Aging and infectious disease

Definition

In the 2020s, persons sixty-five years of age or older made up between 16 and 19 percent of the American population and this percentage was increasing. Infectious diseases are the cause of one-third of all deaths in older adults. The most common infectious diseases among older people include urinary tract infections (UTIs), respiratory tract infections (RTIs), skin and soft tissue infections (SSTIs), and gastrointestinal tract infections (GTIs).

Risk Factors, Etiology, and Pathogenesis

As a person ages, their immune system weakens and becomes less effective (immunosenescence). Studies have shown that increasing age is associated with a decline in the number of (or with functional alterations in) CD8+ cells, naive T cells, and B cells, all of which are involved in fighting infections. Research has also revealed that aging and the pace at which individuals' immune systems decline do not occur in a linear, steady manner. Rather, human aging occurs in major waves with peaks around age forty-four and sixty. At these peak points, individuals become more likely to contract infections. Other causes may include the impact of other diseases (comorbidities) and a decline in bodily functions. Malnutrition may also play a role, as approximately 10 to 25 percent of older persons have nutritional deficiencies and up to 50 percent of older adults who are hospitalized have some kind of caloric or micronutrient deficiency. Malnutrition is a risk factor for infection, and infection can lead to malnutrition, particularly in older adults.

UTIs are one of the most common infections in older adults. Although urine is normally sterile, older persons are more likely to have bacteria in their urine (bacteriuria). Women over seventy experience a prevalence between 10 and 30 percent, while between 3 and 20 percent of men experience a UTI. Factors contributing to this increased bacterial colonization include reduction in bladder capacity, decreased urinary flow, incomplete voiding, prostatic disease in men, and prolapsed bladder and lower estrogen levels in women.

An indwelling urinary catheter, common among institutionalized and older adults, is another risk factor for UTIs, as catheters contain stagnant urine in a warm environment, which promotes the growth of microorganisms. Thinning of the urinary epithelium also contributes to increased bacterial colonization, particularly in women, as does a higher vaginal pH and deficiencies in vaginal and periurethral antibodies that occur with age. Regular urination and strong urinary flow are protective against infectious bacteria, but the aging bladder is less able to sense the need to void. Urinary flow rates are slower in older adults, and they are more likely to experience incomplete bladder emptying.

Escherichia coli is the main pathogen responsible for UTIs in women, but about one-third of older persons have polymicrobial infections, which are rarely seen in younger persons. Infection with multiple organisms is more common in catheterized persons.

RTIs such as pneumonia and influenza are the second-most common infections in older adults. The Centers for Disease Control and Prevention estimates that 70 to 85 percent of the deaths from influenza each year occur in individuals over sixty-five. Older people are at an increased risk relative to younger people because they frequently have deficiencies in protective airway reflexes (such as coughing) and mucus clearance. Decreased elasticity of the alveoli (air sacs in the lung), poorer lung capacity, smoking, and preexisting conditions such as chronic obstructive pulmonary disease (COPD) and congestive heart failure are also common risk factors for lung infections. Older adults are also more prone to active tuberculosis (TB) infections. Latent (inactive) TB is prevalent in all ages, but decreasing immune function with age can lead to the infection becoming active.

The epithelial cells of the skin, bladder, bronchus, and digestive system form a physical barrier to bacteria, fungi, and viruses that may become compromised with age. For example, the skin becomes thinner, dryer, and more easily breached, leading to a higher risk of skin infections. Skin also loses collagen over time, affecting the ability to resist trauma. Epidermal renewal time (the time it takes the body to make all new skin cells) increases from twenty days in younger adults to thirty days in older people, delaying wound healing and making wounds more likely to be colonized by microorganisms. Cellulitis, a bacterial infection often seen in the legs, is much more common in older adults, especially those with diabetes. Shingles are caused by the reactivation of the varicella zoster virus (chickenpox), which is dormant after the initial infection (usually in childhood) but can flare up in old age.

GTIs, including gastroenteritis and colitis, are also more common in older adults. In a 2020 study, 25 to 35 percent of individuals diagnosed with inflammatory bowel conditions were over sixty. Predisposing factors include pH changes in the stomach, decreased intestinal movement, and changes in the composition of the gut bacteria. The risk of gastrointestinal infections is also affected by the presence of Helicobacter pylori, which is found in more than 50 percent of older people. H. pylori causes chronic gastritis in about one-third of those infected, which can lead to lower acid levels in the stomach and a higher risk of infections from other pathogens. Treatment with antibiotics and proton pump inhibitors can change the composition of the stomach’s normal bacteria, which can lead to susceptibility to infectious organisms such as Clostridium difficile.

