The Hygiene Hypothesis, allergies and autoimmune diseases

What the hygiene hypothesis says is that our bodies are ecosystems that have been damaged by a rapid reduction in number and variety of microorganisms that populate our bodies and immediate environment. That alteration of those environments has been so rapid in genetic terms that we have not had time to adapt, and the consequence is the rise of autoimmune diseases and immunological disorders.

When one learns that we would die if all bacteria, molds, etc. were eliminated from our intestines, or that sixty percent of our stool is bacteria by weight, this becomes more obvious. Recent research into the function of the appendix hints that the associaton between us and the organisms we harbor may be even stronger than we now think. It has recently been proposed that the function of the appendix is to regulate our intestinal flora and fauna.

Viewed through the lens of the hygiene hypothesis the modern obsession or preoccupation with sterlity, equating it with cleanliness and goodness, is revealed for what it is. An unhealthy cultual artifact that arose as a consequence of the more immediate and apparent benefits from adopting modern sanitary practices and technologies. Eliminating typhoid and cholera has saved millions of lives in the aggregate since sewers and clean drinking water was introduced in North American and Western Europe for instance.

The idea that so radically altering our environments in this way should have unexpected and unwelcome consequences is perhaps more acceptable an idea now what we know how mankind has damaged the global ecosystem and some of the results of that.

While the results on a global scale are deforestation, extinction of species, global warming, acid rain, etc., on a personal level they are Crohn's disease, asthma, allergies, Multiple Sclerosis, Type I Diabetes, Sjogren's Syndrome, etc.

While no one wants to go back to elevated levels of infant mortatlity and reduced life spans as prevailed before modern sanitation was introduced, a more nuanced approach does seem necessary.

Perhaps we can accomplish with modern methods what used to be handled naturally by living in close proximity to the soil. By inoculating our children and ourselves regularly with benign, attenuated or weakend organisms so that we gain the benefits of exposure to these organisms (bacteria, viruses, molds and parasitess) without any morbidity or pathology. We get our cake and we eat it, too.

Background

Even a casual reader of newspapers and of web sites will have noticed a massive increase in the reporting of allergies and of immune and autoimmune diseases like peanut allergies, Crohn's Disease, asthma, multiple sclerosis and autism over the last ten or twenty years. But has there really been an increase in the incidence of these condtions or just in the reporting of them?

Rates of these conditions did increase rapidly, it wasn't just the news media blowing things out of proportion. But, epidemiologists suspected initially that heightened public awareness and improved diagnostic testing were the causes of the increase, not an actual rise.

Research has conclusively proven otherwise. Multiple studies have confirmed that seasonal rhinitis (hay fever) and allergic rashes, as wel as diseases like Crohn's and MS, are far more common today than previously, particularly in the industrialized world. Nonindustrialized countries and rural areas have lower rates, in some undeveloped areas allergies are still unknown.

In stark contrast to those undeveloped regions where seasonal allergies are unknown research has shown that in the USA seasonal allergies increased by 31% from 1985 to 1995. It shows that childhood peanut allergies doubled from 1997 to 2002, and that asthma has increased from 35 cases per 1000 in 1982 to 56 cases per 1000 in 1994.

Up to half of the people in developed countries in certain areas suffer from some form of allergy. Compare this to the fact that in 1967 a researcher went to Ghana to study asthma and found not one case, despite looking for three months.

Why the increase and why was Ghana free of asthma?

Pollen, dust mite feces, and animal dander account for approximately 90% of allergies, the vast majority of allergic reactions.

Sensitivity to food and its additives, to insects, latex, chemicals, industrial ingredients, and particulates in the air is responsible for the majority of the remaining 10%. But, because a person can develop an allergic response to anything doctors can find identifying the cause of allergy symptoms in patients tough.

Why the immune system mounts an attack against seemingly harmless agents is the cause of some debate amongst researchers, the issue is far from settled.

