Is the hygiene hypothesis also relevant for autoimmunity?
The hygiene hypothesis states that the increasing incidence of diseases like asthma, eczema and allergy in the western world are linked to better hygiene and the elimination of childhood diseases. Is better hygiene also important for the rising prevalence of autoimmune diseases, like type 1 diabetes?
A study in Northern Europe hopes to shed light on this issue. In this EU-funded project, called Diabimmune, babies as well as children between three and five years old from Finland, Estonia and Russia are followed during three years. This collaboration between these counties is a unique advantage. The incidence of type 1 diabetes in Finland is the highest in the world. In Russia, where standards of living and hygiene are poorer, the incidence is six times lower. Estonia, being a rapidly developing country, makes up the middle. Therefore, these populations in close geographical proximity with comparable genetic susceptibility are ideal for this study.
The hygiene hypothesis is attributed to David P. Strachan upon his publication in British Medical Journal in 1989 describing that allergic diseases are less frequent in children from large families that are also more exposed to sources of infection. Epidemiological research supports this hypothesis. For example, children growing up on farms are less prone to allergy and asthma. In non-Western countries, the incidence is lower but migration to the Western world increases the chance of developing allergy. It is suggested that the ratio between Th1 and Th2 cells is important for the risk of developing an allergy. It is now recognized that the concept of reduced microbial exposure applies to a broader range of chronic inflammatory diseases and these probably include autoimmune diseases, like type 1 diabetes.
Mouse models and diabetes
Some evidence for a link between hygiene and type 1 diabetes has been found in studies with mouse models. Research by King et al published in Cell showed that mice that are sensitive to developing type 1 diabetes are less likely to develop diabetes when they are exposed to sources of infection than mice kept in a sterile environment.
Although the results of the Diabimmune study are not clear yet, the researches emphasize that even if the results are in line with the hypothesis it is certainly not the idea to make babies dirty on purpose. Instead, this research might lead to development of harmless pathogens, for instance in a pill, or a liquid, that can instruct the immune system and thereby prevent these diseases.
Sources: Youris, British Medical Journal, Immunology and Allergy Clinics of North America, and Cell