Concepts of Epidemiology
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Concepts of Epidemiology Differences among individuals are, of course, very important in determin- ing the diseases to which they are susceptible and their reactions to the diseases once contracted. But epidemiology, or the study of patterns of disease involving aggregates of people (Fig. 1.4), provides yet another important dimension. Information may be gained by examining the occurrence, incidence, prevalence, transmission, and distribution of diseases in large groups of people or populations.
clinic in Sweden. Given cultural variations that affect definitions of normal and abnormal, the resulting pattern of behaviors or clinical manifestations affects what the culture labels as illness.
Age Differences Many biological factors vary with age, and the normal value for a person at one age may be abnormal at another. Physiologic changes, such as hair color, skin turgor (tension), and organ size, vary with age. In general, most organs shrink; exceptions are the male prostate and the heart, which enlarge with age. Special sensory changes, such as severely diminished near-sight, high-tone hearing loss, and loss of taste dis- criminations for sweet and salty, are normal in an elderly adult and abnormal in a middle-aged adult or child. There are fewer sweat glands and less thirst perception in an elderly person than in a young adult or child. Elderly persons have diminished temperature sensations and can therefore sustain burn injuries—from a heating pad or bath water—because they do not perceive heat with the same intensity as do middle-aged adults. A resting heart rate of 120 beats per minute is normal for an infant but not for an adult.
Gender Differences Some laboratory values, such as levels of sex and growth hormones, show gender differences. The complete blood cell count shows differences by gender in hematocrit, hemoglobin, and red blood cell (RBC) count. For example, the normal range of hemoglobin concentration for adult women is lower than that for adult men—for adult women, the normal hemoglobin range is 12 to 16 g/100 mL of blood, whereas for adult men the normal range is 13 to 18 g/100 mL of blood. There are also gender differences in the erythrocyte sedimentation rate (ESR). Normally, in males, the ESR is less than 13 mm/hr; it is slightly higher in females. There are differences by gender in creatinine values too. For females, the normal serum creatinine level is 0.4 to 1.3 mg/dL; for males, the normal range is 0.6 to 1.5 mg/dL. Research into gender differences also suggests that, on average, males snore more; have longer vocal cords, better daylight vision, and higher metabolic rates; and are more likely to be left-handed than females. Research suggests, too, that females and males may have different communication styles and respond differently to similar conditions.
Situational Differences In some cases, a deviation from the usual value may occur as an adaptive mechanism, and whether the deviation is considered abnormal depends on the situation. For example, the RBC count increases when a person moves to a high altitude. The increase is a normal adaptive response to the decreased availability of oxygen at a high altitude and is termed acclimatization. A similar increase in the RBC count at sea level would be abnormal.
Time Variations Some factors vary according to the time of day; that is, they exhibit a circadian rhythm or diurnal variation. In interpreting the result of a particular test, it may be necessary to know the time at which the value was determined. For example, body temperature and plasma concentra- tions of certain hormones (such as growth hormone and cortisol) exhibit diurnal variation. Reflecting fluctuation in plasma levels, the peak rate in urinary excretion for a particular steroid (17-ketosteroid) occurs between 8 AM and 10 AM for persons who customarily rise early in the morning and is about two to three times greater than the lowest rate in the same people, which occurs between midnight and 2 AM, usually during sleep. The urinary excretion of ions (e.g., potassium) also exhibits diurnal variation. Fig. 1.3 illustrates circadian rhythms of several physi- ologic variables for persons living on a standard day-active schedule.\
FIG 1.3 Circadian rhythms of several physiologic variables in a human subject depict the effect of light and dark. In an experiment with lights on (open bars at top) for 16 hours and off (black bars at top) for 8 hours, temperature readings and plasma growth hormone, plasma cortisol, and urinary potassium levels exhibit diurnal variation. (Redrawn from Vander AJ et al: Human physiology, ed 7, New York, 1998, McGraw-Hill.)
KEY POINTS • Determining whether clinical findings are normal, abnormal, or a normal
variation is an essential, but often difficult, process in evaluating for the presence or absence of disease.
• Normal ranges for laboratory tests are typically defined as the mean ±2 standard deviations; thus 5% of the normal population may fall outside the normal range despite the absence of disease. Laboratory tests must be evaluated in concert with clinical information.
• The predictive value of a clinical test is the extent to which it can differentiate between the presence and absence of disease in an individual. Tests with high sensitivity and specificity generally have better predictive value.
