June 2, 2022 – Sorry, cat people and only children: Having a dog as a toddler and growing up in a large family are two things linked to a significantly lower chance of getting Crohn's disease later in life, according to a new study.
Children who lived with a dog between the ages of 2 years and 4 years were 37% less likely to have Crohn's disease, the study says. And those who lived with at least three other family members during the first year of life were 64% less likely to have this form of inflammatory bowel disease (IBD).
"In this study, we're interested in environmental exposures and which ones are associated with Crohn's disease onset," Williams Turpin, PhD, said in a media interview May 23 at Digestive Disease Week (DDW) 2022, held in San Diego, CA, and virtually.
Turpin and colleagues looked at other things in the environment – including living on a farm, drinking unpasteurized milk or well water, and growing up with a cat – but they did not have a significant link to a higher risk.
Two other things were associated with a slight increase in risk: having a sibling with Crohn's disease and living with a bird at time of the study. But the number of bird owners was small; only a few people in the study had a pet bird when they enrolled.
The link to living with a dog as a toddler "was more robust," said Turpin, a project manager at Mount Sinai Hospital in Toronto.
The study included 4,289 healthy first-degree relatives of people diagnosed with Crohn's disease. They provided urine, blood, and stool samples and did surveys about environmental exposures at different stages of life.
Investigators followed them an average of 5.6 years, during which time 86 people got Crohn's disease.
Living with a dog early in life likely means more exposure to different microbes, boosting the strength of a person's immune system against later challenges. This theory was supported in the study comparing the gut microbiome in people who did and not have a dog in the home early in life.
Turpin and colleagues genetically sequenced the gut microbiome of the people in the study and found differences in bacteria between groups.
"Our study also shows that just by living with a dog, it impacts your gut microbiome composition, which may have an impact on the immune response later in life."
The researchers also looked at the health of the gut by measuring certain factors in the urine. One factor was higher in people who did not live with a dog at any point.
Mediated by the Microbiome?
Living with a dog between ages 2 and 4 years and a large family size (more than three people) in the first year were significantly associated with a lower risk of Crohn's disease onset.
It is unknown if the results apply to other populations; the researchers studied first-degree relatives of people with Crohn's disease.
"The study needs to be replicated and validated," Turpin said.
Future research could evaluate people who never had a dog and look for changes in their microbiome after they get one.
"It's a really interesting study from a good group. It's novel in terms of getting at what really drives environmental risk factors," says Brigid Boland, MD, a gastroenterologist at UC San Diego Health in California, who was not affiliated with the study.
Autoimmune diseases are really complicated, in part because the risk of getting an autoimmune disease is low, and you're going back in time to look at what put people at risk.
"The study was well-crafted in choosing siblings and family members of people with IBD," Boland says, agreeing with Turpin that more research is needed to understand this.