A new study suggests that being around ‘man’s best friend’ from an early age may have a health benefit as well, lessening the chance of developing schizophrenia as an adult.
In the study, Yolken and colleagues at Sheppard Pratt Health System in Baltimore investigated the relationship between exposure to a household pet cat or dog during the first 12 years of life and a later diagnosis of schizophrenia or bipolar disorder.
For schizophrenia, the researchers were surprised to see a statistically significant decrease in the risk of a person developing the disorder if exposed to a dog early in life. Across the entire age range studied, there was no significant link between dogs and bipolar disorder, or between cats and either psychiatric disorder.
Lead author Robert Yolken, chair of the Stanley Division of Pediatric Neurovirology and professor of neurovirology in paediatrics at the Johns Hopkins Children’s Center, said: “Serious psychiatric disorders have been associated with alterations in the immune system linked to environmental exposures in early life, and since household pets are often among the first things with which children have close contact, it was logical for us to explore the possibilities of a connection between the two.”
Links to animal exposure in early childhood
Previous studies have identified early life exposures to pet cats and dogs as environmental factors that may alter the immune system through various means, including allergic responses, contact with zoonotic (animal) bacteria and viruses, changes in a home’s microbiome, and pet-induced stress reduction effects on human brain chemistry.
Some investigators suspect that this ’immune modulation’ may alter the risk of developing psychiatric disorders to which a person is genetically or otherwise predisposed.
Surprisingly, Yolken says, the findings suggest that people who are exposed to a pet dog before their 13th birthday are significantly less likely, as much as 24%, to be diagnosed later with schizophrenia.
“The largest apparent protective effect was found for children who had a household pet dog at birth or were first exposed after birth but before age three,” he says.
Yolken adds that if it is assumed that the hazard ratio is an accurate reflection of relative risk, then some 840,000 cases of schizophrenia (24% of the 3.5 million people diagnosed with the disorder in the United States) might be prevented by pet dog exposure or other factors associated with pet dog exposure.
Yolken said: “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.”
For bipolar disorder, the study results suggest there is no risk association, either positive or negative, with being around dogs as an infant or young child.
Overall for all ages examined, early exposure to pet cats was neutral as the study could not link felines with either an increased or decreased risk of developing schizophrenia or bipolar disorder.
“However, we did find a slightly increased risk of developing both disorders for those who were first in contact with cats between the ages of 9 and 12,” Yolken says. “This indicates that the time of exposure may be critical to whether or not it alters the risk.”
One example of a suspected pet-borne trigger for schizophrenia is the disease toxoplasmosis, a condition in which cats are the primary hosts of a parasite transmitted to humans via the animals’ faeces.
Pregnant women have been advised for years not to change cat litter boxes to eliminate the risk of the illness passing through the placenta to their foetuses and causing a miscarriage, stillbirth, or potentially, psychiatric disorders in a child born with the infection.
In a 2003 review paper, Yolken and colleague E. Fuller Torrey, associate director of research at the Stanley Medical Research Institute in Bethesda, Maryland, provided evidence from multiple epidemiological studies conducted since 1953 that showed there also is a statistical connection between a person exposed to the parasite that causes toxoplasmosis and an increased risk of developing schizophrenia.
The researchers found that a large number of people in those studies who were diagnosed with serious psychiatric disorders, including schizophrenia, also had high levels of antibodies to the toxoplasmosis parasite.
“A better understanding of the mechanisms underlying the associations between pet exposure and psychiatric disorders would allow us to develop appropriate prevention and treatment strategies,” Yolken says.
The study was largely supported by grants from the Stanley Medical Research Institute.