Part 1 BACKGROUND ON EPIDEMIOLOGY Each civilization, it has been said, makes its own diseases. The first half of the 20th century, with its many social, industrial, and economic changes, confirms that present civilization follows the time-tried pattern. A list of significant changes in the 20th century might include: Newly discovered and improved types of transportation; Development of new knowledge for infectious disease control; Increasing numbers of older people; and Emerging importance of chronic disease as a health factor. These changing patterns of living are universal. They contribute to the worldwide problem of disease in a wide variety of ways. The network of sources, causes, and carriers of disease is so intricate that it suggests the need for a special effort (a) aimed at defining the questions and (b) pinpointing the answers to diseases and disabling conditions that occur on a worldwide basis. One of the most interesting and hopeful avenues of approach to the control and eradication of today's major diseases is through world epidemiology. 1 A. EPIDEMIOLOGY DEFINED Epidemiology has been defined as a science of mass phenomena of diseases and human defects. It describes significant varieties of these phenomena in time and place. It draws upon statistical methods and theory, all phases of medicine, and the natural sciences to give a true picture of the occurrence, distribution, and types of diseases or defects. As an illustration, the epidemiological study of hypertension (high blood pressure) seeks-as in similar studies-to evaluate the influence of diverse genetic and environmental factors upon the occurrence and severity of the disease. No one has succeeded in demonstrating specifically what causes high blood pressure. Yet marked differences in the incidence and severity of the disease in different population groups support the hope that epidemiological studies may succeed in defining the causative factors. By contrast with clinical medicine, the unit of study in epidemiology is the population or group, not the individual. Deaths, or any other event, are studied only if information can be obtained or inferred about the population in which the events occurred. The population may be a small and well-defined group in a municipality, may include a whole country or even an entire continent. The epidemiologist, therefore, as opposed to the clinician who deals in cases, is concerned with cases as they occur in their population. He may start with a 1 Other terms frequently used to denote this general discipline, but implying somewhat differing meanings are medical geography and ecology. The more accepted term, epidemiology, is, however, used throughout this publication. Ecology is referred to on p. 11. population and find the cases in it, or he may start with a number of cases and refer them to a population or group that can be taken to represent a population. In addition to observing the occurrence of cases, the epidemiologist must be keenly aware of the importance of variations in human traits and must understand what effect environmental factors might have on individuals under his observation. B. USES OF EPIDEMIOLOGY Epidemiology was originally the study of epidemics-usually of one particular disease. From those early studies it is possible to establish a framework of questions to seek out the characteristics of the disease: where and when it occurs, the frequency and intensity of the occurrences, the persons most likely to be affected, and whether such conditions as climate or weather are a factor. Armed with the answers to these and other questions, which together describe the pattern of the disease, it is then possible to advance theories about the source and mode of spread of the disease and to put these theories to the test by clinical, field, or laboratory study. Finally, it may be possible to establish a logical plan for the control of the disease under study. One of the principal uses of epidemiology is to discover populations or groups with high and low rates of disease, in the hope that causes of disease and of freedom from disease can be found. The epidemiological approach to a search for the cause of a disease may often suggest steps that may be taken for prevention and control. Thus, it is possible to reduce the impact of a major disease before the exact nature of the disease is understood. C. EXAMPLES OF HISTORICAL CONTRIBUTIONS John Snow established an important landmark in epidemiological research with his conclusions about the source of cholera. Long before bacteria were discovered, the features of cholera epidemics had been explored thoroughly, and the existence of minute living causative organisms had been theorized. In 1854, Snow through masterly detective work in the Broad Street pump epidemic in London, held that a contaminated water source was responsible for the outbreak. Later, the well-known Hamburg-Altona epidemic of 1892 indisputably established the theory of waterborne disease. Smallpox Edward Jenner, an English physician, observed that milkmaids throughout the countryside who contracted cowpox while milking were subsequently immune to smallpox. In 1796, he vaccinated against smallpox by transferring material from a cowpox pustule on the hand of a milkmaid to the arm of a small boy. Six weeks later the boy was inoculated with smallpox and failed to develop the disease. Although it was to be many years until the virus of smallpox was discovered and the exact nature of this highly infectious disease understood, the epidemiological method pointed to an effective means for curbing the spread of smallpox. By 1800 the scientific basis of this method, still in use today, was firmly established. Childbirth fever Ignaz Semmelweis, a Hungarian obstetrician, noted that the death rate of women at childbirth was higher in certain clinics where the medical students and physicians came directly to the wards from the morgue or gross dissection room. After making other observations, he concluded rightly that puerperal fever was an infectious disease. He required physicians on his own wards to clean their hands scrupulously before examining patients, and the mortality dropped immediately. Unfortunately, his views met with strong opposition and were not accepted for 20 years after his death. Typhus Charles Micalle, French physician and director of the Pasteur Institute, observed that even though typhus patients were lodged in a common ward, no other patients caught the disease. He also noted that their families and the hospital admitting personnel who relieved them of their clothing caught the disease. He reasoned that the disease agent was something attached to the skin or to the patient's linen-something which yielded to soap and water. He concluded that the carrier could only be the body louse, which he verified by reproducing the malady in animals in 1902. D. HISTORICAL SUMMARY OF USES OF EPIDEMIOLOGY A British scientist2 has summarized some of the classic illustrations of epidemiological research as follows: The main use of epidemiology is to discover