The hygiene theory of acquired immunity and chronic periodontitis

It is now well established that periodontal disease results from the interaction of the host’s defense mechanisms with microorganisms, such as Porphyromonas gingivalis, Tannerella forsythia (previously T. forsythensis), and Treponema denticola, in the dental plaque biofilm.1 It has also been shown that there is a high degree of volatility with respect to the numbers of these organisms able to be detected within an individual over time; it seems that they are more widespread in the community than previously believed.2 It is now recognized that many people carry the organisms without manifesting disease progression. 2 In this context, it is clear that most people are in balance with their biofilm, and it is only when this balance is disturbed that disease results. Such disturbances may occur as a result of environmental influences leading to an opportunistic increase in the numbers of organisms, a depression of the host’s defense mechanisms, or both. Therefore, the innate susceptibility of the host reflects the interplay between the bacteria, the host’s immune system, and environmental factors.3,4 In this issue, Hujoel et al.5 report a study in which they related patterns of medication use over a 7-year period to the severity of periodontal disease. These investigators selected 12,631 individuals from a health maintenance organization database who had been diagnosed with early, moderate, or advanced chronic periodontitis according to the American Academy of Periodontology classifications.6 Hujoel et al.5 examined the prescribed medication use of these individuals over a 7-year period from 1996 to 2002. By examining the ‘‘fill rate’’ (i.e., the number of years out of 7 during which at least one prescription was filled), they were able to show that subjects with moderate or advanced chronic periodontitis had significantly less use for a range of respiratory medications (antihistamines, decongestants, and other cough/cold medications) and anti-infective medications (antibiotics and urinary anti-infectives) compared to those with early chronic periodontitis. Although the investigators were well aware of the limitations of their study, they speculated that this finding could be viewed within the context of the ‘‘hygiene theory of acquired immunity.’