Allergies Parasites and the Hygiene Hypothesis

Allergies, Parasites and the Hygiene Hypothesis

The objective of this study is to explain the relationship between allergies emergence due to parasites based on the hygiene hypothesis and the current information stating how valid this hypothesis is. Towards this end, this study will conduct a brief but intensive review of literature in this area of inquiry. The work of Liu and Leung (2006) report that the hygiene hypothesis has a long history. Categories of allergy and asthma are stated to include specific categories as follows: (1) infections; (2) microbial components; (3) gastrointestinal colonization; (4) soil microbiota; and (5) forces that reduce microbial burden. (Liu and Leung, 2006, p.1063) Stated as fundamental to the hypothesis is "that the microbial exposure of interest can potentially cause both healthful and harmful outcomes." (Ibid, 2006, p.1063) Reported is that this dual way of viewing this is in defiance of the "simplistic tendency to view the relationship as either one way or the other." (Ibid, 2006, p.1063) Reported as the multidimensional determinants of health or disease outcomes in regards to microbial exposures include the following differentiations; (1) microbes or their components; (2) disease phenotypes; (3) elements of time; (4) dosage and exposure combinations; (5) contributions of genetics; and (6) exposure routes. (Ibid, 2006, p.1064) Maizels (2005) reports that evidence has been increasing that "helminth infections can protect the host against Th2-mediated allergic pathologies, even though helminthes themselves are strong Th2 inducers." (p.656) Allergies are reported as being traditionally held as "Th2-mediated inflammatory diseases, involving in the case of allergic asthma, eositophil infiltration into the airway epithelium, the release of long-lasting inflammatory mediators, the elaboration of goblet cells and their production of secretory mucitis, and incremental tissue damage and remodeling which causes longer-term compromise of airway function." (Ibid, 2005, p.656) Therapies that were based on a switching of the "allergen-specific response towards a Th-1 phenotype have met with some success in murine models, but less so in humans." (Ibid, 2005, p. 656) Interesting is that the Th-1 response can also result in inflammation of the airway. It is the opinion of Maizels that the human and animal data model indicates that Tseg cells "are activated in helminth infection and that cells of a similar phenotype can actively suppress airway allergic inflammation…" (Ibid, 2005, p. 656) The work of Schaub, Lauener, and Mutius (2006) reports that there has been a great deal of attention paid to the "advancement of one field in allergy research that investigates the potential link between exposures to microbial sources and the development of allergic diseases." (Ibid, 2006, p. 969) One example stated is that "urticaria is an easily recognizable skin condition, but the variety of factors eliciting these appearances range from infectious stimuli to allergic mechanisms to neoplastic illnesses." (Ibid, 2006, p.969) Bach (2002) states that it is a paradox that infectious agents "…can also suppress allergic and autoimmune disorders." (p.911) It is reported as reasonable to assume "that not a single cause but many will underlie the clinical manifestation." (Schaub, Lauener, and Mutius, 2006, p.969) It is held that "the increased exposure to viruses in a child's environment might foster a milder form of wheezing by suppressing the atopic component." (Ibid, 2006, p.969) This idea is given further validity by studies that investigated the effect of "day care and rhinitis exposure early in life which all showed a protection against atopy in the exposed children." (Ibid, 200, p.971) The work of Okada, Kuhn, Felett and Bach (2005) reports on the hygiene hypothesis for autoimmune and allergic diseases stating that 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." (p.1) The hygiene hypothesis is based upon epidemiological data, particularly migration studies that show that individuals migrating from low-incidence to a high-incidence country acquire immune disorders with a high incidence of the first generation." (Ibid, 2005, p.1) However, there is no causal link demonstrated between infections and immune disorders. (Ibid, 2005, p.1, paraphrased) Reported as well is that there are multiple underlying mechanisms and that these are complex in nature and incident "decreased consumption of homeostatic factors and immunoregulation, involving various regulatory t cell subsets and Toll-like receptor stimulation." (Ibid, 2005, p.1) It is believed that the origination of the mechanism may arise from microbiotia changes caused by changes in lifestyle, most specifically in inflammatory bowel diseases." (Ibid 2005, p.1) The work of Cooper (2009) reports that recent studies have found an association between helminth parasite infections and allergy." (p.29) Reported specifically by Cooper (2009) is that individuals who are exposed to helminth infection "may have allergic inflammatory responses to parasites and parasite antigens. Individuals with limited exposures to helminthes such as expatriates or recent migrants often develop allergic-type clinical manifestations, a probable host response to isolate and kill the parasites." (p.30) While Helminth antigens were found to "stimulate allergic inflammatory responses directed against the parasite in the human host and this inflammation may be actively suppressed during chronic infection. A distinct question is whether helminth infection may also modulate allergic inflammatory responses directed against nonparasite allergens such aeroallergens and affect allergic sensitization and the expression of allergic diseases." (Cooper, 2009, p.30) The work of Elston (2006) reports that the "prevalence of asthma is highest among children living on farms that raise swine and those that add antibiotics to animal feed. Similarly, a questionnaire-based study in Hanoi, Vietnam, found a positive relationship between allergic conditions and pig ownership." (p.172) Elston (2006) additionally reports "Helminths, unlike most bacteria and viruses, selectively stimulate Th2 function and are among the most potent stimulators of mucosal Th2 responses. In general, induction of type 2 cytokine responses imparts resistance to intestinal nematodes, whereas type 1 responses can exacerbate the infection." (p.174) Romagnani (2004) writes that rhinitis, asthma and atopic dermatitis result from systemic inflammatory reaction "triggered by type 2T helper (Th2) cell-mediated immune responses against 'innocuous' antigens or complex genetic and environmental origin" and that various epidemiological studies have indicated that "the increase in the prevalence of allergic disorders that has occurred over the past few decade is attributable to a reduced microbial burden curing childhood as a consequence of Westernized lifestyle (the 'hygiene hypothesis'). However, the mechanisms by which the reduced exposure of children to pathogenic and nonpathogenic microbes results in enhanced responses of Th2cells are still controversial." (Ibid, 2004) Reported as the initial interpretation proposed is a "missing immune deviation of allergen specific responses from a Th2 to a type 1 Th (Th1) profile, as a result of the reduced production of interleukin-12 and interferons by natural immunity cells which are stimulated by bacterial products via their Toll-like receptors." (Ibid, 2004) It is reported that the experimental evidence thus far indicates that both of the mechanisms are likely involved. The work of Ring, Kramer, Schafer and Beherendt reports that factors of modern society arising from the "…physical, chemical, biological or psycho-social environment play a role in the development of allergy. It is crucial to distinguish between causal factors and adjuvant/enhancing factors as well protective inhibitory factors that may be lost." (2001) The evidence presented in this study indicates that the hygiene hypothesis has great support in research findings as causative in allergy infection but that this is likely to be combined with some other explanatory and causative factor.