Living with animals is thought to have a profound effect on the human immune system, potentially reducing the risk of allergies, eczema and even autoimmune diseases.
Since the Amish first emigrated from Central Europe to North America in the 18th century, they have been known for their unique way of life. Today, they rely on the same methods of dairy farming and horseback transportation that their ancestors followed for centuries.
For decades, the Amish have stirred the imagination of Hollywood screenwriters, documentary filmmakers, and social scientists. But over the past 10 years, their lifestyle has become of increasing interest to the medical world as well, as they seem to defy one particularly disturbing modern trend.
While rates of immune-related diseases that begin in childhood, such as asthma, eczema, and allergies, have risen dramatically since the 1960s, this is not the case with the Amish. The reason for this is due to a frank understanding of how the immune system works, and how profoundly animals are affected when living together with humans.
A diverse community
To understand why the Amish have lower rates of certain immune diseases, a team of scientists in 2012 spent time in an Amish community in Indiana and in another farming community, known as the Hutterites, in South Dakota. In both cases, they took blood samples from 30 children and studied their immune systems in detail.
Many similarities have been found between the two groups. Like the Amish, Hutterites also live off the land, are of European descent, have minimal exposure to air pollution, and follow a diet low in processed foods. However, they have four to six times higher rates of asthma and childhood allergies than the Amish.
The difference between the two communities is that while the Hutterites have fully embraced industrialized farming techniques, the Amish have not, which means that from a very young age they live in close contact with animals and the many germs they carry.
"If you look at drone photos of Amish settlements and compare them to Hutterite communities, the Amish live on a farm with animals, whereas the Hutterites live in small villages and the farm may be several kilometers away," says Fergus Shanahan, emeritus professor of medicine at University College Cork, Ireland.
In 2016, a group of scientists from the U.S. and Germany published a now landmark study that Amish children have a lower risk of developing allergies because of the way the environment shapes their immune system.
Specifically, the scientists found that the Amish children in the study had more finely tuned so-called regulatory T cells than children in the Hutterite family. These cells help curb unusual immune responses.
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When the researchers tested dust samples collected from the homes of Amish and Hutterite children for bacteria, they found clear evidence that Amish children were exposed to more germs, likely from the animals they lived among.
Around the world, other researchers have reached similar conclusions. A group of immunologists reported that children raised on alpine farms, where cows usually sleep in close proximity to their owners, were protected against asthma, hay fever (allergic rhinitis or pollinosis) and eczema.
Studies have shown that a child's risk of developing allergies between the ages of 7 and 9 decreases in proportion to the number of pets that were in the house during the early years of their life, which has been dubbed the "mini-farm effect."
"It's not a universal panacea, and every time I give a lecture on the subject, someone says, 'I grew up on a farm and I have allergies,' but we know that if you grew up physically interacting with farm animals, you're about 50 percent less likely to develop asthma or allergies," says Jack Gilbert, a professor at the University of California, San Diego, who participated in the Amish study.
Gilbert also co-founded the American Gut Project, a citizen science project that studies how people's lifestyles affect their microbiome. "Even if you just grew up with a dog in the house, your risk of disease is reduced by 13-14%," he says.
The pets protect
Since the results of the Amish study were published, the potentially protective effects of socializing with animals during childhood have been the subject of much interest, and the New York Times even published an article questioning whether pets are the new "probiotic."
So what's going on? Perhaps unsurprisingly, given the tactile nature of humans and the love of petting and caressing pets, germs from their fur and paws get on our skin - at least temporarily.
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This has led to speculation that the microbiome may be populated by pet microbes. This is a set of huge colonies of microbes that live on the skin, in the mouth and, above all, in the human gut, where a significant proportion of the body's immune cells are concentrated.
According to Nasia Safdar, a professor of infectious diseases at the University of Wisconsin in the US, the concept has caught the interest of the pet food industry. The idea, she said, is to develop products that promote the growth of beneficial bacteria in the bodies of cats and dogs, which can then be given to their owners.
"This area has proven attractive for funding because most of us are interested in the human condition," Safdar says. "So what role can animals play in this?".
Safdar says she is considering a study that would collect fecal samples from both pets and their owners when they come in for repeat veterinary appointments to see if their guts become more microbiologically similar over time. She also wants to see if she can identify similar types of bacteria that may provide health benefits.
