Dogs and cats: what pets do to your gut, immune system and nerves

Healthy state

Living with an animal means sharing more than the sofa. In a study of 60 families, people who lived together had more bacteria in common than with strangers, most clearly on the skin. With a dog in the household, adults shared even more skin bacteria, and dog owners shared more with their own dog than with other dogs. Whether these bacteria stay or just pass through is unclear. Dogs also carry soil and plant microbes into the home: house dust in homes with dogs is composed differently.

This leaves traces in babies' guts. In the Canadian CHILD study of 746 infants, babies whose family kept a furry pet during pregnancy or afterwards, mostly a dog, were more than twice as likely to have high levels of the bacterial genera Ruminococcus and Oscillospira. Other studies link these to fewer allergies and less excess weight; that they do so in these children has not been shown. In adults the effect is small. In a study of 332 adults, gut bacterial diversity did not differ with or without a pet, only single bacterial groups did. In older dog owners, researchers found somewhat more bifidobacteria at the same diversity, and in cat owners shifts in single bacterial families.

What early contact with animals does to the immune system is intriguing. In mice, house dust from homes with dogs protected against allergic reactions of the airways. Their gut bacteria changed a lot, among other things the bacterium Lactobacillus johnsonii increased. This bacterium alone already gave partial protection, also against a respiratory virus. So the gut trained the lungs along with it. Observations in humans fit this. In a Swedish birth cohort, 48 percent of children without a pet in their first year had an allergy at age eight to nine, 35 percent with one pet and 21 percent with two or more. In Japan, among 66,215 children, those with a dog in the home during pregnancy had a food allergy by their third birthday somewhat less often. In Finland, babies with a dog at home were ill less often in their first year, had fewer ear infections and needed antibiotics somewhat less often.

There are also hints for inflammatory bowel disease Inflammatory bowel disease Umbrella term for Crohn’s disease and ulcerative colitis: long-term, relapsing inflammation of the gut that needs medical treatment. More in the chapter “Immune system and inflammation”. A meta-analysis Meta-analysis An analysis that combines the results of several studies. It is only as good as the studies it includes. of 29 studies found about a quarter lower risk of Crohn's disease or ulcerative colitis in people with a pet, and an even lower risk with contact to farm animals. A Canadian study followed 4,289 healthy relatives of people with Crohn's disease for about five and a half years; 86 developed the disease. Those who had lived with a dog between ages 5 and 15 fell ill about a third less often. They also had a tighter gut barrier, measured with a sugar test in the urine, and a richer gut flora Microbiome, gut microbiota The community of all microbes in the gut, mainly bacteria, plus viruses, fungi and archaea. Most live in the colon. More in the chapter “Building microbial diversity”. No such link was found for cats.

Pets also act through the nerves. When a dog gazes at its person for a long time, the bonding hormone oxytocin rises in both; with hand-raised wolves this did not happen. In a stress test with a speech and mental arithmetic, people with a friendly, unfamiliar dog beside them had less cortisol and a calmer pulse than people with a friend or alone, although they felt just as tense. In humans the vagus nerve Vagus nerve The tenth cranial nerve and the most important nerve of the rest-and-digest system. It connects the brainstem and the abdominal organs; most of its fibres tell the brain what is happening in the gut. More in the chapter “The vagus nerve: the data line between belly and head” is usually measured indirectly through heart rate variability, that is, how much the intervals between heartbeats vary: more variation suggests a more active vagus. In a small Japanese study it rose in older people while walking a dog and even while petting one. In eleven older dog owners it was higher when the dog was nearby. Because the vagus and the stress axis help steer gut movement and pain sensitivity, this is a possible route to the gut. Nobody has measured it directly at the gut yet.

Then there is exercise. In England, dog owners were four times as likely to reach the recommended 150 minutes of activity per week, and dog walking came on top of other exercise rather than replacing it. A meta-analysis of almost 3.8 million people found a 24 percent lower risk of death in dog owners, and a clearly lower risk still in people after a heart attack. The American Heart Association therefore considers a dog a reasonable contribution to heart health, but advises against getting one for that purpose alone.

Disrupted state: causes and effects

The most important caveat first: almost all human data are observational. Families with dogs often live more rurally, move more and have more children. Families with allergies get a pet less often or give it away again. Both can suggest a protection that does not really exist. Nobody has assigned families a dog by lot, and the route via house dust and gut bacteria has so far only been shown in mice.

For inflammation markers in adults the picture is sobering. In an English study of 8,785 older people, dog or cat owners had neither better inflammation markers in the blood nor better results for strength, walking speed, memory or mood. And not every animal is the same: in the Canadian Crohn's study, people with a bird at home had an almost three times higher risk and more calprotectin Calprotectin A protein from white blood cells measured in stool. Raised levels point to inflammation in the gut. More in the chapter “Gut tests: what they show and what they do not” in their stool, an inflammation marker of the gut. Why is unclear. It is a single finding that still needs confirmation.

Animals can transmit gut germs. In an Australian study of children under three, a puppy in the home was the strongest risk factor for infection with Campylobacter, one of the most common bacterial causes of diarrhoea. Young animals and animals with diarrhoea shed particularly many germs. Roundworm eggs from dog and cat faeces can reach humans via soil, sandpits and unwashed vegetables, mainly children. Cats that hunt or eat raw meat can shed Toxoplasma. A first infection during pregnancy can harm the unborn child.

A newer problem is raw feeding. In a British study of 432 dogs, raw-fed dogs shed E. coli resistant to important last-resort antibiotics much more often, including so-called ESBL producers. A small study found that the resistance genes in dogs' guts were more similar to those of their own owners than to those of strangers. Whether resistant germs actually pass to humans and make them ill has not been measured.

If you already react to pet hair, more contact will not help. The protective associations concern early childhood, that is prevention, not the treatment of an existing allergy.