The occurrence of bacterial infection is a function of several major variables: (1) the virulence of the bacterial pathogen, that is, its ability to cause severe disease; (2) how the pathogen is transmitted to the “host”—for example, whether it is airborne, foodborne, blood borne, etc.; and (3) host susceptibility—i.e. how well the host can defend itself against the bacterial pathogen. Increased susceptibility, in turn, may result from two different processes: a bigger infectious dose in a given case of disease may cause a more severe infection, and physical characteristics particular to an individual host may render him or her less able to limit the spread of infectious microorganisms from the intestinal tract to the bloodstream.
Morbidity and mortality in the elderly from infectious disease is far greater than in other populations. For instance, death rates for infectious diarrheal disease alone are five times higher in people over 74 years of age than in the next highest group, children under four years of age, and fifteen times higher than the rates seen in younger adults. Published studies attribute the elderly’s heightened risks, both of infection and mortality due to enteric infectious disease, to several factors: (1) the aging of the gastrointestinal tract (reduced gastric acidity/reduced gastric mobility); (2) a higher prevalence of underlying medical disorders (co-morbidity factors); and (3) malnutrition and a decline in the immune response that leaves the host less able to defend itself against infectious agents.
1. Aging of the Gastrointestinal Tract—An Invitation to Infection
Inflammation and shrinkage of the gastric mucosa increase with age. These changes lead to low gastric acidity. In patients with gastric ulcer disease, the drugs used to treat the condition further block gastric acid production. Because stomach acids play an important role in limiting the number of bacteria that enter the small intestine, low gastric acidity increases the likelihood of infection if a pathogen is ingested with food or water.
Gastrointestinal mobility (peristalsis) decreases with age. Peristalsis, which is the mechanism that propels the stomach contents through the intestinal tract, is also the mechanical means for removing ingested, life-threatening pathogens. The risk of infection by potentially invasive pathogens corresponds with the duration of contact between the pathogen and the intestinal mucosa. Thus, a decrease in peristalsis delays the clearance of the pathogen from the intestinal tract and contributes substantially to the increased prevalence and severity of infection in the elderly. If the pathogen is E. coli O157:H7, decreased peristalsis allows the bacteria to multiply and produce more of the toxin that is absorbed in the gastrointestinal tract and leads to the aforementioned complications of an E. coli O157:H7 infection.
2. A Higher Prevalence of Underlying Medical Conditions—Co-Morbidity Factors
Underlying medical conditions or disease (co-morbid factors) also contribute to the morbidity and mortality of infection in the elderly. Among hospitalized patients, those older than 65 develop pneumonia twice as often as younger patients due to poor nutrition, neuromuscular disease (poor cough reflex and aspiration), pharyngeal colonization, depressed level of alertness, endotracheal intubation, intensive care unit admission, nasogastric tube use, and antacid use. Pneumonia is the leading infectious cause of death in the elderly.
Atherosclerosis, another common co-morbid disease, compromises circulation and blood flow to the peripheral tissues and the skin, particularly in elderly individuals who are hospitalized and bedridden with an infectious illness. Unfortunately, it is the skin and the previously discussed mucous membranes that serve as the body’s first line of defense against invasion by infectious microorganisms. Loss of the integrity of the skin may result in the development of pressure ulcers, which are warm, moist mediums for infectious microorganisms to rapidly multiply and are associated with a number of infectious complications.
When an infectious microorganism, regardless of source, gains access to the bloodstream, the patient may develop systemic sepsis, also know as bacteremia. Bacteremia is most common in people who are already affected by, or are being treated for, some other medical problem (co-morbid disease); conversely, people in good health with strong immune systems rarely develop bacteremia. The main sources of bacteremia in elderly patients are the urinary tract, gastrointestinal tract, respiratory tract, and the skin. Other potential sources include surgical wounds, invasive tubes and catheters, intravenous lines—virtually any site where an invasive medical procedure has occurred. Bacterial organisms most likely to cause bacteremia include members of the Staphylococcus, Streptococcus, and Escherichia coli genera. Because bacteremia is far more prevalent in those with co-morbid conditions, which group is substantially populated by the elderly, the presence of co-morbid conditions is clearly a determinant of the mortality associated with infectious disease.
3. A Weakened Immune System—the Inability to Fight Off Infection
With advancing age come progressive weakness, decline, and dysfunction of the immune system. Many of the body’s natural physiologic responses to infection are therefore blunted in the elderly; and the intensity of many clinical signs and symptoms in an elderly patient with an infectious process are muted when compared to those in a younger person. This age-related decline contributes significantly to the increased risk of severe illness and mortality in elderly persons with infectious disease. The effect of a weakened immune response on the health of an elderly person often manifests most apparently during periods of intense stress (e.g., surgery, sepsis, multiple organ failure, malnutrition, dehydration).
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