Does early childhood exposure to germs and bacteria reduce the chance of developing asthma and allergies? A new animal study casts doubt on this popular belief, finding that exposure to multiple microorganisms in early childhood may have little effect on allergic immune responses.
In the late 1980s, epidemiologist David Strachan proposed a new hypothesis to explain the dramatic rise in asthma and hay fever rates over the course of the 20th century. Strachan observed a correlation between reduced household size and the incidence of allergies. He believes that smaller families and improved hygiene standards have resulted in children being exposed to fewer microorganisms and infections at a young age. This ultimately leads to children becoming less tolerant to allergens later in life.
This idea is called the "hygiene hypothesis." In the ensuing years, researchers have iterated on this idea many times, from the "old friends" hypothesis (which holds that humans have co-evolved with bacteria over thousands of years of evolution, giving rise to microbial populations critical to health) to the more recent "biodiversity hypothesis" (which holds that our immune systems need exposure to a diverse natural environment to prevent allergic and inflammatory diseases).
A new study published in Science Immunology, led by researchers at Sweden's Karolinska Institutet, focused on testing the biodiversity hypothesis. The experiment selected two groups of genetically identical mice - one group was raised in a sterile laboratory environment, and the other group was raised in semi-natural conditions and exposed to a variety of microorganisms.
Both groups of mice were then exposed to a range of known allergens to study their immune responses. The results surprised the researchers, showing that both groups of mice developed similar inflammatory responses when exposed to allergens. Corresponding author Stephen Rosshart said the findings do not necessarily overturn the hygiene hypothesis, but do suggest that immune responses to allergens are more complex than previously assumed.
"This is, I believe, the first proof-of-concept study to demonstrate that polymicrobial exposure and infection are not the only or primary factors driving the dramatic increase in allergic disease," Rosshart explained. "[These findings] may help recalibrate how scientists think about the hygiene hypothesis, prompting the field to look more closely at other factors such as indoor living, physical activity, pollutants and chemicals present in the modern world."
In the study, the researchers noted that there is "indisputable evidence" that certain microorganisms can suppress allergic inflammation. For example, studies have found that certain types of parasitic worms can suppress inflammatory diseases such as asthma.
Jonathan Coquet, a co-author of the study, said the findings suggest that "it's not as simple as 'a dirty lifestyle prevents allergies, while a clean lifestyle may trigger allergies'. Instead, specific microbes may modulate allergic immune responses, but it's not clear which microbes are at work, or what exposures are beneficial."
Susanne Nylén, another co-author of the study, said the next step in the research will be to explore the specific impact of the parasite on the immune response of the mice. It's unclear whether lifelong exposure to certain microorganisms is necessary to suppress allergic reactions, or whether short-term exposure in early childhood confer additional benefits.
"This area of research can provide important insights into how infections and microbes can be harnessed to promote health, but is still in its infancy," said Nylén. "Our study is a reminder that general and widespread exposure to microbes may not produce the clear beneficial effects we hope for." ""
The new research was published in the journal Science Immunology.