• Variations in physiologic processes may be a result of factors other than disease or illness. Age, gender, genetic and ethnic background, geographic area, and time of day may influence various physiologic parameters.
• Trends and changes in a particular individual are more reliable than single observations.
CHAPTER 1 Introduction to Pathophysiology 7
Age. In one sense, life is entirely different during the 9 months of gestation. The structures and functions of tissues are different: they are primarily dedicated to differentiation, development, and growth. Certainly the environment is different; the individual is protected from the light of day, provided with predigested food (even preoxygenated blood), suspended in a fluid buffer, and maintained at incubator temperature. This is fortunate because the developing embryo or fetus has relatively few homeostatic mechanisms to protect it from environmental change. (The factors that produce disease in utero are discussed in Chapter 6.) Diseases that arise during the postuterine period of life and affect the neonate include immaturity, respiratory failure, birth injuries, congenital malformations, nutritional problems, metabolic errors, and infections. These conditions are discussed in separate chapters.
Accidents, including poisoning, take their toll in childhood. Infections in children reflect their increased susceptibility to agents of disease. Consideration of other childhood diseases is addressed in each chapter as appropriate and given separate consideration throughout the text. The study of childhood processes and of changes that occur in this period of life is the domain of pediatrics; specific diseases that occur during maturity (ages 15 to 60) are emphasized in this text.
The changes in function that occur during the early years of life are termed developmental processes. Those that occur during maturity and postmaturity (age 60 and beyond) are called aging processes. The study of aging processes and other changes that occur during this period of life is called gerontology. The effects of aging on selected body systems are so important physiologically that they also receive separate consid- eration throughout the text. The immune, cardiac, respiratory, musculoskeletal, neurologic, special sensory, endocrine, gastrointestinal, and integumentary systems are among those affected by the process of aging.
Ethnic group. It is difficult to differentiate precisely between the effects of ethnicity on patterns of disease and the socioeconomic factors, religious practices, customs, and geographic considerations with which ethnicity is inseparably bound. For example, carcinoma of the penis is virtually unknown among Jews and Muslims who practice circumcision at an early age (avoiding the carcinogenic stimulus that arises from accumulation of smegma about the glans penis).
However, comparisons reveal significant differences in the occur- rence of certain disease states in ethnic groups that seem to be more closely related to genetic predisposition than to environmental factors. For example, sickle cell anemia has a much higher rate of occurrence in African populations, whereas pernicious anemia occurs more frequently among Scandinavians and is rare among black populations worldwide.
The study of racial and ethnic group variation in disease states is the domain of medical anthropology. Disease-specific differences that relate to racial or ethnic group differences are a developing research frontier. In clinical practice, recognition of diversity in disease risk by racial or ethnic group is useful in disease diagnosis, prevention, and management. Ethnic group–specific differences, where important, are presented in individual chapters.
Gender. Particular diseases of the genital system obviously show important differences between the sexes; men do not have endometriosis, nor do women have hyperplasia of the prostate, and carcinoma of the breast is more common in women than in men. Pyelonephritis is more common in young women than in men of comparable age (before they develop prostatic hyperplasia) because the external urethral orifice of women is more readily contaminated, and bacteria can more easily travel up a short urethra than a long one. Less obviously related to the reproductive system, the onset of severe atherosclerosis in women is delayed nearly 20 years or more over that in men, presumably because of the protective action of estrogenic hormone.
Endemic, Pandemic, and Epidemic Diseases A disease that is native to a local region is called an endemic disease. If the disease is disseminated to many individuals at the same time, the situation is called an epidemic. Pandemics are epidemics that affect large geographic regions, perhaps spreading worldwide. Because of the speed and availability of human travel around the world, pandemics are more common than they once were.
Almost every flu season, a new strain of influenza virus quickly spreads from one continent to another. The 2014–2015 Ebola epidemic in West Africa provides an excellent example of an epidemic that spread worldwide, becoming pandemic. The first case was reported in Guinea in March 2014, and the disease spread in the neighboring countries of Liberia and Sierra Leone. Over the span of a year, the Ebola pandemic caused more than 10 times as many cases of Ebola than the combined total of all those reported in previous Ebola outbreaks. As the outbreak became more widespread, travel-associated cases appeared in Nigeria, Mali, Senegal, and countries outside Africa, including the United States.
