Are pets 'probiotics' for humans?

Veterinarians assess potential benefits of microbe-sharing across species

Published: February 15, 2024

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Microorganisms transferred from pets to people's skin, gut and other parts of their bodies may provide health benefits, research indicates.

Most people know that pets hold a special place in many a human heart, providing a source of comfort and bolstering emotional well-being. Fewer might be aware that pets also can improve the composition of our gut flora, potentially bringing a host of unique health benefits to owners — and perhaps even to veterinary professionals.

It's a phenomenon that scientists are only just beginning to fully comprehend, as a mounting body of research demonstrates that the type of inhabitants that occupy our guts and other body parts is highly influenced by whom we share our lives with.

Within each of us exists a community of trillions of microorganisms such as bacteria, viruses, archaea and fungi that comprise what's known as a microbiome.

The idea that regular contact with dogs, cats and other animals enriches that community is gaining prominence as cutting-edge DNA-sequencing technology enables scientists to map the microbiome and better understand its effects on our health.

The likelihood of suffering from illnesses like asthma, obesity, inflammatory bowel disease, diabetes, cardiovascular disease, arthritis, cancer and depression has been shown to be influenced by the composition of the microbiome, albeit to varying degrees and with much still to learn.

"The long and short of it is that the microbiome, and especially the gut microbiome, affects practically almost every aspect of human and animal health," said Dr. Laurel Redding, an assistant professor of epidemiology at the University of Pennsylvania's School of Veterinary Medicine.

Redding has been studying the benefits that pets can provide to the human microbiome for years. Her latest research, now in the recruitment phase, is investigating how owning a pet could help older folks better tolerate a course of antibiotics.

In brief

Determining which diseases might specifically be curbed by exposure to non-human animals also is a work in progress. Redding points out that research to date has indicated conditions such as asthma, atopy (a tendency to develop allergic diseases), obesity and certain bacterial infections could be prevented by an inter-species exchange of microorganisms.

When two clouds merge

The makeup of an individual's microbiome, while unique from person to person, is influenced to a large extent by that of their mother's microbiome. However, new inhabitants are picked up throughout the course of our lives, for better or for worse. Consequently, an individual's microbiome may more closely resemble that of a friend's or lover's than that of a stranger's.

Buzz around the extent of those connections was heightened last year with the publication of a landmark Italian study based on more than 9,700 human stool and saliva samples collected from people on five continents. Participants in the same households shared a "significant " portion of bacteria strains of 12% and 32% in their guts and mouths, respectively, the study found. Cohabitation affected sharing "more than age or genetics," it also concluded.

"The germaphobes might not appreciate this, but everyone is like a little cloud of bacteria, and every time you come into contact with a surface or another animal or human, you're interacting with that other cloud of microbes," said Dr. Celeste Allaband, a veterinarian and microbiologist at the University of California, San Diego. "For the most part, there's very little transfer, but the longer you're in contact and the more you're in contact, the more things are likely to swap back and forth."

Although distinct microbiomes exist in (or on) many parts of an animal's body, most research has gravitated toward studying the gut microbiome, in part due to the sheer abundance of lifeforms that reside there.

"The gut microbiome is sort of like the Amazon rainforest," said Allaband. "It has the most diversity; it has the most stuff going on. It interacts with our diet, our immune system, our metabolism."

Allaband's mushing-of-clouds description is more illustrative than literal. The transfer of microorganisms from animal to human occurs through a variety of pathways that could be as simple as skin-to-fur contact altering the skin microbiome. The digestive tract gets involved when pets, say, lick their owner's face or when owners pat their pet before eating a sandwich or bowl of fries. There's also exposure to things that pets touch, like clothes, bedding, leashes and bowls. "Some of these taxa can even be found on less frequently touched surfaces like walls and television screens," Allaband said.

Bad bugs, of course, get transferred to the microbiome, too. Pet owners, for example, risk exposure to unwanted lodgers such as Campylobacter and Salmonella, some of which may have developed a resistance to drugs designed to kill them.

"Where the sweet spot sits of avoiding the bad bugs and getting the good bugs is definitely not known at this point," Redding said.

Benefits from pets may be pronounced in early childhood

The extent to which the human microbiome can be altered by exposure to another animal's microorganisms is unclear, but there are certain beneficial microorganisms that are found more in pets than in people. Compared with humans, for instance, dogs tend to host more Lactobacillus, a bacteria widely considered in human medicine circles as a probiotic — microorganisms introduced to the body for their apparent benefits. Lactobacillus has anti-inflammatory properties that could help combat disease associated with the immune response, like asthma, atopy and obesity, according to Redding.

