Does dirt not protect against allergies after all?

New study raises doubts about hygiene hypothesis

Does playing in dirt really make children healthier and protect against allergies? The new study conducted by a team of researchers at FAU and the Karolinska Institute in Stockholm has now raised doubts about this assumption. The researchers have published their findings in the research magazine Science Immunology.

The assumption that contact with dirt in childhood trains the immune system and protects against allergies or related illnesses later in life is known as the hygiene hypothesis. This is based on the observation that there has been a dramatic increase in cases of allergies and autoimmune diseases such as asthma, hay fever or eczema, whereas people who have been brought up on a farm, for example, seem to suffer less from such diseases. The suspected cause: excessive cleanliness. If the immune system does not have to fend off external intruders, it turns on itself, with the known consequences.

Although the hygiene hypothesis has been commonly shared by researchers since 1989 and is supported by statistic health data, not enough experiments have been conducted to date to check its verity. In their experiment, the team of researchers, including the microbiomic professor Dr. Stephan Rosshart from FAU, compared a group of standard laboratory mice born and brought up under virtually sterile conditions with wildling mice. Wildlings are a new strain of laboratory mice developed by FAU researcher Stephan Rosshart.

Genetically, they are identical to standard laboratory mice but have the microorganisms of genuine wild mice and grow up in a semi-natural environment, in cages with hay and other materials, meaning that they are subjected to a microbial load similar to that encountered in the normal environment. The results were surprising: Both groups of mice reacted in a similar way to allergens. “The wildlings were not protected against allergens, as we had originally expected because of the hygiene hypothesis,” explains Stephan Rosshart. “On the contrary, they even developed more pronounced symptoms.”

Has the hygiene hypothesis been disproved? “Not directly,” says Stephan Rosshart. “But our study does shed a very different light on the hygiene hypothesis. It shows that exposure to a wide variety of microbes and infections are not the only factor, let alone the main factor, for the dramatic increase in allergic illnesses. The explanation must be more complex, there are probably very many different factors that have to be taken into account.

It is worth noting, for instance, that asthma cases have increased significantly among children in Europe and the USA since approximately the 1960’s, although today’s hygiene standards were developed before 1920 and a large number of people were already living in these conditions from then on.

“Our study will make a contribution to changing science’s understanding of the hygiene hypothesis and encourage researchers to investigate other factors such as living indoors, physical activity, toxins and chemical compounds in the modern world in more detail.

Further information

DOI: https://doi.org/10.1126/sciimmunol.adf7702

Background information on wildling mice: https://www.science.org/doi/10.1126/science.aaw4361

Prof. Dr. Stephan P. Rosshart
stephan.rosshart@uk-erlangen.de