Other factors that increase the risk of infectious diseases among older adults include a higher likelihood of being bedridden, which increases the risk of pressure ulcers and subsequent skin infections, and more frequent institutionalization and hospitalization, which increase the risk of nosocomial (hospital-acquired) infections and higher exposure to pathogens in confined settings. In addition, older people are more likely to have comorbid conditions such as diabetes, cancer, and heart disease; both the diseases and their treatments (for example, chemotherapy) can weaken the immune system and lead to a higher risk for infections.

Symptoms

Older people often do not have the same symptoms associated with infections that younger people do. For example, the classic symptoms of infection include fever, inflammation, pain, chills, and vomiting. However, older people with infections often have nonspecific symptoms such as delirium, confusion, fatigue, loss of appetite, decline in function, mental status changes, incontinence, falls, or subnormal temperature. This atypical presentation may delay diagnosis and treatment, especially because the same symptoms are also present in noninfectious diseases in older adults. The average body temperature for older adults is often lower too, meaning that if a baseline temperature is unknown, a fever may be missed. In an institutional setting, cognitive comorbidities increase the risk of a missed infection. For example, about one-half of nursing home residents have dementia and are unable to describe symptoms at all.

UTIs generally cause symptoms such as an urgent need to urinate, increased frequency of urination, and pain. Fever may also be present. However, these symptoms may be hidden by preexisting incontinence. In some cases, delirium, confusion, and rapid functional decline are the main symptoms of a UTI, and these infections may even manifest with respiratory symptoms such as cough or shortness of breath. Diagnosis relies on symptoms, urinalysis, and urine culture, although older persons with symptomatic UTIs may have lower bacterial counts than younger persons: Although 105 or more colony-forming units (CFU) per milliliter (mL) of urine is the standard definition, bacterial counts in older adults may be only 102 to 103 CFU/mL.

RTIs can affect the nose, throat, airways, and lungs and are typically associated with cough, fever, weakness, sore throat, irritability, difficulty breathing, and aches and pains. Often, any type of RTI is attributed to the flu because the different types of infections are difficult to distinguish; other types of infections have not been studied as thoroughly. In persons with COPD, even a simple cold can cause an acute exacerbation, leading to hospitalization and even death.

SSTIs such as cellulitis generally present with redness, warmth, and swelling. The primary symptom associated with shingles is pain, and even after the infection clears, persons frequently experience postherpetic neuralgia, or nerve pain, which can last up to one year or longer.

GTIs are typically associated with gastrointestinal pain, diarrhea, fever, cramping, nausea, and vomiting. Diarrhea may be bloody in the case of E. coli infections but typically is not bloody among persons infected with C. difficile. As with other infectious diseases, GTIs may be hard to distinguish from other conditions in older adults, including incontinence, irritable bowel, or medication-induced diarrhea. Initial infection with H. pylori is associated with nausea, upper abdominal pain, vomiting, and fever lasting anywhere from three days to two weeks; after the original infection subsides, the bacteria tend to colonize the gastrointestinal tract, triggering subsequent gastritis episodes, unless treated.

Prevention and Treatment

UTIs may be prevented through personal hygiene, avoidance of catheterization wherever possible, and possibly certain nutritional approaches such as cranberry juice. Although asymptomatic bacteriuria is very common, the guidelines of the Infectious Diseases Society of America do not recommend screening for or treating the condition because of a lack of proof that doing so prevents future UTIs or reduces morbidity; in addition, overtreatment for asymptomatic infections may contribute to antibiotic resistance. In persons with symptomatic UTIs, existing catheters are removed, and the infection is treated with an oral antibiotic specific to the pathogen involved. If the infection is serious, intravenous antibiotic therapy may be required. Polymicrobial infections may require a broad-spectrum antibiotic.

The most reliable ways to prevent RTIs include smoking cessation and vaccination. In addition, vigilance on the part of healthcare providers and caregivers is required because symptoms can be subtle, particularly for TB. Vaccines for pneumonia and influenza are available and recommended for all adults over sixty-five and fifty, respectively. The United States has one of the world’s highest rates of influenza and pneumonia vaccination in older persons, although the rate was less than the government target of 90 percent.