When the immune system treats a harmless substance or agent as a threat, B lymphocytes (a type of white blood cell or macrophage) manufacture an antibody to attack it. The antibody, or cytokine, they produce is immunoglobulin E (IgE). IgE attaches to mast cells such as those present in the nasal and bronchial passages in staggering numbers when this happens (billions of molecules in a sqare millimeter of tissue), resulting in massive histamine release when the antibodies bind to the agent they are targeting and cause the mast cell to burst, releasing massive quantities of histamines. Inflammation and allergy symptoms follow in the familiar and miserable way.

Once sensitized, an individual's mast cells automatically release histamine when the invading agent reappears. Because IgE can sensitize cells anywhere in the body, individuals' symptoms vary (eg, a cat may produce hives in one person and generate sneezing or asthma in another person).

The Increase in Allergies

That genetics plays a role in an individuals susceptibility to allergies and ot autoimmune disorders is well established. But a shift in the human gene pool is not the explanation for the increased prevalence of allergies because the change in the incidence of these diseases in the population has occurred in such a brief, genetically speaking, period of time.

Shifts in population-based traits require several generations to emerge; yet as is obvious from the statistics quoted at the beginning of this page we have seen massive increases in the incidence of these diseases within a decade or two. Crohn's disease was unknown until just before World War II for instance, but today affects 014% of the US population, or half a million Americans. In one study that illustrates how rapid this change in our reactions has been, for example, frozen blood samples were randomly selected from syphilis screening initiatives conducted in 1987 and compared with those randomly selected from a 1998 HIV screening. The presence of IgE to common allergens in the 1998 sample was double that of the earlier sample.

Two of the numerous theories proposed have empirical support, that is they are backed up by observation and testing, by research. They are the allergen exposure hypothesis and the hygiene hypothesis.

Allergen Exposure Hypothesis

Simply stated, increased exposure to allergens leads to increased sensitivity. Today, people eat diverse and exotic foods, many containing artificial preservatives and additives; spend more time indoors, increasing contact with dust, synthetic materials, and household chemicals; are more likely to maintain pets indoors; and are exposed to more pollution. The exposure theory is supported by the increased prevalence of allergies primarily in developed countries, and mostly in urban environments, as opposed to rural communities. This seems on the face of it to be true, however many of these factors have been true in at least pockets of the population for longer than many of our immunological disorders and allergies manifested. So this cannot be the whole explanation.

Hygiene Hypothesis - Why Ghana was free of asthma

This theory proposes that inadequate exposure to genuinely harmful agents leads to immune dysfunction. Under normal circumstances, the immune system is exposed to various viral, bacterial, fungal, or parasitic challenges and becomes properly calibrated to these real threats following successful defenses. Today's public health successes, coupled with the increased use of antibiotics and vaccines, minimize opportunities to mount successful attacks against genuine attackers. IgE is still present, however, and, in the absence of harmful agents, it begins attacking harmless environmental substances.

There is an enormous amount of scientific evidence for this. Children who have had early infections manifest far less atopy, allergy and autoimmune disease. Populations in which parasitic infestation is common manifest lower levels of hay fever and asthma, or as in the case of Ghana in 1967 none that could be found. People who have had measles have fewer allergies, as do children with multiple siblings, presumably because they have experienced far more childhood diseases because of exposure through their siblings.

Animal studies also support the hygiene theory; mice treated with antibiotics showed greater sensitivity to mold allergens, lab animals kept in sterile conditions develop Type I Diabetes. The hygiene hypothesis, however, cannot satisfactorily explain the high prevalence of allergies in poor inner-city housing, where potential exposure to harmful agents is increased. Similarly, total antibiotic sales do not correlate with shifts in the prevalence of allergies or autoimmune diseases.

So, for the Ghanaians of 1967 they were free of asthma becasue they lived and grew up in an environment much closer to the one we evolved in than that found in the sterile west. One filled with childhood disease, free of antibiotics and vaccinations and one in which parasites are common.