But other researchers say the idea that microbes from dogs, cats, or any other nonhuman animals could be incorporated into the microbiomes of humans is questionable. "There's no evidence for that," Gilbert says. "We don't detect long-term accumulation of canine bacteria on our skin, in our mouths, or in our gut. They really don't linger."
In response, Safdar says she still finds the study very useful, stating that it is likely that gut microbes can be transmitted from pets to their owners and vice versa. "This is worth exploring, and so far it hasn't been the subject of much attention," she says.
Gilbert believes that pets play a different but equally important role. According to his theory, as man's distant ancestors domesticated various species of animals, his immune systems evolved to be stimulated by the microbes they carry. These microbes don't live with us all the time, but our immune cells recognize familiar signals when they pass through us, allowing the immune system to evolve in the right direction.
"For many millennia, the human immune system has gotten used to seeing bacteria from dogs, horses and cows," Gilbert says. "And when it sees these bacteria, it triggers beneficial immune development. She knows what to do," he says.
Studies have also shown that people who share a home with a pet have more similar gut microbiomes, and Gilbert suggests that the animal likely serves as a vehicle for the transmission of human microbes between its owners.
At the same time, regular contact with your pet's own microbes stimulates their immune system to be more active and better manage the bacterial population in their own gut and skin microbiome, keeping pathogens out and encouraging the growth of beneficial bacteria.
Ancient microbes
All of this is good news for animal lovers: research continues to suggest that living with pets throughout life may be beneficial to the human immune system.
After reading a study on the Amish and Hutterites, Shanahan was inspired to conduct his own research on Irish Travellers, a marginalized group of people who typically live in confined spaces among a variety of animals, from dogs and cats to ferrets and
horses.
Shanahan sequenced their gut microbiomes and compared them to the microbiomes of the Irish, who lead a more modern lifestyle, as well as the microbiomes of indigenous people in Fiji, Madagascar, Mongolia, Peru and Tanzania, who still lead a lifestyle similar to human hunter-gatherer ancestors.
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He found that the microbiome of the Irish travelers was more similar to that of the indigenous people. Their microbiome also has similarities to the microbiome of people from the pre-industrial world, which other scientific groups have been able to study by collecting ancient fecal samples preserved in caves, he said.
"Irish travelers preserved an ancient microbiome," Shanahan says. "It's much more like what you see in tribes in Tanzania who still live as hunter-gatherers, or Mongolian horsemen who live in yurts, close to their animals."
Shanahan believes this may explain the low rate of autoimmune diseases among Irish travelers: conditions such as inflammatory bowel disease, Crohn's disease, ulcerative colitis, multiple sclerosis and other conditions that, like asthma and allergies, have become increasingly common in recent decades.
"This doesn't mean their health is good," Shanahan says. "Irish travelers die much earlier than sedentary residents. But they die from things like alcoholism, suicide and accidents, which is caused by poverty, marginalization and the destruction of their culture. But if you go to an Irish rheumatologist and ask him if he has ever seen a traveler with systemic lupus erythematosus (an autoimmune disease), he will say no never."
Researchers are now trying to find out whether bringing animals back into people's lives in different ways can benefit lifelong health. Researchers at the University of Arizona in the US have investigated whether bringing dogs back into the homes of older people can help improve their physical and mental health by strengthening their immune systems.
And results from an Italian research group that created an educational farm where children from families without pets could regularly pet horses under supervision showed that the children's gut microbiome began to produce more beneficial metabolites.
According to Gilbert, it's likely that this may well be a means of boosting childhood immunity. "If you're exposed to more types of bacteria, you stimulate your immune system in more diverse ways, which may improve its ability to manage microbes on your skin and in your gut," he says. "But you don't get colonized by animal bacteria, that doesn't happen."
Researchers note that having pets throughout life may help microorganisms interact more with the immune system in other ways. For example, having a dog increases the likelihood of regular walks, notes Liam O'Mahony, professor of immunology at APC Microbiome Ireland, a microbiome research center at University College Cork.
"If you have a pet, you're getting out in nature and walking in the park," O'Mahony says. "You're also exposed to microbes from the park, soil and other places that can be beneficial."