Although the preventive efficacy of influenza and pneumonia vaccines is lower in older persons, vaccination has been shown to reduce the severity of cases in terms of length of hospital stays and number of mortalities. Some healthcare institutions maintain standing orders for vaccinations for older adults. This strategy removes the physician from the equation and allows pharmacists, nurses, and physician assistants to provide routine vaccinations after a simple screening. Treatment for RTIs varies from simple bed rest to complex antiviral or antibiotic regimens lasting weeks.

SSTIs are best prevented through awareness and good hygiene by older people and their caregivers. Pressure ulcers may be prevented by regularly repositioning persons restricted to their beds, using supportive devices and surfaces, and keeping skin hydrated. People with diabetes or poor circulation who are at higher risk of cellulitis can wear supportive stockings and keep the lower extremities elevated whenever possible to prevent swelling. A vaccine is approved for herpes zoster and is indicated for all adults fifty years and older, regardless of their history of zoster infection. The vaccine has proven both effective and cost-effective in older adults.

GTIs are best prevented through scrupulous personal hygiene, proper food-safety measures, and avoidance of antibiotics and proton pump inhibitors unless necessary. Institutional settings should ensure against transmission from infected persons, including visitors and staff, to healthy residents. Risk-based food-safety programs and ongoing food-safety education for staff are necessary. Treatment for GTIs includes hydration, supportive care, and the discontinuation of any antibiotic that may have caused the problem. Treatment with antidiarrheal agents is not recommended in infections related to C. difficile or E. coli. Oral metronidazole or vancomycin may be used to treat C. difficile infections. Alcohol-based hand sanitizers are not effective at killing C. difficile, so soap and water should be used if that is the infectious agent. Treatment regimens for H. pylori infections may include proton pump inhibitors, amoxicillin, clarithromycin, and metronidazole.

Impact

Infectious diseases are major causes of death, disability, morbidity, cost, and health-services utilization in older adults. The infectious disease hospitalization rate in the United States increased by about 12 percent from 1998 to 2006 and was about four times higher among older adults than among younger adults. As the COVID-19 pandemic infected millions across the globe, the number of hospitalizations of adults over sixty-five increased drastically between 2020-2021. Because of fear of contracting COVID-19, around 41 percent of Americans avoided medical care, leading to a 20 to 30 percent drop in reported non-COVID-19 illnesses.

UTIs accounted for 13 percent of infections in 2011 and 9.5 percent of infections in 2019. Among institutionalized older adults prior to the COVID-19 pandemic, prevalence ranged from 0.1 to 2.4 cases per 1,000 residents, and 12 to 30 percent of residents had a minimum of one UTI every year.

RTIs such as pneumonia, influenza, and chronic bronchitis are the fourth-leading cause of death in this age group, after heart disease, cancer, and stroke. An estimated one million individuals over sixty-five are hospitalized yearly. COPD, which is an umbrella diagnosis that includes chronic bronchitis and emphysema and is generally related to smoking, is found in more than 15 million adults in 2021, though it is more common in older persons. According to the CDC, COPD was the cause of 1.2 million emergency room visits, 42.9 deaths per 100,000 people, and 4.2 percent of all doctor visits in 2021. Most were a result of acute exacerbations of the disease, which are caused by viral, bacterial, or fungal infections in about two-thirds of cases. People sixty-five years and older make up more than 50 percent of cases of active TB in the United States, and nursing home residents have higher infection rates than community-dwelling older people.

Bacterial, viral, and fungal SSTIs that are common in older adults include shingles (herpes zoster), cellulitis, pressure ulcers, scabies, and chronic fungal infections of the nails (onychomycosis). Other SSTIs that have a higher incidence in older people include necrotizing fasciitis, methicillin-resistant Staphylococcus aureus infections of the skin, and surgical site infections. The incidence of herpes zoster in patients older than sixty-five has been estimated at 3.9 to 11.8 per 1,000 persons each year. It was estimated that approximately two-thirds of people seventy years and older have a minimum of one skin problem.

GTIs caused by H. pylori are common in older adults, and if left untreated, chronic infection with H. pylori can lead to gastritis, gastric ulcers, and even stomach cancer, which is one of the most frequent causes of cancer-related deaths worldwide. The incidence and severity of C. difficile-associated diarrhea began increasing in the 1970s when it was first identified. It became endemic to hospitals and long-term care facilities by the twenty-first century. An antibiotic-resistant strain associated with a high rate of recurrent infection was also later identified.

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