Epidemiology of Aging and Infectious Diseases

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Thomas T. Yoshikawa

1. INFECTIOUS DISEASES AND HISTORY OF MANKIND

Throughout recorded history, mankind has always faced the scourges of infectious diseases. Infections were the major causes of mortality prior to the modern era of antimicrobial chemotherapy, and even today, infectious diseases worldwide account for over one-third of the deaths. Until the mid-20th century, such diseases as typhus, plague, typhoid fever, cholera, diphtheria, smallpox, and tuberculosis caused major outbreaks of illnesses and accounted for deaths of millions of people throughout the world. Moreover, rheumatic fever, scarlet fever, measles, mumps, pertussis, poliomyelitis, and syphilis not only resulted in mortality but also caused disability, deformities, limitation in functional capacity, and social rejection (1). During the Civil War, infections caused more deaths than battle injuries—for both the Confederate and Union armies; similar statistics have been described for combatants in World War II (2). Poor sanitation, close contact, lacking in acquired immunity to diseases, and high stress levels left military combatants vulnerable to typhoid, malaria, dysentery, tuberculosis, smallpox, and measles.

With the establishment of the germ theory of diseases, medical advances followed, i.e., sanitation, public health measures, antisepsis, antibiotics, and immunization, that reduced the mortality and morbidity of infectious diseases after the middle of the 20th century. The modern era of antimicrobial therapy beginning in the last half of 20th century and vaccination successes of smallpox and poliomyelitis appear to herald the "conquering" of the lethal effects of infections (2). However, in 1981, the first cases of acquired immunodeficiency syndrome (AIDS) were reported by the Centers for Disease Control and Prevention, which in 1984 would be identified to be caused by the human immunodeficiency virus (HIV) (2). This infection has become a global health care problem, striking the young, old, rich, poor, men, women, and people of all ethnic and racial backgrounds. HIV infection and its complications have clearly become the single most publicized and (perhaps) important disease of recent modern times. Finally, newer pathogens have emerged during this past one or two decades, e.g., Hanta virus, Ebola virus, and herpes simplex type 6, as well as mutant strains of antibiotic-resistant organ-

From: Infectious Disease in the Aging Edited by: Thomas T. Yoshikawa and Dean C. Norman © Humana Press Inc., Totowa, NJ

isms such as methicillin-resistant Staphylococcus aureus, penicillin-resistant Streptococcus pneumoniae, vancomycin-resistant enterococci, and multiple drug-resistant gramnegative bacilli (3). The impact of these and other new and changing infectious disease agents on both young and old will be in the future the interest and focus of many infectious diseases specialists and clinicians caring for children, adults and the elderly.

2. DEMOGRAPHICS OF AGING

In 1900 the average life expectancy at birth in the United States (U.S.) was approximately 47 years (46 years for males and 48 years for females) (4). Only 4% of the total U.S. population was aged 65 years and older (5). With the reduction in childhood mortality, due primarily to infectious diseases, the average life expectancy dramatically increased during the latter half of the 20th century. Presently, the average life expectancy at birth in the U.S. is approximately 75 years (73 years for males and 80 years for females) (4). Furthermore, the elderly (aged 65 years and older) now account for approximately 13% of the entire U.S. population (5). It is anticipated that over the next 30 years, those persons 65 years and older will account for 21% of all Americans, with the older elderly (80 years and older) experiencing the most rapid growth based on percentage of elderly persons.

3. EPIDEMIOLOGY OF MORTALITY, INFECTIONS AND AGING

3.1. Causes of Death

Up until the beginning of the 20th century, half of the top 10 causes of death in the U.S. was attributed to infections. Children, unfortunately, were disproportionately affected. As stated earlier, with the advent of immunization, sanitation, public health practices, antisepsis, and antibiotics, many of the lethal infectious diseases were prevented or mitigated. With the reduction of childhood mortality, life expectancy drastically increased (see earlier discussion). Similarly, there were decreases in infectious disease deaths and complications in adults as well. With adults living longer, other diseases have now become common and prevalent. For the entire U.S. population, heart disease, cancer, and stroke are the most common causes of death. In the elderly population, these same three diseases hold the same level of prominence. However, in the elderly, pneumonia and influenza are the fourth leading cause of death; diabetes mellitus and its complications including infections is the sixth leading cause of death; and bacteremia is responsible for the ninth most common cause of death (6).

3.2. Common Infections in the Elderly

Although older persons are at greater risk for acquiring infections (see also Chapter 2,), there are little data to indicate that aging is associated with greater susceptibility to all infections. Whether aging alone versus age-related diseases (that adversely impact host resistance to infections) is responsible for vulnerability to infections remains controversial and unproven (7).

There are considerable data indicating that certain infections appear to occur more often in older persons and/or are associated with higher mortality and morbidity (8-11). These infections include lower respiratory infections, primarily bacterial pneumonia; urinary tract infections; skin and soft tissue infections including infected pressure ulcers; tuberculosis; infective endocarditis; sepsis with known and unknown causes; intraabdominal infections, primarily cholecystitis, diverticulitis, appendicitis, and ab-

Table 1. Common Infections in the Elderly and Comparative Mortality with Younger Adults

Infections

Mortality rate in elderly vs. young adult

Pneumonia Tuberculosis" Urinary tract infection6 Infective endocarditis Intraabdominal infection a

Cholecystitis

Appendicitis Bacterial meningitis Bacteremia/sepsis Septic arthritis

*Indicates that mortality rate is three times greater in elderly compared with young adult.

