Allergy & Asthma Advocate: Winter 2004
The Hygiene Hypothesis
By Stanley Goldstein, MD, FAAAAI
“You can never
be too clean” is a well-accepted tenet of daily life. However, it may be
a cause for the increased incidence of asthma and allergies. This is
what is known as “The Hygiene Hypothesis.”
Our immune system is made up of two types of cells: TH1 and the TH2
lymphocytes. At birth, our immune system has a preferential number of
TH2 cells which, when stimulated, results in the development of allergic
disease, such as asthma and allergic rhinitis. However, according to the
theory, if our immune system is exposed to certain bacteria, known as
endotoxins, and viruses early in life, TH1 lymphocytes could be
stimulated, leading to the development a non-allergic or normal immune
system.
Researchers are beginning to believe that there may be a negative
consequence from too much cleaning, sterility and overuse of
antibiotics. They argue that our immune system is not as strong as it
would be if children were interacting more with dirt. Researchers are
not suggesting that Americans stop cleaning, but as they gather more
evidence, they might better understand how much and what kind of
cleaning will make us less allergic and less asthmatic.
The hygiene hypothesis was developed by epidemiologist, E.P. Strachen,
who wrote in 1989 in the British Medical Journal that the rise in
allergy cases may be linked to declining family sizes, reduced exposure
to germs and higher standards of cleanliness providing less exposure to
bacteria, which contain endotoxins. The under-stimulated, developing
immune system would then be stimulated by harmless substances like
pollen, house dust and animal dander, resulting in the development of
allergies. Since the 1989 report, the Center for Disease Control and
Prevention (CDC) reported that the prevalence of asthma cases in the
United States has doubled in the previous two decades from 6.8 million
to more than 15 million in 1998. About 5% of the US population has
asthma. The highest proportion is among children ages 5 to 14. The CDC
has also been reporting increasing prevalence of allergies for the last
four decades.
Articles appearing in medical literature have continued to support this
theory. The best true-to-life experiment supporting the Hygiene
Hypothesis comes from comparing the prevalence of allergies in the East
and West German populations before and after unification. Prior to
unification, East Germany had greater pollution, more children growing
up on farms exposed to farm animals and their feces and many growing up
in larger families. Prior to unification, the prevalence of allergies
was much lower in East Germany than West Germany. After unification,
East Germany developed a more western culture, resulting in decreased
pollution, less farming and more sterility. Since unification, the
prevalence of allergies in East Germany has been increasing and now
approaches that of West Germany.
A study appearing in the New England Journal of
Medicine in 2000 examined the prevalence of asthma in relationship
to the number of siblings in the home and whether they attended daycare
centers during infancy. They found that the presence of older siblings
at home and the attendance at daycare centers during the first six
months of life protected against the development of asthma.
A study appearing in the May 2000 issue of the
American Journal of Respiratory and Critical Care Medicine evaluated
1,200 rural Canadian teenagers. It concluded that those who grew up on
farms and were exposed to animals were 40% less likely to have asthma
than those who did not. Studies have shown a protective effect from
living on a farm, particularly if there is contact with poultry,
livestock or domestic animals.
What do daycare, siblings, farms and animals have in common? It may be
the endotoxins found in bacteria and in animal feces. The present
recommendation is not that children should be exposed to feces as an
antidote to developing allergies. However, this information will lead to
studies designed to learn how children should grow up with respect to
bacteria and virus exposure. The present data suggests that germ-rich
childcare facilities may not be so bad. In fact, they may be helpful to
some degree in immunizing children against allergies and asthma.
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