Pet Exposure in Early Life Reduces Risk of Schizophrenia, Bipolar Disorder

A household dog or cat can pay dividends by driving down the risk of schizophrenia or bipolar disorder as a child.

A team, led by Robert Yolken, MD, Stanley Division of Developmental Neurovirology, Johns Hopkins School of Medicine, examined the relationship between household pet exposure during the first 12 years of life and having a subsequent diagnosis of schizophrenia or bipolar disorder.

“Exposure to household pets during infancy and childhood may be associated with altered rates of development of psychiatric disorders in later life,” the authors wrote. “Exposure to a household pet dog at birth and during the first three years of life is associated with a significantly decreased hazard and relative risk of a subsequent diagnosis of schizophrenia.”

They also detected trends in associations between childhood household exposure to a pet cat and relative risks of schizophrenia and bipolar disorder. This could be caused by socio-demographic, neuro-immune, or other biological factors or combinations of factors.

The cohort study included 396 individuals with schizophrenia, 381 patients with bipolar disorder, and 594 control patients. They used Cox Proportional Hazard and multivariate logistic regression models, including socio-demographic covariates to calculate the hazards of developing schizophrenia or bipolar disorder associated with first exposure to a household pet cat or dog.

“We found that exposure to a household pet dog was associated with a significantly decreased hazard of having a subsequent diagnosis of schizophrenia (HR, .75; P <.002),” the authors wrote. “Furthermore, a significant decreased relative risk of schizophrenia was detected following exposure at birth and during the first years of life.”

Overall, 220 (55.7%) of the individuals with schizophrenia were exposed to a pet dog before their 13^th birthday. There were also 248 (65.2%) individuals exposed to a pet dog before their 13^th birthday with bipolar disorder and 389 (62.1%) of the control group.

For household cats, 139 (35.1%) of the individuals with schizophrenia were exposed to a pet cat before the 13th birthday as well as 157 (41.2%) of the participants with bipolar disorder and 206 (34.7%) of the control individuals.

However, the investigators did not find a significant relationship between household exposure to a pet dog and bipolar disorder and no significant associations between exposure to a household pet cat and the subsequent risk of either a schizophrenia or bipolar disorder diagnosis.

Cox proportional hazard analysis indicated that exposure to a pet dog during the first 12 years of life was associated with an approximately 25% decreased hazard of having a subsequent schizophrenia diagnosis, which was not explained by a range of demographic factors that may impact household pet exposure, including age at evaluation, gender, race, place of birth, and level of parental education.

Despite these findings, there were trends towards an increased risk of both psychological disorders at defined periods of exposure.

In the past, investigators have linked psychiatric disorders like schizophrenia and bipolar disorder, to environmental exposures in early life, as well as a high rate of familial associations. However, contact with household pets such as cats and dogs, often serve as a source of environmental exposure during this time period.

Extensive genetic studies have also found a large number of genomic regions associated with increased risk of developing both disorders. However, few genes of large effect have been identified, with substantial differences relating to genetic risk factors among different populations.

The study, “Exposure to household pet cats and dogs in childhood and risk of subsequent diagnosis of schizophrenia or bipolar disorder,” was published online in PLOS One.