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.
The clinical consequences of exposure to different allergens in early life have long been a matter of discussion, especially if infants are exposed to pets such as cats and dogs during their first year of life. Early pet-keeping was previously considered to be a risk factor for allergy development, but several studies from the last 20 years have highlighted that this is probably not the case [1–8], even in individuals with a strong family history of atopy . Today, early pet-keeping is generally not considered to be a risk factor for allergy in families with otherwise healthy infants.
Conversely, pet-keeping during early life may instead protect from later allergy , especially exposure to more than one dog or to both a cat and a dog [3, 4]. We were the first to demonstrate, in 1999 , that children in families keeping (a) cat(s) or (a) dog(s) during the child´s first year of life had less asthma at 7–9 years as compared to children with no such animals, and that this difference remained also after adjusting for selection mechanisms due to allergy among parents or siblings.
The existence of an allergy-protective effect from pet-keeping is also supported by immunological data. In studies analysing the effect of cat exposure on asthma and allergy development, a high-dose exposure to cat allergens , or keeping of cats , were associated with clinical tolerance and cat-specific IgG4, but not IgE.
Immunological tolerance facilitated by keeping of cats and dogs during early life is, however, still a hypothesis, despite some support for this assumption in the aforementioned studies. Not all studies report a long-term protective effect , and if such an effect exists, it is still not known how induction of this immunological tolerance is mediated.
In principle, we hypothesized that two different mechanisms–not mutually exclusive–could contribute to a protective effect of pet-keeping. First, exposure to cat or dog dander, containing massive amounts of allergens from the respective species, could induce high-dose clinical tolerance to the allergens, i.e. reduced risk of cat-allergy in the children exposed to cats and dog-allergy in children with dogs.
Second, cohabiting pet animals could provide a “mini-farm” environment, with microbes or other immunoregulatory factors that provide a broad modifying effect on immune development in the child, leading to tolerance not only to the pet itself, but also to food and airborne allergens. In this study we try to address this question, hypothesising that high-dose allergen exposure should induce tolerance only to that specific type of animal, whereas a mini-farm induced tolerance is supposed to be protective not only to a specific animal but also to other environmental allergens.
Most often research focus on identifying risk factors for allergy development. But in modern society, finding lifestyle factors that could protect from allergy has become equally important. The main aim of this study was to investigate if pet-keeping during early life affects later allergy development and, if so, whether a dose-response association was detectable.
Second, if the protective effect was species-specific, suggesting an allergen-driven tolerance induction, or, if it is species-unspecific suggesting an allergy-protective “mini-farm” environment. We used data from a cross-sectional cohort and a birth cohort for the analyses to minimize influences from common methodological shortcomings, e.g. selection bias and reverse causation.
More information: Robert Yolken et al, Exposure to household pet cats and dogs in childhood and risk of subsequent diagnosis of schizophrenia or bipolar disorder, PLOS ONE (2019). DOI: 10.1371/journal.pone.0225320