A new paper by a British-Kazakhstan-Armenian group nicely summarizes why we need good hypothesis papers.
As a deterring example they use the hygiene hypothesis
the hygiene hypothesis that was originally proposed by David Strachan in 1989. David Strachan studied the epidemiology of hay fever in a cohort of 17,414 British children and concluded that declining family size and improved personal hygiene had reduced the chances of cross infections in families, resulting in epidemics of atopic disease in post-industrial Britain. Over the past four decades, several related hypotheses have been proposed to expand the potential role of symbiotic microorganisms and parasites in the development of human physiological immune responses early in life and protection from allergic and autoimmune diseases later on.
I agree with the description although David never claimed to be the first author writing about the hygiene hypothesis. When I spoke to him the last time London he wasn’t even happy with this popularity.
The chain certainly started also much earlier than 1989 with “continuing activity of an immune system made redundant by man’s cleanliness” (Godfrey, 1975) continued with Gerrard 1976, Frick 1986, Busse, 1989 until David Barker argued in 1985 that there is a “decreased incidence of infection among children, especially in wealthier families, that changed their pattern of immunity”.
The harm inducted by the hygiene hypothesis is described as
The misunderstanding of the hygiene hypothesis that primarily aimed to shed light on the role of the microbiome in allergic and autoimmune diseases resulted in decline of public confidence in hygiene with dire societal implications, forcing some experts to abandon the original idea. Although that hypothesis is unrelated to the issue of vaccinations, the public misunderstanding has resulted in decline of vaccinations at a time of upsurge of old and new infections.
I fear the authors are right – some people may have been harmed by the hygiene hypothesis leading them back in the pre-Semmelweis era.
A new paper in PLoS Negl Trop Dis this week reports
The hygiene hypothesis is accepted by many in the global scientific community […] However, aspects of this hypothesis are based on assumptions that may not fully account for realities about human helminth infections. Such realities include evidence of causal associations between helminth infections and asthma or inflammatory bowel disease
Talking about realities may help proponents of the hygiene hypothesis who are stuck in a never ending loop publishing observational studies.
Even the NEJM contributes to the hygiene hysteria hype this week. Although the allergic rhinitis prevalence isn’t so much lower in old order Amish (who are even a heterogenous group), the difference to Hutterite is being highlighted. I do not even understand the study design here as it is neither cross-sectional, nor case-control nor cohort study. Is it just an exposure description in two different groups? Conclusions like
sustained microbial exposure was also reflected in the phenotypes of peripheral innate immune cells in the Amish.
are strange if we believe that we humans carry more bacterial than human cells. And every smoker encounters a 120fold endotoxin concentration compared to ambient air – without getting rid of asthma or allergy.
The results of our studies in humans and mice indicate that the Amish environment provides protection against asthma
is certainly wrong – nobody knows if this is an environmental or genetic or iatrogenic factor. The key finding is IRF7 expression but unfortunately IRF7 plays a critical role in the innate immune response against viruses – and not bacteria/endotoxin…
The hygiene hypothesis usually assumes an “underemployed” system that is directed against self-defined allergen targets – a rather mechanistic view of a half filled barrel. A new Nature paper on memory CD8 T-cells now explains why this view is rather odd Continue reading Another challenge of the hygiene hypothesis
The authors put a question mark at the end of the above statement while I would not hesitate to put an exclamation mark there. Writing this as a comment to a new study in the IJE they summarize the evidence that the “epidemics” of asthma in Western countries has begun to decline – as hygiene standards are not declining this might indicate the end of the hygiene hypothesis. Continue reading The end of the hygiene hypothesis