There are a number of factors that increase the risk of infection in older adults when compared to young adults. The relationships between these risk factors, whether they are comorbidities, waning immunity, or age itself, may be very complex. For example, many older individuals have latent infection with Mycobacterium tuberculosis (i.e., asymptomatic infection) and do not manifest clinical illness despite an aging immune system and the presence of various comorbid conditions. However, superimposing malnutrition, perhaps caused by an intervening stress, may be the last insult necessary to tip the scales toward illness, resulting in clinical manifestations. This complex interplay of risk factors makes it difficult to determine the attributable risk of any one characteristic, and any risk factor in isolation cannot be considered “the cause” of infectious risk in the elderly. However, several well-recognized features associated with advanced age clearly do increase risk for clinical infection; these are reviewed in this chapter.
In the elderly individual, the increased incidence of infection and mortality for many infectious diseases (Figure 124-1) is likely a direct result of the comorbid conditions (e.g., diabetes, renal failure, chronic pulmonary disease, edema, immobility) that accompany advanced age. Comorbidity most often results in reduced innate immunity, defined as those responses that are not specific to a given organism or antigen. These include nonspecific barriers such as skin integrity, cough, and mucociliary clearance, and those immune responses triggered by recognition of patterns of microbial products (e.g., endotoxin, lipoteichoic acid) without the need for prior exposure such as complement, tissue phagocytes, and toll-like receptors (TLRs). Perhaps the best clinical example in which comorbidity contributes heavily to infection risk is chronic obstructive pulmonary disease (COPD). This disease, most often caused by prolonged exposure to tobacco smoke, has a high prevalence in older adults. The impaired mucociliary clearance, alveolar macrophage dysfunction, and suppressed cough mechanism that accompany COPD substantially increase the risk for lower respiratory tract infection in the elderly. Comorbid diseases in elderly individuals with infection can also be more important predictors for worse outcomes. For example, community-acquired pneumonia (CAP) is typically treated on an outpatient basis and rarely causes mortality in patients younger than 50 years of age. However, multiple comorbid conditions and advanced age greatly increase the risk of mortality associated with CAP. In fact, while age itself dominates many CAP prognostic indices, advanced age alone is not a predictor of mortality in those persons older than 75 years of age where comorbidity dominates. Furthermore, cognitive decline and other barriers that reduce adherence to medical regimens often necessitate hospitalization of older adults in circumstances where their younger counterparts are often treated as outpatients, further increasing costs and enhancing the rate of complications.
Multiple determinants model of infection risk in older adults. The relative contribution of host versus social factors varies by individual and it is the accumulation of multiple ...