Hygiene Hypothesis - Supporting Evidence

Supporting Evidence

The hygiene hypothesis is supported by epidemiological data, but there is currently no plausible explanation for the inverse relationship between infections and certain diseases. Studies have shown that various immunological and autoimmune diseases are much less common in the developing world than the industrialized world and that immigrants to the industrialized world from the developing world increasingly develop immunological disorders in relation to the length of time since arrival in the industrialized world.

Studies in mice have shown that exposure of young mice to viruses can result in a decreased incidence of type I diabetes.

In Cell : Homeostatic Expansion of T Cells during Immune Insufficiency Generates Autoimmunity they showed that when short lived T cells were replaced during a state of too few long lived T-cells (Memory T cell), because of lack of infections, the risk of developing autoimmune diseases will increase. They showed that in a state of too few long lived T-cells, because of lack of infections, not enough short lived T-cells could be produced by long lived T-cells during homeostatic expansion. Therefore, more auto reactive T-cells will divide in such a state, causing multiplying auto reactive T-cells with a greater risk of causing autoimmune diseases like type I diabetes or multiple sclerosis.

One conclusion is that a clean environment, with lack of infections (like early life infections) increases the risk of an autoimmune disorder.

T_h2 immune disorders such as asthma and other allergic diseases are probably related to the hygiene hypothesis. A baby has many T_h2 cells, which stimulate the production of antibodies. When not sufficiently stimulated with early life diseases, the immune system will have too many T_h2 cells present, leading to a greater risk of T_h2 immune disorder. If a child is exposed to infection diseases then the cell defense will be stimulated via T_h1 cells causing a reduction of T_h2 cells and subsequently a reduction of antibody stimulation by T_h2 and therefore a lower risk of developing an allergic disease such as asthma. Unfortunately, vaccination only uses the T_h2 mechanism.

In developed countries where childhood diseases were eliminated, the asthma rate for youth is approximately 10%. In the 19th century, asthma was a very rare disease.

Longitudinal studies in the third world demonstrate an increase in immunological disorders as a country grows more affluent and, presumably, cleaner. The use of antibiotics in the first year of life has been linked to asthma and other allergic diseases. The use of antibacterial cleaning products has also been associated with higher incidence of asthma, as has birth by Caesarean section rather than vaginal birth. However, the studies investigating these links showed only tenuous correlations between the factors described and the conditions they are hypothesized to cause.

Several pieces of experimental evidence also support the hygiene hypothesis. Work performed in the laboratory of Professor Anne Cooke at the University of Cambridge showed that mice of the NOD strain (which spontaneously develop type 1 diabetes mellitus) had a significantly reduced incidence of this disease when infected with the helminth parasite Schistosoma mansoni.

In November 2009 a group of researchers at the School of Medicine at University of California, San Diego, found that Staphylococci helped reduce inflammation.

A double blind study performed on 2500 pregnant women in Uganda showed that infants of the women treated with anthelminthic medication for worm infections had double the rate of doctor-diagnosed infantile eczema.