Scientists Uncover Intriguing Connection Between Cats and Schizophrenia
Although there's a wealth of information about the world that science has unlocked for humanity, those who work in the field will find that this rarely happens quickly. The scientific method calls for findings to be retested and confirmed over and over again, and new factors in studied matters can send researchers back to the drawing board at any time.
It's for that reason that the link between cat ownership is schizophrenia has attracted research for over 40 years with new conclusions over how dangerous this link is apparently contradicting the ones they followed. That's why a new and major meta-analysis aims to set the matter straight.
The story of cats and schizophrenia starts with dogs
According to Wired, the very idea of linking neurological conditions with pet ownership began in the 1970s, when psychiatrist E. Fuller Torrey learned that it's possible for dog viruses to trigger multiple sclerosis in humans.
As he said, "That got me thinking about which animals host which infectious agents." When the question inevitably turned to cats, Torrey discovered that this question would soon lead him to one of the most pervasive and mysterious bacteria in the world.
The quiet infection
According to the Centers For Disease Control, Toxoplasma gondii is a bacteria commonly found in cat feces and while there are many ways to be infected with it, accidental ingestion after cleaning the litter box is a common one.
Although severe cases of toxoplasmosis can damage the eyes, brain, and other organs, most people who contract it will remain asymptomatic. Since it's so easy for the pathogen to fly under the radar, it's not a surprise that it's so prevalent.
There are some high-risk individuals
While most people don't necessarily have to take special precautions against toxoplasmosis, that is not true for pregnant women and people whose immune systems are compromised. According to the Centers For Disease Control, both are at risk of severe health consequences when exposed to Toxoplasma gondii.
In the case of pregnant women, a particular concern is the risk of passing the infection onto their unborn children. Although toxoplasmosis rarely manifests at birth, the consequences are severe when it does and they can develop symptoms later in life when it doesn't.
What does this have to do with schizophrenia?
After Torrey started learning about Toxoplasma gondii, he started combing through research and discovered multiple studies, which found that those living with schizophrenia tend to have more toxoplasma antibodies in their blood than those without the condition.
When he started conducting his own research, Torrey surveyed a large number of people living with schizophrenia and found that most of them had lived with cats in their lives. However, this wasn't quite enough evidence for the purposes of Torrey's eventual theory.
A misunderstood hypothesis
It's easy to see the direction of Torrey's research and come to the conclusion that he thinks Toxoplasma gondii outright causes schizophrenia. However, Wired explained that he was more interested in when people living with the condition lived with cats, rather than whether they did.
That's because he was really arguing that the bacteria alters the development of the brain when a person is young in a way that can make contracting the illness more likely. Even then, he figured this was probably only true of those who were already predisposed to schizophrenia.
A controversial hypothesis
Although Torrey has been studying this possible correlation for well over 40 years, others have also investigated the matter and not all of them agree with him. According to CNN, a team led by Francesca Solmi at University College London described previous studies as having methodological issues due to their reliance on subjects' childhood memories.
To address this problem, the British researchers conducted an ambitious study that followed 5,000 people from birth until they reached the age of 18. Their birth information was used to determine the presence of cats in their houses and they were psychologically evaluated at 13 and 18.
One of multiple refutations
Once her research concluded, Solmi stated, "Once we controlled for factors such as household over-crowding and socioeconomic status, the data showed that cats were not to blame. Previous studies reporting links between cat ownership and psychosis simply failed to adequately control for other possible explanations."
As such, the team concluded that they found no link between cat ownership and the development of schizophrenia. And since Torrey admitted to Wired that even he isn't totally convinced his hypothesis reflects the reality of the world, that means the case is closed, right?
Torrey fires back
Despite his own uncertainty, however, Torrey took issue with the statistical analysis Solmi's team included in their study. As he saw it, the overcrowding and poverty factors they outlined actually increased the likelihood of exposure to cats and felt that boiling their question down to cat ownership rather than cat exposure was a mistake.
Torrey also pointed out that cats aren't quite neat enough to limit their bathroom breaks to litter boxes. Any loose soil is potentially contaminated by Toxoplasma gondii, including sandboxes. Indeed, Torrey noted that cats love sandboxes for this reason.
Torrey also felt the timing was off
Although the length of this study was even more ambitious than the 12-year production of the movie Boyhood, Torrey also told Wired that he felt the British researchers concluded this study before it gave them their most illuminating results.
Schizophrenia typically doesn't manifest until between the ages of 19 and 25, which meant that the 18-year cutoff point was likely too early to see any rates of contraction. Indeed, the study's authors were aware of this and used the reporting of "disturbed thoughts" as a substitute. As Torrey said, "It's a great cohort, and in 10 years they'll have great data."
There's one thing both sides can agree on
Although Torrey and Solmi's team disagree heavily on Torrey's cat hypothesis, both acknowledge that Toxoplasma gondii likely has an inflammatory effect on the risk of developing psychotic disorders like schizophrenia. There's a preponderance of evidence to suggest that it does.
