Early Studies

Epidemiology was learned and practiced empirically long before it was named. For example, people have known for ages that malaria is a disease of wet lowlands, so they have avoided living in such places. But there was little true understanding about conditions that led to disease. The periodic epidemics that swept Europe from the Middle Ages until fairly recent times, for example, were often seen as religious signs rather than as a result of squalid living conditions. It was from the more rational study of these epidemics that epidemiology evolved to its present form. Samuel Pepys, who wrote vivid chronicles of life in seventeenth-century London, used the Bills of Mortality, the forerunner of modern death certificates, to measure the onset and decline of a plague epidemic in London in 1665. Percivall Pott’s Treatise of the Chimney Sweep’s Cancer, in 1775, described the unusually high occurrence of scrotal cancer among chimney sweeps and is thus an early scientific description of an occupational hazard.

In 1854 John Snow, a medical practitioner in the Soho area of London, went so far as to control an outbreak of cholera by the application of his epidemiologic conclusions. His investigation took place some years before the germ theory of disease was understood, so Snow began by trying to identify the features common to those who died from the disease. After he mapped out the residences of those who had died (Fig. 14-1), his subsequent inquiries disclosed that all of the victims had used water from the same source. That source, in the days before indoor plumbing, was a pump on Broad Street (now Broadwick Street, where the site of the pump is currently occupied by a public toilet). Snow reached the rational conclusion that something in the water was responsible for the spread of the disease. Snow’s method of controlling the epidemic, still without knowing its cause, was to persuade the authorities to remove the pump handle. The epidemic soon subsided.

Fig. 14-1 John Snow’s map of the Soho area of London, showing deaths from cholera during the epidemic of 1854.¹⁸

(From Longmate, N. Alive and well: Medicine and public health, 1830 to the present day. Hammondsworth, Penguin, 1970.)

Snow’s subsequent investigations on the relation between cholera and the source of water supply are epidemiologic classics. The results of the patiently executed research of nineteenth-century workers such as Snow still benefit present-day society. Their investigations led to gradual but profound improvements in sanitation and personal hygiene, and the development of the public health codes for housing, water supply, and food processing that are now taken for granted. The fact that infectious diseases such as cholera, typhoid, yellow fever, plague, scarlet fever, and relapsing fever are now rare in high-income countries is not simply chance but is due largely to the pioneering work of these early epidemiologists. Their work continues today: our understanding of the mode of transmission of the human immunodeficiency virus and its translation into public health education to prevent acquired immunodeficiency syndrome followed remarkably quickly on the first identification of the virus in 1983.^21,30 That, too, is epidemiology.

The types of study design used in epidemiology were described in Chapter 13 and the various uses of the epidemiologic method in research are shown in Box 14-1.

The Concept of Measuring Disease

The good clinician thinks in qualitative terms. During a diagnostic examination the dental practitioner not only looks for existing disease but also tries to look ahead to the possibility of future disease. Measuring the oral health of a population, however, requires a more standardized and objective approach. Specific diagnostic criteria, written explicitly for clinical, radiographic, microbiologic, or pathologic examination, provide an objective framework for the practitioner’s judgment. These criteria are applied to judge the condition of the oral tissues as they are at examination time, not as they might be in the future. This objective application of diagnostic criteria is the most important philosophical difference between the epidemiologic examination and the examination carried out for treatment planning.

Measurement, the quantification of observations, is the crux of science. Measurement variability is inherent in all fields of science; it is one reason why laboratory experiments are repeated before their findings can be accepted. In studies of oral disease, a count of carious lesions in a population is almost never duplicated; a repeat examination of the same group of patients frequently results in a different count of carious lesions. Any one count of disease in a group is therefore an estimate of conditions, rather than absolute truth. As long as criteria are applied consistently, however, valid estimates will still result, because diagnostic drifts in one direction will be balanced by drifts the other way.

Acute diseases, such as measles, are characterized by a sudden onset of symptoms, so that the patient progresses rapidly from a state in which the disease is clearly absent to one in which the disease is clearly present. Remission of the acute phase of the disease is equally rapid, so little time is spent in the “gray areas.” Chronic diseases, however, are usually characterized by a much slower onset. It is difficult to establish exactly when arthritis, alcoholism, mental illness, dental caries, and periodontitis become definitely established; there is a considerable gray area. In dentistry this problem is handled by counting as lesions only those that meet specific criteria.