BALTIMORE, Md. - Dogs have long been protectors of their human companions, and it turns out they may be protecting us in more than just the physical sense.
There’s a reason that our canine companions have earned the title “man’s best friend.” Ever since humans domesticated the dog anywhere from an estimated 18,000 to 32,000 years ago, they have proven to be loyal, obedient and protective friends capable of fending off loneliness and providing emotional support to their human caregivers.
This long-standing partnership may have additional health benefits — growing up with a dog may lessen a person’s risk of developing schizophrenia in adulthood, according to a new study from Johns Hopkins Medicine.
Researchers were curious if being raised around either dogs or cats during early developmental years had any sort of impact on the risk of developing psychiatric disorders later in life.
They found a statistically significant decrease in a person’s risk of developing schizophrenia as an adult if that person grew up in a household with a dog early in life.
By comparison, there was no significant link between dogs and bipolar disorder, or between cats and either psychiatric 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,” said 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 team of colleagues from Sheppard Pratt Health System in Baltimore carried out a study to analyze the relationship between having a household cat or dog during the first 12 years of life and a later diagnosis of either schizophrenia or bipolar disorder.
They analyzed data for nearly 1,400 adults between the ages of 18 and 65, including 396 people with schizophrenia diagnoses and 381 people with bipolar disorder diagnoses.
Each participant was asked if they had a household pet cat or dog or both during their first 12 years of life. Those who reported that a family pet was in the house at the time they were born were considered to be exposed to that particular animal since birth.
Researchers then used a statistical model to determine the risk of each group by comparing the number of people that had pets and developed psychiatric disorders with the number of people in the control group who later developed psychiatric disorders.
Yolken and his team said they were surprised to see that their findings suggest that people who are exposed to a pet dog before their 13th birthday are as much as 24 percent less likely to be diagnosed with schizophrenia later in life.
“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 said.
Previous studies have found that having a pet cat or dog in the household can be an environmental factor with the potential to alter the immune system through a variety of ways, including allergic responses, contact with animal bacteria and viruses and pet-induced stress reduction effects on human brain chemistry.
Yolken suspects that it is this modulation of the immune system that may be altering the risk of developing psychiatric disorders in people who are genetically predisposed.
“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 said.
Early exposure to pet cats was found to have a neutral effect on risk for both disorders, though they did note a slight increase in risk for developing schizophrenia in children who were introduced to a pet cat between the ages of nine and 12.
“This indicates that the time of exposure may be critical to whether or not it alters the risk,” Yolken said.
Some experts suggest that the disease toxoplasmosis, a condition resulting from the Toxoplasma gondii parasite being transmitted to humans via contact with cat feces, could be a potential pet-borne trigger for schizophrenia. Yolken said that this is why much of the previous research investigating links between childhood pet exposure and schizophrenia has been focused on exposure to cats. He said that the most recent study is among the first to consider exposure to pet dogs as well.
More research is needed to better understand the underlying mechanisms resulting in links between childhood pet ownership and later development of psychiatric disease.
“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 said.
This story was reported from Los Angeles.