Aggregate Factors Principal factors affecting patterns of disease in human populations include the following: (1) age (i.e., time in the life cycle), (2) ethnic group, (3) gender, (4) socioeconomic factors and lifestyle considerations, and (5) geographic location.
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FIG 1.4 A, The aggregate focus in disease: influence of crowds on disease transmission. Crowd gathered at a public market in Russia. B, Crowds gathered to purchase goods at a public market in Guangzhou, China. (Photographed by L-E Copstead.)
8 UNIT I Pathophysiologic Processes
hospital-acquired infections have become resistant to at least one of the drugs commonly used to manage them, largely attributable to the overprescribing of antibiotics. Staphylococcus, the leading cause of hospital infections, is now resistant to 95% of first-choice antibiotics and 30% of second-choice antibiotics. Poor hand hygiene is considered the leading source for infections acquired during hospitalizations. Unfortunately, efforts to convince health care personnel to reduce transmission of infection through practices as simple as more frequent and thorough handwashing have met with only modest success.
The incidence of many parasitic diseases is closely tied to socioeco- nomic factors and lifestyle considerations. Worm infections, for example, are related to the use of human feces as fertilizer. In some areas, such as parts of Asia, Africa, and tropical America, the frequency of schis- tosomiasis (a parasitic infestation by blood flukes) is directly related to the widespread use of irrigation ditches that harbor the intermediate snail host. There is adequate opportunity for transmission of schisto- somiasis because children often play in these ditches, and families wash their clothes in ditch water (Fig. 1.5).
Trichinosis, a disease caused by the ingestion of Trichinella spiralis, occurs almost entirely from eating inadequately cooked, infected pork. People who are fond of raw meat and inadequately cooked sausage are at highest risk.
Education is often very effective in changing lifestyle patterns that contribute to disease. In Tokyo, for example, mass public education about minimizing the use of sodium—a common ingredient in most traditional Japanese cooking—has been effective in changing dietary practices.
Examples of educational efforts directed at lifestyle modification in the United States are numerous. Antidrug, antismoking, and profitness messages fill the media and are prevalent on the Internet. Choosing
However, some gender-specific factors defy explanation. For example, systemic lupus erythematosus is much more common in women. Toxic goiter and hypothyroidism are also more common in women. Rheu- matoid arthritis is more common in women, but osteoarthritis affects men and women with equal frequency. Thromboangiitis obliterans (a chronic, recurring, inflammatory peripheral vascular disease) occurs more commonly in men. Gender differences in predisposition to cancer and other diseases, where relevant, are presented throughout the text.
Socioeconomic factors and lifestyle considerations. The environ- ment and the political climate of countries determine how people live and the health problems that are likely to ensue. The importance of poverty, malnutrition, overcrowding, and exposure to adverse environ- mental conditions, such as extremes of temperature, is obvious. Volumes have been written about the effects of socioeconomic status on disease. Sociologists study the influence of these factors. Social class influences education and occupational choices.
Disease is related to occupational exposure to such agents as coal dust, noise, or extreme stress. Lifestyle considerations are closely related to socioeconomic factors. Many people living in the United States, for example, consume too much food, alcohol, and tobacco and do not exercise enough. Childhood obesity is a problem in the United States and is becoming a global problem as well. Arteriosclerosis; cancer; diseases of the kidney, liver, and lungs; and accidents cause most deaths in the United States. By contrast, people living in developing nations suffer and frequently die of undernutrition and infectious diseases.
However, infectious disease is not limited to developing countries. The Centers for Disease Control and Prevention (CDC) estimates that 2 million people annually acquire infections while hospitalized, and 90,000 people die as a result of those infections. More than 70% of
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FIG 1.5 Risk factors for schistosomiasis include the widespread use of irrigation ditches that harbor the intermediate snail host. (Photographed in China by L-E Copstead.)
CHAPTER 1 Introduction to Pathophysiology 9
bedding, needles, syringes/sharps, or medical equipment) that have been contaminated with infected body fluids. Additionally, people can become sick with Ebola after coming in contact with infected wildlife. In Africa, Ebola may spread as a result of handling bushmeat (wild animals hunted for food) and contact with infected bats.
The World Health Organization (WHO) and the National Institutes of Health (NIH) have been deeply concerned with geographic problems in disease. Consult WHO and NIH home pages on the World Wide Web for additional information. (Web locations are provided on the Evolve website.)