New findings challenge ‘hygiene hypothesis’ behind allergy development
Does early childhood exposure to a broad variety of germs and bacteria make one less likely to develop asthma and allergies? A new animal study challenges this popular idea, finding diverse microbial exposure when young may have little effect on allergic immune responses.
In the late 1980s epidemiologist David Strachan proposed a novel hypothesis to explain the dramatic increase in rates of asthma and hayfever over the 20th century. Strachan observed a correlation between declining family size and prevalence of allergies. He suggested small families and improved hygiene standards had led to children being exposed to fewer microbes and infections when young. And this ultimately resulted in a decreased tolerance to allergens in later life.
The idea was dubbed the ‘hygiene hypothesis.’ Over subsequent years researchers proposed a number of iterations to the idea, from the ‘old friends’ hypothesis, which argues humans have co-evolved with bacteria over thousands of years to harbor microbial populations crucial for good health, to the more recent ‘biodiversity hypothesis’ that suggests our immune system requires contact with diverse natural environments in order to prevent allergies and inflammatory disorders.
A new study, published in Science Immunology and led by researchers from the Karolinska Institutet, has focused on testing the biodiversity hypothesis. The experiment took two genetically identical groups of mice – one group raised in a sterile lab environment and another group reared in semi-natural conditions with diverse microbial exposure.
The two groups of mice were then exposed to an array of known allergens in order to study their immune responses. The findings surprised the researchers, with both groups of mice developing similar inflammatory responses when exposed to the allergens. Corresponding author Stephen Rosshart says the findings don’t necessarily debunk the hygiene hypothesis but they do indicate immune responses to allergens are a little more complicated than previously assumed.
“I believe that it is the first proof-of-concept study confirming that diverse microbial exposures as well as infections are not the sole nor primary factors driving the dramatic rise of allergic diseases," Rosshart explained. "[The findings] may help to recalibrate scientists' view on the hygiene hypothesis, prompting the field to have a closer look at other factors such as indoor living, physical activity, pollutants and chemical compounds present in the modern world," he added.
The researchers do note in the study there is “incontrovertible evidence” demonstrating certain microbes can dampen allergic inflammation. Some types of parasitic worms, for example, have been found to suppress inflammatory diseases such as asthma.
Study co-author Jonathan Coquet says the findings demonstrate, “it's not as simple as saying, 'dirty lifestyles will stop allergies while clean lifestyles may set them off.'” Instead, it is likely specific microbes may modulate allergic immune responses but it’s unclear which microbes are doing this or what kinds of exposures are beneficial.
Susanne Nylén, another co-author on the study, says the next step for the research will be to explore the specific effects of parasites on mouse immune responses. It’s also unclear whether lifelong exposure to certain microbes is necessary to subdue allergic responses or whether short-term exposure while young confers extended benefits.
"This field of research can provide important insights into how infections and microbes can be used to facilitate health, but it is still in its infancy,” Nylén said. “Our study is a reminder that general and broad exposures to microbes may not have the clear beneficial effects that we wish them to have.”
The new study was published in Science Immunology.