How dogs are helping us understand human allergies
If your dog has allergies, chances are you do too. Thanks, microbes
Ah, fresh air. I prop my office window open to enjoy a temperate June day by the Charles River in Boston. A soothing breeze packed with the allergens of the urban jungle whirl around the room. A few minutes later, it hits. An onslaught of wheezing and sneezing forces me to reseal the office so the neighbors avoid thinking someone is having a fit about their research.
We share some of these allergic disorders with man’s best friend: our dogs. Around 10 percent of dogs and children experience eczema. Using our canine companions, researchers are dissecting why we these allergies develop and how we help ameliorate them.
When I inhale, my immune system kicks into overdrive upon encountering the allergens I’m sensitive to. A fleet of proteins called Immunoglobin E (IgE) specific to these allergens flood the affected cells, causing the typical symptoms millions of Americans experience every season: sneezing, coughing, rashes, and difficulty breathing. For those with food allergies, the symptoms can be life-threatening.
While respiratory allergies have held steady for decades, food and skin allergies showed sizable increasessince the late 1990s. Public health experts and scientists continue to refer to the growing impact of allergies as an epidemic. Finding the causes is a critical public health effort.
The hygiene hypothesis
I never had any pets growing up in a typical northeastern suburban community. My amazing mom kept our house spotless. And, I must admit, I was an indoors kid with the exception of baseball games until high school. And to this day, my skin suffers from eczema and loathes grass. Unfortunately, this makes me the poster child for the “hygiene hypothesis,” a theory that emerged in 1989 from epidemiologist David Strachan. He highlighted the idea that a loss of microbial diversity could “lead to allergic disease.” Now, the idea has advanced to the biodiversity hypothesis, focusing on the entirety of microbes in our living environments, not just in the home. As Dr. Jenni Lehtimäki, researcher at the University of Helsinki, told me in an interview, “The decrease in biodiversity relates to the increase in non-communicable diseases, and the link between these two are the environmental microbes.”
This hypothesis is a hot topic in the field. Several epidemiological studies have highlighted a clear link between microbes and disease, but the complexity of these relationships make it difficult to parse out underlying causes. There are a variety of factors involved in allergy development: your genetic make-up, what your mother ate while pregnant, the percentage of non-human family members, and countless other possibilities. Our busy, complicated lifestyles make it difficult to parse out the impact of all these factors and get to the bottom of it. Lehtimäki and her colleagues have been tackling this challenge.
Their question was simple, but daunting: does our living environment – the microbes, animals, plants, co-workers, family around us –affect the development of allergic disease and if so, how?
Studies focusing on allergies or the microbiome studies are notoriously difficult, and now they wanted to tackle both together. Lehtimäki and her team looked to dogs, similar enough to humans but easier to control for aspects that may influence their studies, like lifestyle factors. Lehtimäki and her colleagues collected microbes from the skin of 169 dogs and information about their living environments, lifestyles, and allergies. They used genetic sequencing to catalog the diversity of microbes on the dogs’ skin. The living environment of dogs associated with their skin microbiota and allergies, suggesting the answer to whether our living environment influences allergy development is yes.
At the same time, Lehtimäki and her collaborators recognized the wealth of data that could be accessed from dog owners about this question. After surveying over 6,000 owners in Finland, another clear association appeared. Allergic dogs were more likely to have allergic owners, and that living in an urban environment increases the risk of allergies.
Both findings have a clear message for us living in urban environments: have frequent, intimate encounters with nature. And bring your dog with you.
As the evidence for the biodiversity hypothesis continues to grow, researchers are increasingly focusing on identifying why microbe diversity could lead to a lower risk of allergies. One primary hypothesis is that these microbes are our personal trainers for our immune system, building it for encounters with pathogens and allergens.
Lehtimäki hinted at future work identifying these biological links, but some current research suggests that this hypothesis may be true. In another study at the University of Helsinki focused on eczema, diversity of a specific microbial species was correlated with healthy individuals and individuals with higher concentrations of IL-10, an anti-inflammatory protein secreted by cells, in their blood. A similar phenomenon was seen in mice when administered the microbes directly into the skin.
Still, robust evidence for microbial links remain elusive, particularly in the context of disease. Though one mantra continue to rings true: microbes can be friends, not foes. Dogs will continue to help us understand this.