By Dr. Karen Shaw Becker
One of the many benefits of pet ownership seems to be improved health in babies and children who have a furry family member. For example, a study published earlier in the year suggests that babies born into families with pets have higher levels of two types of gut microbes linked to a lower incidence of both allergies and obesity.1
"There's definitely a critical window of time when gut immunity and microbes co-develop and when disruptions to the process result in changes to gut immunity," Anita Kozyrskyj, Ph.D., study co-author, University of Alberta pediatric epidemiologist, and a leading researchers on gut microbes, told ScienceDaily.2
These findings are part of a long-running research project using fecal samples from 746 babies registered in the Canadian Healthy Infant Longitudinal Development study that shows kids who grow up with dogs have less asthma than those who grow up in homes without pets (more about that shortly).
According to Kozyrskyj, their research stems from the theory that exposure early in life to dirt and bacteria from, for example, a dog's paws and fur, can create early immunity.
Study Uncovered Two Types of Health-Enhancing Gut Bacteria in Babies Exposed to Pets
For the study, Kozyrskyj and her colleagues looked at infants whose mothers were pregnant between 2009 and 2012. The mothers were asked if they had a pet at home during their second and third trimester, what kind (most had dogs, cats came in second) and whether it was still in the home during the baby's first 3 months.
More than half the infants were exposed to at least one pet with fur during either the mother's pregnancy or after birth. Eight percent were exposed during pregnancy only, and almost 47 percent were exposed before and after birth.
The researchers discovered that exposure to pets while in the womb or up to 3 months of age increased the levels of two strains of gut bacteria. One is Ruminococcus, which is linked to a reduction in childhood allergies. The other is Oscillospira, which is linked to childhood obesity.
The levels of these two bacteria were twice as high in babies with exposure to a pet in the home. The route of exposure is indirect, from dog to mother to baby during pregnancy as well as the first 3 months of life. This means that even if the dog was no longer around at the time of the baby's birth, his or her microbiome would still benefit from the exchange of health-giving bacteria.
In addition, the exchange occurred even in higher risk birth situations including a C-section delivery, when the mother was taking antibiotics at the time of birth, and when the mother didn't breastfeed.
Yet another protective benefit is that the presence of pets in the home reduced the incidence of transmission of vaginal GPS (group B strep) during birth, which causes pneumonia in newborn infants (and is the reason mothers are sometimes given antibiotics during delivery).
School-Age Kids With Dogs Have Significantly Lower Rates of Asthma
In a 2015 study, a team of Swedish researchers set out to try to quantify the reduced incidence of asthma in kids who grow up with dogs.3 The researchers looked at the medical records of over 1 million children born in Sweden between 2001 and 2010.
There were around 275,000 school-age children included in the 1 million, and the researchers found that school-age kids who had dogs at home had a 13 percent lower rate of asthma than the children from homes without dogs. According to Sujata Gupta writing for the journal Nature:
"The idea that pets can enhance the microbiome makes even more sense when viewed in light of the old friends hypothesis, a refinement of the hygiene hypothesis. In this view, humans' co-evolution with livestock and animals has made us dependent on their microbes for our health and even survival. Losing contact with these 'old friends' might tip the delicate evolutionary balance."4
Some researchers speculate that because humans and canines have such a long history working and living together, our microbiomes may be somehow intertwined. It could be that a baby without a dog — or a puppy without a human — is on some level incomplete, according to Gupta.
Human Members of Dog-Owning Families Have Similar Skin Bacteria
So how, exactly, is friendly bacteria exchanged between dogs and humans? No one's really sure yet. A study conducted at the University of Colorado-Boulder demonstrated that dog owners have both more and different skin bacteria than non-dog owners.5 The microbes in question are a blend of harmless bacteria from doggy tongues (betaproteobacteria) and paws (actinobacteria).
Study participants included 159 people and 36 dogs from 60 families, separated into four groups, including families with children aged 6 to 18, families with no children but one or more dogs, families with both children and dogs and families with no kids and no dogs.
The researchers collected samples of skin (from the forehead, palms or paws and tongues) and stool samples from all the participants (two- and four-legged) to determine what bacteria were present in each location. Analysis of the samples showed that human family members share similar microbes in the stool, on the skin and in the mouth.
The researchers found that the skin microbiota of people in families with a dog were more similar to each other than the microbiomes of members of dog-free homes. This suggests it was the dogs spreading friendly microbes around.
The researchers noted that much of the common bacteria shared between humans and their dogs is transmitted through licking. Another method of transmission occurs when dogs track bacteria in from outdoors and humans in the household pick it up. The paws and foreheads of dogs are a rich source of a great number of diverse microbes.
Does Having Dogs Around Benefit Adult Gut Bacteria?
Whether or not the microbiomes of dogs influence not just children's but also adult microbiomes is unclear. A small study conducted by psychiatrist Dr. Charles Raison of the Raison Research Group suggests there is no effect once a person has reached a certain age.6
Raison's study involved 20 adults between ages 50 and 80 who were given shelter dogs to care for over a three-month period (with the option to adopt at the end of the study). The study participants' blood, skin, saliva and stool samples were tested before they received the dogs, and again at one-month intervals for three months.
Interestingly, while the temporary dog guardians experienced a number of emotional and physiological benefits from having the dogs around, their microbiomes were unaffected. "The dogs clearly impacted people's emotions and immune systems, but not through the microbiome," said Raison.7 These findings align with prior studies showing that in humans, the gut microbiome forms within the first few years of life.