Sometime during the Last Glacial Maximum, or period when ice sheets covered most of North America, Europe and Asia, man got a great idea. He realized that it was cold, really dry and that food was getting harder to come by. He was competing with other animals for that food and he was freezing his butt off.
One of his food competitors was the wolf. One of our ancestors got the brilliant idea to make wolves his best friends. Maybe by dancing with them. Okay, ridiculous reference. No promises, but I’ll try to avoid them in the future. So this genius figured out a way to make wolves into pets. Since that day, “man’s best friend” has been progressively domesticated.
In fact, modern dogs and their wolf ancestors still share DNA. Yes, even that poodle with the weird haircut and those Chinese Crested things. You know, these guys:
Kind of reminds you of a wolf, right? Yeah, me too.
Maybe that isn’t exactly how wolves got domesticated into dogs. But we all know modern dogs as obedient (mostly,) protective and faithful. Dog owners enjoy a relationship filled with companionship, fun and emotional well-being. But it turns out those aren’t the only benefits of being a “doggie parent.”
The good science folks at Johns Hopkins Medicine think they’ve found another one. This one is interesting and may be important. Kids who are around dogs from an early age have a reduced risk of schizophrenia. According to the research, however, there doesn’t seem to be evidence of the same result for exposure to cats.
“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.
Yolken and his band of merry researchers at Sheppard Pratt Health System in Baltimore looked at the connection between exposure to a pet dog or cat during a child’s first 12 years and a diagnosis of schizophrenia or bipolar disorder later in life. While there was no evidence to suggest a significant link between dogs and bipolar disorder risk reduction, nor for cats and reduction of risk of either psychiatric disorder. What surprised the scientists was the statistically significant decrease in the risk of developing schizophrenia among those exposed to a dog in their household early in life.
Yolken and company have stated that more studies are needed in this area. More studies could confirm these findings, dig into what’s behind the linkage and more accurately define the actual change in the level of risk of developing psychiatric disorders for kids under 13 exposed to household dogs and cats.
There are some 94 million pet dogs and 90 million pet cats in America, according to a recent survey by the American Pet Products Association. Early life exposure to pet dogs and cats represents an environmental factor that may alter the human immune system. The mechanisms for these changes include allergic responses, contact with zoonotic bacteria and viruses (zoonotic is a fancy word for animal,) pet-induced stress reduction and it’s effects on human brain chemistry and even changes to a home’s microbiome. I’ve written about environmental impact on microbiomes here and here.
These factors can all contribute to what’s called “immune modulation.” This may change the risk of developing a psychiatric disorder, even if someone is genetically or otherwise predisposed to one.
Yolken’s study included 1,371 men and women, all between 18 and 65. Of these, 396 already had schizophrenia, 381 had bipolar disorder and 594 represented controls, having neither condition.Those with bipolar disorder and schizophrenia came from the inpatient, rehabilitation programs and day hospital programs of Sheppard Pratt Health System. Control group members were recruited from local Baltimore communities and screened to rule out past or present psychiatric disorders. Each participant had information documented that included gender, age, birthplace, ethnicity/race and parents highest education level (used to indicate socioeconomic status.)
Each was asked whether they had a dog or cat as a pet between birth and 12 years old. Those with a dog or cat present in the house when they were born were noted as such.
Researchers used a hazard ratio, which is a measure over time of the frequency of specific events or types of events – in this case, exposure to a pet and the development of a psychiatric disorder – occur in a specific group compared to that of a control group. They measured the relationship between the age of first exposure to a household pet and any subsequent psychiatric diagnosis. When using a hazard ratio, a measure of 1 indicates no difference between the study groups and the control group. A ratio of more than 1 indicates an increased likelihood of a later diagnosis of bipolar disorder or schizophrenia. Conversely, a ratio of less than 1 indicates a decreased risk.
The researchers analyzed pet contact in four age ranges: birth to 3 years, 4 to 5 years, 6 to 8 years and 9 to 12 years.
They were surprised to find that exposure to a pet dog before age 13 correlated with as much as a 24% lower likelihood of a later schizophrenia diagnosis.
“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.
If Yolken and his team have come up with accurate reflections of relative risk, then about 840,000 cases of schizophrenia could be prevented each year in the US simply by early life exposure to a pet dog. About 3.5 million people are diagnosed with schizophrenia in the US each year.
“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.
No risk association, either positive or negative, was found for childhood dog exposure with relation to bipolar disorder.
Thinking that a cat would be just as good as a dog for preventing later diagnosis of bipolar disorder or schizophrenia? Not so much. Early exposure to cats was neutral with relation to both diagnoses, with neither a positive nor negative correlation found.
“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.
So the bottom line here seems to be that dogs are a better choice for kids than cats when it comes to preventing schizophrenia later in life. In fact, if the toxoplasmosis connection is true, you might want to keep your kids away from cats all together! Okay, that’s a bit of an overreaction, but there is cause for concern.
The take-away? When it comes to mental health, dogs may really be “man’s best friend.”
Keep the faith and keep after it!
Journal Reference – Robert Yolken, Cassie Stallings, Andrea Origoni, Emily Katsafanas, Kevin Sweeney, Amalia Squire, Faith Dickerson. Exposure to household pet cats and dogs in childhood and risk of subsequent diagnosis of schizophrenia or bipolar disorder. PLOS ONE, 2019