The parasite has been linked to personality changes, the appearance of psychotic symptoms, and some neurological disorders, including schizophrenia.
However, a link does not prove that T. gondii causes these changes or that the parasite was transmitted to a human from a cat.
The new analysis of 17 studies found "a significant statistical association between cat ownership in the broad sense and an increased risk of schizophrenia-related disorders," summarizes the University of Queensland.
"After adjusting for covariates, we found that individuals exposed to cats were approximately twice as likely to develop schizophrenia," the study's team writes.
The authors highlight, however, that 15 of the 17 studies were case-control studies. This type of research cannot prove cause and effect and often does not look at issues that could have affected both the exposure and the outcome. Several of the studies analyzed were of poorly verified quality, the authors also warn.
Findings were inconsistent across studies, but those of higher quality suggested that the associations in the unadjusted models could have been due to factors that could have influenced the results.
One study found no significant association between owning a cat before age 13 and developing schizophrenia later, but did find a significant link when limiting cat ownership to a specific period (ages 9 to 12). This inconsistency suggests that the crucial period of exposure for cats is not clearly defined.
A study conducted in the United States, involving 354 psychology students, did not find a connection between cat ownership and schizotypy scores. However, those who had received a cat bite had higher scores compared to those who had not.
Another study, which included people with and without mental disorders, found a connection between cat bites and higher scores on tests measuring particular psychological experiences. But they suggest that other pathogens such as Pasteurella multocida may be responsible.
Researchers agree that better, broader research is needed before we can make firm interpretations.
"In conclusion, our review supports the association between cat ownership and schizophrenia-related disorders," the authors write.
"More high-quality studies, based on large and representative samples, are needed to better understand cat ownership as a possible risk-modifying factor for mental disorders," the authors suggest.
