Monday, April 16, 2007

Hygiene hypothesis


I just did some quick poking about pubmed about the hygiene hypothesis. The hypothesis is that the increase in some diseases especially atopic diseases in some countries is a direct result of high quality hygiene. The diseases are type 1 diabetes, multiple sclerosis, Crohn's disease and allergic diseases (atopic diseases such as asthma). All have increased incidence in countries with high quality sanitation and an overactive immune system plays a role. The idea is that a developing immune system that is not exposed to enough innocuous antigens does not have sufficiently repressed TH1 and TH2 reactions to allergens and self-antigen. It sounds quite reasonable, however, it seems as though it is far from established. I found this decent review of the evidence for the hygiene hypothesis. Correlations such as the early use of antibiotics and incidence of asthma have to make you wonder if there is something to this hypothesis.


6 comments:

Bayman said...

Wouldn't one prediction of the hygeine hypothesis be that lab rats, mice and other animals living in near sterile conditons should have an increased incidence of these types of diseases compared to their counterparts in the wild? I've never heard or seen that to be the case but it would be interesting to really test that prediction. But maybe it's not hygeine generally, but rather specific agents administered to children like antibiotics as mentioned in the post, or vaccines, that contribute to these immunopathologies.

Bayman said...

So maybe they should start calling it the antibiotic hypothesis instead of the hygeine hypothesis. Who knows if it even has anything to do with pathogens? Maybe it's an off-target effect of antibioitcs that has nothing to do with its effect on bacteria. For example, by inhibiting Treg cell development...

Anonymous Coward said...

The answer as always is in the bayblab...

Hip said...

I think we should also consider the reverse: the UNhygienic Hypothesis.

Recent science is indicating that diseases like type 1 diabetes, multiple sclerosis, Crohn's disease, breast cancer, cervical cancer, prostate cancer, are caused by persistent viral or microbial infections.

Most of these virus or microbes are passed from human to human, just by normal social contact. We pick up many viruses from our early days at school, and we harbour them in our tissues for decades, sometimes, before they manifest as a disease like MS. For example, most of us will have picked up the herpes 6 virus by the age of three; this is thought to be a causal factor in MS. Prostate cancer has recently been connected to a gamma retrovirus, which may pass from human to human just via saliva. Many of these viruses will infect us without us knowing, as they enter our bodies asymtomatically.

Unfortunately, as we get increased urbanisation, which results in living in ever more crowded conditions, we get a daily exposure to more and more people and, therefore, to more contagious viruses, etc.

IN traditional, more rural life, there were far less people living on top of each other, compared to modernity. IN addition, modern global travel enables new viruses and microbes to circumnavigate the globe very quickly, to infect more people.

So rather than saying we are living in more hygienic environments, in fact the reverse is true: we are probably more exposed to disease-causing pathogens then ever before in history. This fact is more likely why we are seeing a rise in so many diseases. And we can expect this trend to continue as our social milieu continues to become more compressed and overcrowded.

rob said...

Those are actually good points Hip. Thanks.

Bayman said...

Yeah...in fact both "hypotheses" are probably true. For example, eradication of certain viral diseases ("hygiene") would free up new niches for novel "emerging" viruses to occupy as they "emerge".