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Wise et al.

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The most comprehensive collection of evidence evaluating a potential association between the microbiome and the development of allergic disease is from a recent systematic review by Melli et al. 444 Studies included in this systematic review compared intestinal microbiota of allergic patients with healthy controls. A total of 21 studies were noted to report an association between the intestinal microbiota and allergic disease when stool collection was performed prior to the outcome assessments. Only 4 of the analyzed studies had specific outcomes related to AR or sensitization. Penders et al. 447 found that the presence of Clostridium difficile at 1 month of age was associated with an increased risk for allergic sensitization (odds ratio [OR] 1.54; 95% confidence interval [CI], 1.09 to 2.31) until the age of 2 years. Adlerberth et al. 448 noted an increased ratio of gram-negative to gram-positive bacteria at 1 year of age to be associated with IgE levels greater than 100 kU/L at 1.5 years of age. Bisgaard et al. 449 found lower bacterial diversity was associated to higher risk of allergic sensitization ( p = 0.003) and AR ( p = 0.007). Johansson et al. 450 reported lower frequency of colonization with Lactobacilli and Bifidobacterium bifidum in allergic children. 15 Ultimately, Melli et al. 444 found that most of the studies linking the microbiome to the development of atopic disease were varied and difficult to interpret due to differing methodologies, samples sizes, and culture techniques. There are some thoughts that the composition and/or dysbiosis of the microbiota (viruses, fungi, and/or bacteria) of other sites such as the nasopharynx, lungs, and sinonasal cavities may also play a role in the development of allergic disorders. However, these studies are in their infancy and little can be concluded at this time. 451 A thorough understanding of the role of the microbiome and how it influences allergic disease has not been fully elucidated. Although some data suggest associations between allergic disease and the microbiota, based on the current evidence it is difficult to distinguish between protective microorganisms and those that increase risk for allergic disease. 446 Future research should provide an enriched and diverse understanding of the human microbiome and the way it impacts AR. A variety of population-based surveys have been used to estimate the prevalence of AR within the adult population. Prevalence estimates largely rely on self-reports of “hay fever” or “nasal allergies,” or of nasal symptoms “when you did not have a cold or the flu.” Questions on seasonality (to separate seasonal from perennial rhinitis) are sometimes asked, but there are few large-scale well-conducted population-based studies that have evaluated persistent (lasting more than 4 days/week for more than 4 consecutive weeks) vs intermittent symptoms. Because many surveys differ in terms of disease definitions, geography, and seasonality prevalence estimates drawn from surveys vary widely. One of the earliest studies, conducted in Tecumseh, Michigan, in 1959–1960 included a physician assessment and suggested that the prevalence of hay fever (diagnosed as “upper respiratory symptoms believed to be allergic in origin and occurring predominantly in either spring, summer or autumn”) was about 11% in those aged over 20 years. 452 About 20 years

Author Manuscript Author Manuscript Author Manuscript Author Manuscript

V. Epidemiology of allergic rhinitis V.A. Prevalence of allergic rhinitis in adults

Int Forum Allergy Rhinol . Author manuscript; available in PMC 2020 June 10.