Once anathema, it now seems that a ‘dirty’ environment can enrich a baby’s microbiome and lessen her or his likelihood of developing everything from obesity to asthma. Again, it seems that we can rely on man’s best friend to help us out.
Two decades of research have made it clear that children who grow up with dogs have lower rates of asthma than those who do not. Many researchers attribute this finding to the hygiene hypothesis — the idea that exposure to a little dirt early in life can ward off allergic diseases that can appear later. Without firm data to show a link between owning a dog and improved immune health, however, the idea remained alluring, but unproven.
Perhaps mums who own dogs are healthier or more likely to breastfeed, says Anita Kozyrskyj, a paediatric epidemiologist at the University of Alberta in Edmonton, Canada. “It could be the lifestyle.”
In 2013, Kozyrskyj set out to see if she could pinpoint what might be going on. Kozyrskyj’s team evaluated the various microbes present in 24 faecal samples collected between 2008 and 2009 from 4-month-old infants enrolled in the Canadian Healthy Infant Longitudinal Development (CHILD) cohort study. Of the 24 babies, 15 lived in houses with at least 1 dog or cat1.
The researchers discovered that infants living with pets had a higher diversity of microbes in their guts (as measured in their faeces) than infants without pets. A generation ago, Kozyrskyj’s findings would have been cause for alarm. Microbes — synonymous at the time with germs — were thought to be best kept at bay. A family history of allergies may even have prompted physicians to advise expectant parents to give up the family pet.
But now, says Kozyrskyj, it’s known that the immune system develops alongside the gut microbiome — the genetic material of the community of microorganisms that live in the gut. That means that if infants grow up with limited exposure to microbes, such as those present in dog fur or those tracked into the house on muddy paws, the child’s immune system may deem those particles worthy of attack.
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This lack of exposure to microbes is especially problematic in developed countries, where people spend most of their time indoors. Researchers are beginning to suspect that dogs present one way for people to safely sully a baby’s environment.
It’s premature, however, for doctors to start writing prescriptions for pooches. Kozyrskyj’s study was too small to draw any firm conclusions. Rob Knight, a microbiologist and director of the Center for Microbiome Innovation at the University of California, San Diego, points to the gaps that remain to be filled. “What hasn’t been done is the type of intervention study that would definitively prove that if you got a dog, it would improve your child’s health,” says Knight. “But there are a lot of different lines of evidence that point in that direction.”
In developed countries, the incidence of allergic and autoimmune diseases has been rising for decades. In 1989, David Strachan, an epidemiologist now at St. George’s, University of London, found that children who grew up with more siblings had lower rates of hay fever than those with fewer2. Strachan, who introduced the hygiene hypothesis, proposed that the rise in allergic diseases was the result of cleaner environments brought about, in part, by higher standards of personal cleanliness.
Many researchers have since confirmed that exposure to a little dirt — through siblings, growing up on a farm or living in a developing country — can be beneficial and even ward off disease. For instance, in 2015, capping decades of observations, researchers quantified the reduction in risk of asthma for children growing up with dogs3. The investigators combed through the records of more than one million children born in Sweden between 2001 and 2010. Of the 275,000 or so school-age children in the cohort, the researchers found that the children of dog-owning families had a 13% lower chance of developing asthma than their peers who grew up without a dog.
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.
Researchers suspect that our long association with canines means that human and dog microbiomes may have developed in tandem. The microbiome of a baby growing up without a dog (and of a puppy growing up without a human) is, in a sense, incomplete. “All of the people alive today probably had ancestors who lived in tribes that hunted with dogs,” says Jack Gilbert, director of the Microbiome Center at the University of Chicago in Illinois.
Read full story: https://www.nature.com/nature/journal/v543/n7647_supp/full/543S48a.html?WT.mc_id=FBK_NA_317_AnimalHealth