Immune senescence can create an inefficient immune response. Clinically, immune senescence potentiates the colonization of the elderly, particularly the ill, by more
virulent microorganisms and sets the stage for more serious infections.¹

Several anatomic and physiologic changes occur with aging that compromise the ability to ward off infections. The first line of defense, the skin and mucosal lining, undergo important changes that predispose the host to a variety of infectious insults.²,³ The epidermis thins, resulting in a decreased production of Langerhans cells, interleukin-1, and thymocyte-activity factor. As a result, cellulitis and infected decubitus ulcers become more likely. Mucocutaneous tissue production of secretory Immunoglobin A (IgA), a predominant immunoglobulin of the mucosal immune system, does not appear to be reduced with aging.¹,²

A normal immune response is comprised of two independent processes. Innate (natural) immunity is comprised of neutrophils, macrophages, and natural killer cells, which do not require prior sensitization to respond to a foreign antigen.¹, ², ³ However, repeated exposure to a specific antigen does not enhance these cells’ response. The second type, acquired immunity, requires the activation of the immune system following the exposure to an antigen. The acquired immune response involves cells of lymphoid lineage, T cells (cellular immunity) and B cells (humoral immunity) stimulated to the production of a variety of antigen-specific compounds. Repeated exposure to the antigen enhances the immune response.³

Innate immunity does not appear to be greatly affected by aging. However, there are several changes that occur with acquired immunity. There is a decreased production of interleukin-2 (IL-2) and IL-2-induced T cells. The number of circulating and antigen-responsive B cells also decreases. Dysfunctional B lymphocytes produce less effective and fewer antibodies. As a result, the quality, quantity, and memory of antigen-specific molecules is adversely affected. These effects combine to blunt the response to vaccination, and increase the likelihood of infection in the elderly. Humoral antibody unresponsiveness accounts, in part, for the increased incidence and high mortality associated with pneumonia, influenza, infectious endocarditis, and tetanus in the elderly.⁴

Infectious diseases account for approximately one third of all deaths in individuals older than 65. Ninety percent of all deaths due to pneumonia occur in people older than 65.⁵ Influenza and pneumonia are the fourth leading causes of death in the elderly. Bacteremia is the ninth leading cause.

As the population ages and travel becomes easier, a greater variety of pathogens emerge as potential health concerns in the elderly. Hanta and Ebola virus, herpes simplex type 6, methicillin-resistant Staphylococcus aureus (MRSA), penicillin-resistant Staphylococcus pneumoniae (PRSP), vancomycin-resistant enterococci (VRE), and multiple drug-resistant, gram-negative bacilli (MDRGNB), Cryptosporidium, and severe acute respiratory syndrome (SARS) will trigger a larger portion of morbidity and mortality in the elderly. Compared to young, healthy adults, mortality from any specific infections in the elderly may be 20 to 25 times greater.⁶ Table 40.1 lists common infections seen in the elderly.

Infections in the elderly may present in an atypical, unusual, and nonclassical fashion, yet early diagnosis and treatment greatly lowers the morbidity and mortality in this population.⁷ The signs and symptoms of infection that are common in younger patients are frequently less obvious or even absent in older patients. Only 60% of older adults develop leukocytosis with a serious infection, yet the absence of such response does not rule out an infectious process. Nuchal rigidity may be absent in geriatric patients with bacterial meningitis and peritoneal signs may be absent in elderly patients with intra-abdominal infections. In all elderly patients with any infection, there may be a complete disassociation of the clinical signs and symptoms with the severity of the illness.⁷,⁸

There are several factors associated with the atypical presentation of infection in the elderly. Coexisting diseases may mask or alter the normal immune response to infection in the elderly while increasing the risk of infection. The inability of a patient to cognitively interpret signs and symptoms may be complicated by an inability to communicate. Normally, nonpathogenic or weakly pathogenic organisms may become virulent in the elderly. Lastly, an altered physiologic response can create an atypical presentation.⁹,¹⁰

Fever, like many physiologic functions, normally exhibits a circadian rhythm. Early morning values are lowest, 36.1°C (97.0°F), rising to temperatures of 37.4°C (99.3°F) in the late afternoon.⁵,¹¹,¹² This cyclic variation has two important consequences in the elderly. First, febrile responses associated with a disease state, although superimposed on the normal circadian variation, peak in the late afternoon or early evening. Therefore, the patient cannot be considered to be afebrile until the temperature pattern has been monitored for at least 24 hours. Second, small temperature elevations >37.0°C (98.6°F) are often recorded in healthy individuals. It has been recommended that a temperature elevation of 2°C (1.1°F) from the baseline temperature be used as a true indicator of fever (Evidence Level C). However, in the elderly, a core temperature of 37.8°C (100.0°F) associated with a decline of mental functioning is highly suggestive of an infectious process.¹¹,¹² A blunted or absent febrile response occurs in 20% to 30% of geriatric patients, particularly in individuals 75 years or older. The presence of fever should not be taken lightly, as up to 10%
of febrile patients older than 60 who present to an emergency department die within 1 month, compared to 1% to 5% of younger individuals. The clinician should also keep in mind that fever may occur as a result of noninfectious processes.

Although a thorough history and physical is the cornerstone for the management and treatment of patients, this may be very difficult in the elderly patient. The incidence of confusion and delirium increases in the infected patient and compromises the accuracy of history in many cases.¹³

The white blood cells (WBC) and band percentage have a high predictability of bacteremia (sepsis) in the elderly. Generally, a WBC count of >11,000 with a bandemia of 6% or greater is highly predictive of bacteremia.

A chest radiography (CXR) is useful to confirm the diagnosis of pneumonia, but the predictive value of a sputum Gram stain and culture is debated. Urinalysis is a simple test to obtain, but interpreting the results should be done carefully. Negative urinalysis results are more useful in excluding a urinary tract infection than positive results are in diagnosing an infection. Blood cultures from two separate sites should be obtained on all febrile hospitalized patients. Afebrile elderly individuals presenting with an acute functional decline and leukocytosis with bandemia or pulmonary infiltrate should also have blood cultures obtained.

Pneumonia, a major infectious disease in the elderly, is discussed in detail in Chapter 30.