The Hygiene Hypothesis

He began researching ulcerative colitis and discovered that the prevalence of inflammatory bowel disease — an umbrella term that includes both ulcerative colitis and Crohn’s disease — had increased markedly in the United States over the 20th century. Yet the disease was less common in the developing world. He learned that exposure to dirt and unsanitary conditions early in life seemed to protect against these and other inflammatory diseases later. And then he encountered an explanation for the correlations in the research of a scientist named Joel Weinstock.
Weinstock, a gastroenterologist now at Tufts University, thought that parasites were to blame. But it wasn’t their presence in the human digestive system that was causing the rise; it was their absence. To survive for years in another animal, parasitic worms, known as helminths, counter their hosts’ defenses. Because an out-of-control immune response against native bacteria was thought to drive inflammatory bowel disease, Weinstock’s insight was that parasites’ ability to disarm the immune system might prevent the disorder. The broader implication was that the disappearance of parasites — largely eradicated from American life in the early 20th century through improvements in sanitation — might have left our immune systems unbalanced, increasing our vulnerability to all types of inflammatory disorders. ...
The prevalence of autoimmune and allergic diseases has increased between two- and threefold in recent decades. Roughly one in five Americans has an allergic disease, ranging from seasonal hay fever to life-threatening food allergies. Roughly one in 13 has an autoimmune disease — a disorder in which the immune system tasked with our protection instead attacks our own bodies. These disorders often strike in the prime of life or earlier, causing decades of suffering.
- Moises Velasquez-Manoff, The ParasiteUnderground, NYT, June 16, 2016

According to the ‘hygiene hypothesis’, the decreasing incidence of infections in western countries and more recently in developing countries is at the origin of the increasing incidence of both autoimmune and allergic diseases. The hygiene hypothesis is based upon epidemiological data, particularly migration studies, showing that subjects migrating from a low-incidence to a high-incidence country acquire the immune disorders with a high incidence at the first generation. However, these data and others showing a correlation between high disease incidence and high socio-economic level do not prove a causal link between infections and immune disorders. Proof of principle of the hygiene hypothesis is brought by animal models and to a lesser degree by intervention trials in humans. ...
Diabetes has a very low incidence and may even be absent in NOD mice bred in ‘conventional’ facilities, whereas the incidence is close to 100% in female mice bred in specific pathogen-free (SPF) conditions. Conversely, infection of NOD mice with a wide variety of bacteria, virus and parasites protects completely (‘clean’ NOD mice) from diabetes [2].
- The ‘hygiene hypothesis’ for autoimmune and allergic diseases: an update, Okada, Kuhn, Feillet, and Bach, Clinical & Experimental Immunology, 2010

But Dr. Knip’s hunch is that children growing up in Russian Karelia early on encounter microbes that are absent in Finland. The takeaway, in his view, is this: The human immune system most likely anticipates a microbiome that more closely resembles Russia’s because, for most of human evolution, the world was, microbiologically speaking, more like Russian Karelia than modern Finland. When we don’t encounter the attendant stimulation early in life, the immune system can become unsteady. Thus, in the past half-century, as Finland became a modern state, the incidence of autoimmune diabetes more than quintupled.
- Moises Velasquez-Manoff, Educate Your Immune System, NYT, June 3, 2016

Scientists say they may have found a sort of magic ingredient to prevent asthma in children: microbes from farm animals, carried into the home in dust.

The results of their research, published on Wednesday in The New England Journal of Medicine, were so convincing that they raised the possibility of developing a spray to do the same thing for children who do not have regular contact with cows and horses.
- Gina Kolata, Barnyard Dust Offers a Clue to Stopping Asthma in Children, NYT, Aug. 3, 2016

Beyond the threat of drug-resistant illness, there is evidence of another risk from antibiotic overuse in pigs, poultry and cattle: the possibility that people who consume antibiotic-laced meat will get some of the drugs, as well as resistant bacteria, into their own digestive tracts — with potentially harmful results.

A growing body of scientific research also shows that antibiotics can disrupt our so-called gut microbiome, the bacteria that live happily in our stomach and intestines and that are the key to our ability to properly digest food and process fats. This disruption has been linked to the rise of noncommunicable diseases such as obesity, juvenile diabetes, asthma and allergies. Some researchers also believe that alterations in the gut microbiome have led to an increase in the incidence of autism, Alzheimer’s and Parkinson’s disease. ...
... there is wide agreement that antibiotic overuse in both livestock and in people is destroying our ability to fight certain diseases and infections.
- William D. Cohan, A Burger, Please: Hold the Tetracycline, NYT, May 25, 2018

Our modern germ-free life is the cause of the most common type of cancer in children, according to one of Britain's most eminent scientists. ...
Prof Mel Greaves, from the Institute of Cancer Research, has amassed 30 years of evidence to show the immune system can become cancerous if it does not "see" enough bugs early in life.
- James Gallagher, Missing microbes 'cause' childhood cancer, BBC News, 21 May 2018

See also:

Moises Velasquez-Manoff, An Epidemic of Absence, 2013

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