populations or groups with high rates of disease, and with low, in the hope that causes of disease and of freedom from disease can be postulated. Examples can readily be drawn from the riches of classical epidemiology. The observations on nutritional deficiencies (scurvy, goiter, beri-beri, pellagra). The experience of peoples in relation to climate and season, "the epidemic constitutions of the atmosphere" (malaria; or pellagra), and to geographic and geological features (iodine deficiency). "Dangerous trades," and the industrial pulmonary diseases or the occupational cancers (epithelioma of the skin from cutting oils, and lung cancer associated with asbestosis are among the recent ones identified). The "unhealthiness of towns" (malaria, dysentery; pellagra; respiratory disease). The writer, director of the social medicine research unit of the Medical Research Council, London Hospital, adds: When affected groups are thus characterized by their environment, their living conditions and special ways of life, the epidemiological method is in fact beginning to unravel "causes" of disease, causes about which it may be possible to suggest and do something by way of control and prevention. This may happen before the intimate nature of the disease is understood, as in the achievements of Snow and the cholera, Takaki and beri-beri, Goldberger and pellagra. "Ways of life" can often be described only in simple terms, and the causes of health and disease postulated in them may therefore be in simple terms of the satisfaction of elementary human needs, of gross hazards and trauma. "Ways of life" as they are presently known, are thus often "general" factors, causes of disease or of diseases, rather than of any specific disease. Such are the relations of purity (and abundance) of water supply to bowel infections of many kinds― and not merely the cholera. Or of living space with respiratory infections-as a class. And the manifold connections of income levels with nutrition, and with child growth, development and health. These "general" principles include much of what we know of healthy living. Likewise, it may be possible to define the groups being studied only in broad biological and social categories. But simple studies should not on that account be dismissed. The history of the search for means to prevent disease shows how the reconnaissance and the large scale map, and courageous generalisations from these (the "filth" theory for instance), can be of value. Many of the illustrations in the paragraph above are of this nature. Morris, J. N., "Uses of Epidemiology," E. S. Livingstone, Ltd., Edinburgh and London, 1957, pp. 61-63. With the development of clinical and laboratory diagnosis, of statistics and survey methods, epidemiological inquiry into the causes of disease is increasingly being refined. E. CHANGING WORLD PATTERNS OF DISEASE The conquest of many of the infectious diseases in the We tern World, thanks to epidemiological and many other scientific economic and social advances, has resulted in a dramatic change in the patterns of disease. This change was alluded to in Committee Print No. 4 entitled "The United States and the World Health Organization" (p. 24ff.). The change can be clearly noted in facts such as the following: In 1900, the 10 leading causes of death in the United States included five infectious diseases; the current list has only one. First place in 1900 was occupied by influenza and pneumonia, and second place by tuberculosis; by 1956, influenza and pneumonia had dropped to sixth place and tuberculosis no longer appeared among the 10 leading causes of death in the United States. (Source: National Office of Vital Statistics, Public Health Service: Summary of Health and Vital Statistics, June 1958, pp. 11, 12.) The community problems of the present, as measured in terms of death, are resident in diseases not communicable nor caused by a specific infectious agent. They include heart disease, the cancers, diabetes, and others. The main direction of effort in epidemiology is still toward communicable diseases. But it becomes increasingly evident, as the following pages of this print will show-that a limitation of activities to the infections or to a closely circumscribed area is no longer warranted. That conviction is supported to such an extent that it may now be accepted as a principle, applying with little reservation to the countries of North America and Western Europe. This line of thought reflects the altered conditions of the new world. There should, however, be no misunderstanding that this view implies any slackening of effort against the communicable diseases, which are still the major problem in many areas of the world. F. LIFE EXPECTANCY CLIMBS Along with the changing pattern of disease in the United States, from emphasis on infectious or communicable disease to the so-called chronic diseases, there has been an accompanying increase in the proportion of older people. This pattern is reflected in many other countries of the world in varying degree (fig. 1). In Puerto Rico, for example, life expectancy at birth has increased from 46 years in 1940 to 68.3 years in 1955. In Ceylon, the increase has been from 42.8 years, as late as 1946, to 59.9 years just 8 years later in 1954. Even in India, life expectancy has gone from 26.7 years in the 1920's to 32.1 years in the 1940's. (Source: Statistical Bulletin, Metropolitan Life Insurance Co., April 1958.) The pattern is repeated around the world, and understandably more clearly defined in the better developed nations of Scandinavia, Europe, and the Western Hemisphere. A great part of the earth's population now lives more than 50 years, including that in Central America; the southern half of South America; Ecuador; British Guiana; Spain; Portugal; Eastern Europe; Turkey; the Sudan; South Rhodesia; Ceylon; Thailand; Malaya; and the Philippines. Those areas where persons live to be 65 and over include Russia, Sweden, the Netherlands, the United Kingdom, Denmark, the United States, New Zealand, Israel, Australia, and Norway, which tops the list with a life expectancy of 73. (Source: U.N. Statistical Office.) Concurrently there have been sharp increases in the total numbers of older people in proportion to total populations. These conditions, if ignored, have the potential for posing serious international economic, social, and even political problems; for unless the aging populations are kept productive, through better health, as well as through vocational and avocational opportunities, they can become a weight on their homes and families, communities, and central governments. The genetic factor, climate, occupation, radiation, food and the sanitary standards, general health of the community, social mores, and religion, all are considerations in the worldwide study of aging. At present it is not certain what part these considerations play in the process of aging, but we do know that these conditions vary widely in different parts of the world, and that there are variations in the longevity of different peoples. |