Do Pet Dogs Affect the Risk of Schizophrenia in Humans?

Children who have a pet dog may have a lower risk of schizophrenia.

Posted Jan 06, 2020

Joe from Pexels CC0
Source: Joe from Pexels CC0

A new study from the Johns Hopkins University School of Medicine reports some findings about the relationship between dog ownership and human mental health that are extremely promising, but also somewhat puzzling. It has been well established that having a pet dog improves mental well-being and reduces a number of psychological risk factors. Specifically, people who own a dog are less likely to suffer from stress-related problems and depression. However, this new study suggests that early exposure to a pet dog may significantly reduce the risk of later diagnosis of schizophrenia as well.

One reason why this result is so important is because of the nature of schizophrenia. To begin with, schizophrenia is seldom what you see in Hollywood movies, where an individual lapses into multiple personalities. Rather it is a problem where people have a hard time recognizing reality, thinking logically and behaving appropriately in social situations. In some cases, people may even lose their ability to use language coherently or communicate meaningfully. Schizophrenic patients show reduced longevity and a greatly increased likelihood of suicide. The effects are significant enough so that the World Health Organization ranks schizophrenia as one of the top 10 most disabling conditions that people may suffer from.

For psychologists, a particularly disturbing aspect of schizophrenia is that we are still far from having a cure for it. It is a lifelong illness, and although extreme psychotic symptoms may come and go, the negative emotional symptoms and cognitive disruptions are extremely persistent. According to the American Psychiatric Association's Diagnostic Symptom Manual, for individuals suffering from schizophrenia, "the prognosis is guarded" and "full recovery is unusual." Estimates are that only about 15 to 20 percent of people with schizophrenia have a fully favorable outcome. Furthermore, the incidence of schizophrenia is surprisingly large, and it may affect 1 in every 100 people worldwide.

Given the disruptive nature of schizophrenia, it's relatively high prevalence in our society, and our inability to successfully treat it, anything that suggests a means of lowering the risk for the appearance of the disease is important, and in this newest research, if the results are to be believed, it may be possible to reduce the risk factors by a sizable amount.

The lead researcher in this current study is Robert Yolken of the Pediatric Neurovirology division at the John Hopkins Children's Center. The rationale for this investigation comes from the fact that in earlier research Yolken and his colleagues found a link between serious psychological disorders (specifically schizophrenia or bipolar disorder) and exposure to aspects of the environment in early life that may affect a growing child's immune system. There is a lot of data confirming that having a pet dog during early childhood can strengthen a child's immune system.

This, in turn, can result in visible benefits, such as a lower incidence of allergies in later life. If having a pet affects the immune system, and the immune system may be involved in the appearance of psychological problems, Yolken and colleagues at Sheppard Pratt Health System in Baltimore reasoned that it might be worthwhile to look at the relationship between a child's exposure to a household pet cat or dog and the risk of a diagnosis of schizophrenia or bipolar disorder later on as adults.

By the standards of most clinical research, this was a large study involving 1,371 people between the ages of 18 and 65. It is difficult to gather large numbers of patients with diagnoses for specific psychological disorders, however, this research team managed to test 396 people with schizophrenia and 381 with bipolar disorder. These patients were then compared to 594 control individuals with no diagnosis of current mental problems. Among other information obtained, all of the study participants were asked if they had a household pet dog or cat during the first 12 years of their life.

As is often the case in studies such as this, the statistical analyses were extensive and often difficult to interpret for individuals who have not had analytic training, however, the major findings are quite clear. To begin with, owning a pet cat seems to have no statistically significant effect on either the risk of schizophrenia or bipolar disease. Pet dogs also do not have any effect on the risk of bipolar disorder.

However, when we look at the association between pet dog ownership and later risk of schizophrenia, the results suggest that it is surprisingly large. People who are exposed to a pet dog before their 13th birthday are significantly less likely to be diagnosed later with schizophrenia. The associations are huge, as much as 24 percent risk reduction for the overall sample. For children who had a household pet dog at birth, or who were first exposed to a dog before the age of 3, the reduction in risk of schizophrenia may be as large as 50 percent.

You have to take a moment to wrap your head around the implications of these results. If we can assume that the hazard ratios presented in this report are accurate reflections of the relative risk of contracting the disease, then some 840,000 cases of schizophrenia (that is 24 percent of the 3,500,000 people diagnosed with the disorder in the U.S.) might never have contracted the disease if they simply had exposure to a pet dog in their home when they were young children. Remember prevention is the only way that we can currently combat the incidence of schizophrenia because it is so difficult to cure.

I believe that these researchers were as startled by the size of the associations as I was, and it is clear that they have only a tentative set of hypotheses to explain the results. Dr. Yolken suggests, "There are several plausible explanations for this possible protective effect from contact with dogs—perhaps something in the canine microbiome that gets passed to humans and bolsters the immune system against or subdues a genetic predisposition to schizophrenia."

The best guess of these scientists is that your pet dog tracks lots of stuff into your house from the outside, and transfers some materials to your growing child by licking him. These substances challenge the child's developing immune system and it, therefore, grows stronger. This, in turn, in some way or another, reduces the risk of the later development of schizophrenia.

Obviously, results like these are of such importance that this study needs to be replicated; however, I did send a note to my granddaughter, who has a 1-year-old child and is pregnant with another, which is due to be born in a few months. I suggested that these data provide more than adequate justification for her to bring a pet dog into her household.

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Robert Yolken, Cassie Stallings, Andrea Origoni, Emily Katsafanas, Kevin Sweeney, Amalia Squire, Faith Dickerson, (2019). Exposure to household pet cats and dogs in childhood and risk of subsequent diagnosis of schizophrenia or bipolar disorder. PLOS ONE, 2019; 14 (12): e0225320 DOI: 10.1371/journal.pone.0225320