"Compared to nonhuman immunodeficiency virus-infected persons.

^Kidney infection.

scesses; bacterial meningitis; and herpes zoster (8). (Specific and in-depth details of these infections can be found in the appropriate chapters.)

The majority of these infections are associated with higher death rates in the elderly when compared with younger adults with the same diseases. Table 1 provides a summary of these findings (8,12,13).

3.3. Infections in Long-Term Care Setting

As mentioned earlier, it is anticipated that with the rapid increase in the aging population, there will be disproportionate growth in the very old segment of this group. With extreme old age comes frailty, cognitive impairment, and physical dependence. It has been stated that persons aged 65 years and older have approximately 45% risk during their lifetime to becoming institutionalized in a long-term care facility such as a nursing facility (nursing home) (14). Furthermore, frail elderly residents in nursing facilities are substantially vulnerable to infections because of age-related immune changes and diseases and physical disabilities. A closed, institutional environment also favors constant exposure to microorganisms from frequent contacts with personnel and other residents; limited ventilation, filtration, and removal of recirculated air, which could contain microorganisms; and unrestricted movement of infected residents (15).

It is estimated that approximately 1.5 million infections occur annually in nursing facilities in the U.S. (16). The incidence of infections has been reported to range from approximately 10 to 20 infections per 100 residents per month (17). The most frequently encountered infections in residents of long-term care facilities are lower respiratory infections (pneumonia most often), urinary tract infections, and skin and soft tissue infections (including infected pressure ulcers). These three infections constitute nearly 70-80% of nursing facility-associated infections (17). Moreover, fever is one of the most common reason residents of a nursing facility are transferred to an acute care facility. Furthermore, infections are often the cause of acute confusion or delirium in older persons (18). Thus, the presence of fever and/or an acute change in clinical/functional status of a resident in a long-term care facility should prompt a careful search for an infectious etiology. Finally, the increasing use of antimicrobial agents in residents of long-term care facilities has been associated with an alarming rise in mutant strains of bacteria resistant to a variety of antibiotic agents (3). Stringent adherence to infection control policies and measures and appropriate prescribing of antimicrobial agents will be necessary to prevent major outbreaks of life-threatening and untreatable infections. (See also Chapters 20 and 21).

REFERENCES

1. Lyons, A.S. and Petrucelli, R.J., II (1978). Medicine: An Illustrated History. Harry N. Abrams, New York.

2. Kupersmith, C. (1998) Three Centuries of Infectious Disease. An Illustrated History of Research and Treatment, Greenwich Press, Greenwich, CT.

3. Yoshikawa, T.T. (1998) VRE, MRSA, PRP, and DRGNB in LTCF: lessons to be learned from this alphabet. J. Am. Geriatr. Soc. 46, 241-243.

4. U.S. Dept. of Health and Human Services, Public Health Service, National Center for Health Statistics (1986) Health United States 1985. D.H.H.S. Publication No. (P.H.S.) 861232, Hyattsville, MD.

5. U.S. Bureau of the Census (October 1982) Decennial censuses of population, 1900-1980 and projections of the population of the United States: 1982 to 2050 (advance report). Current Populations Reports Series P-25, No. 922, Washington, DC.

6. National Center for Health Statistics (1996) Leading causes of death and number of deaths according to age: United States, 1980 and 1993. Health United States, 1995. DHHS Pub. No. (PHS) 96-1232, Hyattsville, MD, p. 108.

7. Beeson, P.B. (1985) Alleged susceptibility of the elderly to infection. Yale J. Biol. Med. 58, 71-77.

8. Yoshikawa, T.T. (1981) Important infections in elderly persons. West. J. Med. 135,441-445.

9. Yoshikawa, T.T. (1983) Geriatric infectious diseases: an emerging problem. J. Am. Geriatr. Soc. 31, 34-39.

10. Yoshikawa, T.T. (1997) Perspective: aging and infectious diseases: past, present and future. J. Infect. Dis. 176, 1053-1057.

11. Yoshikawa, T.T. (1999) State of infectious diseases health care in older persons. Clin. Geriatr. 7(5), 55-61.

12. Yoshikawa, T.T. (1994) Infectious diseases, immunity and aging. Perspectives and prospects, in Aging, Immunity, and Infection (Powers, D.C., Morley, J.E., Coe, R.M., eds.), Springer Publishing, New York, pp. 1-11.

13. Norman, D.C. and Yoshikawa, T.T. (1994) Infections of the bone, joint, and bursa. Clin. Geriatr. Med. 10(4), 703-718.

14. Kemper, P. and Murtaugh, D.M. (1991) Lifetime use of nursing home care. N. Engl. J. Med. 324, 595-600.

15. Yoshikawa, T.T. and Norman, D.C. (1995) Infection control in long-term care. Clin. Geriatr. Med. 11(3), 467-480.

16. Alvarez, S. (1990) Incidence and prevalence of nosocomial infections in nursing homes, in Infections in Nursing Homes and Long Term Care Facilities (Verghese, A. and Berk, S.L., eds.), Karger, Basel, Switzerland, pp. 41-54.

17. Yoshikawa, T.T. and Norman, D.C. (1996) Approach to fever and infection in the nursing home. J. Am. Geriatr. Soc. 44, 74-82.

18. Rockwood, K. (1989) Acute confusion in elderly medical patients. J. Am. Geriatr. Soc. 37, 150-154.

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