However, the main point that they disagree on concerns how significantly exposure to cat feces influences this risk. Since the bacteria is so easy to contract from trace amounts of the feces without realizing it, it's hard to know exactly how this risk translates to real-life manifestations of schizophrenia.
Cats aren't the only way to end up with Toxoplasma gondii
Citing the Centers For Disease Control's estimate, CNN reported that as many as 60 million Americans may be infected with Toxoplasma gondii. However, one doesn't become the most successfully infectious bacterium in the world by having only one way to infect people.
According to the Centers For Disease Control, Toxoplasma gondii is also transmissible by eating undercooked, contaminated meat or shellfish, not washing hands after preparing these meals, contamination through kitchen utensils exposed to these foods, and contaminated drinking water. It's even possible to contract it through organ transplants, though this is rare.
A new study enters the fray
By 2023, the number of contradictory studies regarding the connection between cat exposure and schizophrenia wasn't exactly lost on the scientific community. This is why a 2023 study published in Schizophrenia Bulletin sought to systematically analyze all existing research up to that point.
To that end, a team led by John J. McGrath tracked down studies from every source and citation they could find between January 1, 1980, and May 30, 2023. These studies were included in the analysis regardless of the geographical locations or languages they were printed in.
They knew they had to be careful about discussing cats
Considering how much debate there was over the importance of "cat ownership" versus "cat exposure" in previous studies, they pooled and analyzed the other studies' risk assessments based on broad categories that included cat ownership, cat bites, and cat contact.
Once these categories were accounted for, McGrath's team pooled together these studies in order to make the closest apples-to-apples comparisons. They then assessed the risk of bias, study quality, and other nuance considerations of these studies to determine their eligibility for inclusion in the meta-analysis.
It took some significant paring down
By the time the research team had finished collecting all the material they could find, they ended up with 1,915 studies. Naturally, not all of these studies were conducted with the best of scientific rigor or best exemplified the conclusions they reached.
As a result, this meant the team had to be significantly more discerning with the studies that were considered substantial enough for a full text review, which turned out to be 106 in number. From there, the 17 most representative studies to exemplify the most common findings and hypotheses were selected for discussion in their report.
A limitation right off the bat
When McGrath's team started their research, their hypothesis was pretty all-encompassing. Since cat ownership or contact had been associated with schizophrenia-like disorders and what they called "psychotic-like experiences" in past research, their mission was to sort out the evidential basis for this association.
However, they soon realized that they wouldn't be able to aggregate any cogent estimates for psychotic-like experiences because the measures discussed in previous studies were too broad to fit together. There wasn't a real consensus on where to start on this issue among other researchers.
Schizophrenia was a different story
Although the research team's findings about psychotic-like experiences were inconclusive, the trend in their meta-analysis was different when it came to officially diagnosed schizophrenia. As it turned out, there was more quality research to suggest that the connection did exist than didn't.
More specifically, they found that when the estimates of other researchers were pooled together, the data suggested that cat ownership (or at least contact) had the potential to increase the risk of developing schizophrenia symptoms by a multiple of between 2.24 and 2.35 times.
The debate isn't over yet
With this result having been found, McGrath's team concluded that there was a consistent and evidence-based association between cat exposure and the increased risk of developing a disorder from the broad range of conditions associated with schizophrenia.
Although this study sounds like it proves Torrey's theory right, even its authors didn't feel it was quite that simple. Not only was the matter of psychotic-like experiences still inconclusive, but they called for more high-quality research into the matter at large. And it's a good thing they had because others had concerns about their study.
Researching this isn't any easier now than in the '80s
As it turned out, the 2023 study in Schizophrenia Bulletin was also hamstrung by some methodological issues. So much so that by the following year, its original authors were compelled to issue a correction.
The correction was obtained by Oxford Academic and it stated that multiple studies that made it into their analysis needed to be excluded. As a result, the association they found was murkier and hovered between 1.56 times and 2.40 times as likely to develop schizophrenia symptoms after cat exposure.
An important note to prevent overreactions
While this potential association between cat contact and schizophrenia could sound alarming when the message isn't properly delivered, it's worth noting that none of these studies are suggesting that being around a cat too long can cause schizophrenia out of thin air.
Remember that even Torrey, who has devoted over 40 years of his life to studying this exact connection, believes it is likely that only those who are already predisposed to schizophrenia are likely to be affected by Toxoplasma gondii in this way. There's also a reason why he doesn't sound more certain about that.
The problem is with schizophrenia itself
As Wired explained, the half-century spanning struggle to definitively determine whether cat feces is a significant enough driver of Toxoplasma gondii infection to influence schizophrenia symptoms is largely due to the complexities of schizophrenia itself. It's rare enough that collecting data on it is difficult, but that's only the beginning.
The truth is that so much of schizophrenia's biological and genetic roots remains shrouded in mystery. That means that even if it was somehow determined that 100% of cases were caused by Toxoplasma gondii and specifically derived from cat feces, it would remain unknown as to how that manifestation works.