Early-Life Exposure to Dogs Linked to Lower Schizophrenia Risk

Megan Brooks

December 31, 2019

Having a pet dog as a child may protect against the development of schizophrenia later in life, new research suggests.

Results showed that adults who owned a dog during childhood were 25% less likely to have a subsequent diagnosis of schizophrenia. Cats, however, did not show the same association, the researchers note.

There are some plausible explanations for a possible protective effect from contact with a dog, study investigator Robert Yolken, MD, chair of the Stanley Division of Pediatric Neurovirology at Johns Hopkins School of Medicine, Baltimore, Maryland, told Medscape Medical News.

"One is that families that have dogs differ in some way from those who have cats, for example, in terms of where they live or economic resources, and that these differences are relevant to the risk of schizophrenia," he said.

Another explanation is that the effect relates directly to differences between types of microbiota in dogs vs cats and "that components of the microbiota are transmitted to infants through interaction with the pets," Yolken noted.

"We do know that dogs and cats differ in terms of their microbiota so it is possible that the bacteria in the dog microbiome are more protective than that of the cat. Consistent with this possibility are studies indicating that the microbiota of individuals with schizophrenia differ from that of other individuals," he added.

The findings were published online December 2 in PLOS One.

Dogs vs Cats

To evaluate ties between exposure to a pet dog or cat during infancy and childhood and a later diagnosis of schizophrenia or bipolar disorder, the researchers assessed 396 patients with schizophrenia, 381 patients with bipolar disorder, and 594 individuals who acted as the healthy control group.

In Cox proportional hazard analysis, having a pet dog in the house before the 13th  birthday was significantly associated with a decreased risk of receiving a subsequent diagnosis of schizophrenia (hazard ratio [HR], 0.75; 95% confidence interval [CI], 0.63 - 0.90; P < .002).

This association was not explained by a range of demographic factors that may affect household pet exposure, including age, sex, race/ethnicity, parental education, and place of birth.

The apparent protective effect of exposure to a pet dog was most evident when the dog was present at a child's birth or joined the household before the end of the child's second year of life. Exposure to the family dog during this time was associated with about a 50% reduction in relative risk for a schizophrenia diagnosis.

There was no significant association between bipolar disorder and having a pet dog, and having a cat in the house was not significantly associated with a diagnosis of either schizophrenia or bipolar disorder.

Interpret With a "Grain of Salt"

Commenting on the findings for Medscape Medical News, Oliver Freudenreich, MD, codirector of the schizophrenia clinical and research program, Massachusetts General Hospital, Boston, said epidemiologic studies are always to be "taken with a grain of salt, as association does not equate to causation."

In addition, there are often many confounding variables, not all of which may be known, that may cause a "spurious association," said Freudenreich, who was not involved with the research.

Also, this particular study is rather small, he added. "That said, epidemiological studies are often the first step toward the discovery of something new in medicine."

Freudenreich said he would put this study in the category of "increasing our knowledge" about environmental risk factors for schizophrenia.

"Environmental risk factors are probably at least as important as genetic risk factors in brain development and its associated diseases such as schizophrenia, but we are in the infancy of understanding them, including how they confer risk biologically," he said.

The study also shows that the environment contains protective factors "and not just risk factors," Freudenreich noted.

"The dog story is interesting as there is a plausible biological mechanism that the authors note, the 'hygiene hypothesis.' This type of study is good as a starting point for discovery and hypothesis-generating that can then be studied methodically," he concluded.

The study was largely supported by grants from the Stanley Medical Research Institute. Yolken is a paid scientific adviser to the Stanley Medical Research Institute and to the Astellas Research Institute of America and is on the board of the Treatment Advocacy Center. The other study authors and Freudenreich have disclosed no relevant financial relationships.

PLOS One. Published online December 2, 2019. Full text

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