"You have studies showing that children raised on farms surrounded by animals, or people raised in households with furry pets, are less likely to have those conditions, and the thought is that it's modulated by sharing of microbiota," Redding said. "That has not been definitely proven, but that's certainly the working hypothesis."

The situation is complex, not least because various studies indicate that pet ownership can increase the risk of childhood asthma. Timing, it appears, is a crucial factor: The earlier a child is exposed to an animal, the more likely they are to benefit. "The first 1,000 days of life is when your microbiomes are all sort of fluid and just trying to set you up well for the rest of your life," Allaband explained.

A lower incidence of asthma in children growing up on farms, she posits, also could be related to the so-called hygiene hypothesis — the idea that exposure to bugs can be a good thing because it helps our bodies set their defenses. "In a city, we're constantly cleaning," Allaband said. "We're just not interacting with the outside environment as much."

One of several studies linking pet ownership to lower disease risk in children was conducted in Sweden. Published in December 2018, it found that in a cohort of more than 1,200 children, allergy manifestations including asthma, hay fever or eczema were found in 49% of those reported to have no pets in their first year of life, compared with none among the children who reportedly lived with five or more pets.

"The prevalence of allergic disease in children aged 7 to 9 years is reduced in a dose-dependent fashion with the number of household pets living with the child during their first year of life, suggesting a "mini-farm" effect, whereby cats and dogs protect against allergy development," the paper concludes.

Further research holds promise

As for the propensity for beneficial microorganism-swapping with pets to occur throughout life, the science is less definitive but nonetheless promising. Some studies indicate that exchanges with pets are common, such as one published in 2013 that found that dog ownership significantly affected the adult skin microbiome. However, a study published in 2020 and based on an analysis of stool samples from 332 participants in Wisconsin found that only four types of microorganisms were more abundant in participants with pets, while seven were more abundant in participants without pets. "Future research is needed to further elucidate the relationship between the gut microbiome and pets," the researchers concluded.

At the University of Pennsylvania, Redding is indeed trying to probe further. In her latest effort, she is following people who are taking antibiotics for dental implants to see if owning a pet could help their gut microbiome recover faster from the drugs. Taking antibiotics is associated with negative outcomes ranging from mild diarrhea to potentially life-threatening infections of Clostridioides difficile (commonly known as C. diff) to which the elderly are particularly at risk.

The veterinarian led previous research, published in 2020, that found that owning a pet lowers the incidence of recurrent C. diff infection in humans. She hypothesizes that a richer microbiome helps its inhabitants outcompete opportunistic pathogens like C. diff for real estate in the gut.

Allaband, too, alludes to this apparent competition factor, using methicillin-resistant Staphylococcus aureus (MSRA), a bacteria that causes hard-to-treat infections in humans, as an example.

Dogs, she said, host a relatively larger volume of a related bacteria, Staphylococcus intermedius, compared with humans. "They're both Staphs, and they both compete for the same resources," she said. "So having the dog's Staph around can keep a human's S. aureus in check because they're constantly fighting for that same space in the microbiome."

Moreover, research published in late 2022 that was based on data collected by a project called American Gut, a nonprofit research initiative led by scientists at UC San Diego, found that dog ownership was associated with increased abundance of the apparently beneficial bacteria in the families Actinobacteria, Bifidobacteriaceae and Ruminococcaceae in people over age 65.

The American Gut project, conducted in 2012, was followed in 2014 by the British Gut project. Those "citizen science" initiatives, which involved volunteers submitting samples of their skin, saliva or stool for analysis, formed the foundation of the currently active Microsetta Initiative.

Allaband, who worked closely with the American Gut project, said an insignificant number of samples from pets were collected by the researchers at the time. She's hopeful that a greater number of samples from non-human animals will be submitted for any further research.

As our knowledge grows, she wonders if further beneficial microorganism swaps between humans and non-human animals will be confirmed. "There's been particular bacteria that are found more abundantly in the homes of dog owners that are known to produce antioxidants, anti-inflammatory and anticancer-type molecules; [and] cholesterol-lowering and glucose-lowering metabolites," she said.

The microbiome, veterinarian style

As for the idea that veterinarians and veterinary technicians, since they work frequently with a vast array of animals, could have their own unique type of microbiome?

"I sure hope so!" Redding said with a chuckle. "But the short answer is that it has not been investigated."

At the same time, Redding notes that a study published in 2022 and conducted in the Netherlands found that veterinary professionals had a similar carriage of C. diff as the general population, despite a higher exposure risk.

Allaband, for her part, is optimistic that veterinarians have distinctly enriched microbiomes, courtesy of their patients.

"I haven't seen any actual studies looking into that," she said. "But I bet we do."

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