Findings do not link similar contact with cats to either schizophrenia or bipolar disorder
Ever since humans domesticated the dog, the
faithful, obedient and protective animal has provided its owner with
companionship and emotional well-being. Now, a study from Johns Hopkins
Medicine 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.
And while Fido may help prevent that condition, the jury is still out
on whether or not there’s any link, positive or negative, between being
raised with Fluffy the cat and later developing either schizophrenia or
bipolar disorder.
“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,” says Robert Yolken,
M.D., chair of the Stanley Division of Pediatric Neurovirology and
professor of neurovirology in pediatrics at the Johns Hopkins Children’s
Center, and lead author of a research paper recently posted online in
the journal PLOS One.
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.
The researchers caution that more studies are needed to confirm these
findings, to search for the factors behind any strongly supported
links, and to more precisely define the actual risks of developing
psychiatric disorders from exposing infants and children under age 13 to
pet cats and dogs.
According to the American Pet Products Association’s most recent
National Pet Owners Survey, there are 94 million pet cats and 90 million
pet dogs in the United States. 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, Yolken notes, suspect that this “immune
modulation” may alter the risk of developing psychiatric disorders to
which a person is genetically or otherwise predisposed.
In their current study, Yolken and colleagues looked at a population
of 1,371 men and women between the ages of 18 and 65 that consisted of
396 people with schizophrenia, 381 with bipolar disorder and 594
controls. Information documented about each person included age, gender,
race/ethnicity, place of birth and highest level of parental education
(as a measure of socioeconomic status). Patients with schizophrenia and
bipolar disorder were recruited from inpatient, day hospital and
rehabilitation programs of Sheppard Pratt Health System. Control group
members were recruited from the Baltimore area and were screened to rule
out any current or past psychiatric disorders.
All study participants were asked if they had a household pet cat or
dog or both during their first 12 years of life. Those who reported that
a pet cat or dog was in their house when they were born were considered
to be exposed to that animal since birth.
The relationship between the age of first household pet exposure and
psychiatric diagnosis was defined using a statistical model that
produces a hazard ratio — a measure over time of how often specific
events (in this case, exposure to a household pet and development of a
psychiatric disorder) happen in a study group compared to their
frequency in a control group. A hazard ratio of 1 suggests no difference
between groups, while a ratio greater than 1 indicates an increased
likelihood of developing schizophrenia or bipolar disorder. Likewise, a
ratio less than 1 shows a decreased chance.
Analyses were conducted for four age ranges: birth to 3, 4 to 5, 6 to 8 and 9 to 12.
Surprisingly, Yolken says, the findings suggests 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 3,” 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.
“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,”
Yolken says.
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’ feces. 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
fetuses 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, M.D.,
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.
Because of this finding and others like it, most research has focused
on investigating a potential link between early exposure to cats and
psychiatric disorder development. Yolken says the most recent study is
among the first to consider contact with dogs as well.
“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.
Working with Yolken on the research team are the following members
from Sheppard Pratt Health System: Cassie Stallings, Andrea Origoni,
Emily Katsafanas, Kevin Sweeney, Amalia Squire, and Faith Dickerson,
Ph.D., M.P.H.
The study was largely supported by grants from the Stanley Medical Research Institute
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