Counting out pestilence
TRAFFIC cops in Ahmedabad run risk of cancer; Silicosis among refractory plant workers in Orissa; Radiation hazards haunt Geological Survey of India; Asthma in Rajasthan -- Ever wondered what runs common in these headlines? A close look will reveal the theme running through them -- the identification of a group of people who are at risk of being struck down by a particular disease, on the basis of "epidemiological studies".
Epidemiology studies the incidence and distribution of diseases and their control and prevention. Its distinguishing feature is the final numerical measurement of a particular disease in relation to a "population at risk" -- the group of people, healthy or sick, who would be counted if they had the disease. This might give the impression that risk calculations in epidemiological studies are drawn from purely clinical, observed data. In practice, however, things are different.
The notion of a "target population" is implicit in any epidemiological investigation. While clinical observations determine decisions about individuals, epidemiological observations determine decisions about groups, however heterogenous they might be. Epidemiology, in fact, thrives on heterogeneity. For instance, if everyone uniformly smoked 20 cigarettes a day, the link between smoking and lung cancer would never have been detected.
Epidemiology makes it imperative to have pre-defined and standardised methods of measuring the chosen criteria. If 2 interviewers are sent to a person at risk, one noting the ordinary clinical history and the other using a different questionnaire, they might arrive at different prevalence estimates. Cases are defined not by theoretical criteria, but in terms of responses to specific investigative techniques. As a result, epidemiological case definitions are narrower and more rigid than clinical ones.
Studies in possibility Prospective studies, rather than retrospective or cross-sectional ones, have been found to be more reliable. In a prospective study, a group of several thousand people in a specified area -- who are at risk of later developing a common disease -- have their medical histories examined. Their health is monitored over 5 years or longer. If one of the variables in the study is significantly associated with the early development of disease X, it could be concluded that the higher the value of the variable the higher the "risk factor".
The surveillance over time of the increase or decrease in the incidence of a particular disease, and the changes in its distribution, is one of epidemiology's major tasks: for instance, a decline has been noticed in the incidence of small pox and bubonic plague but the incidence of hypertension, heart attacks, backaches and headaches is on the rise.
Epidemiological studies have also revealed an interesting trend, often called the "epidemiological transition", by which the pattern of mortality and disease changes from high mortality among infants due to natural factors like famine to human-made diseases like hypertension. The epidemiological transition at the turn of the century is generally associated with rising standards of living, nutrition and sanitation.
However, epidemics in the Third World have been found to be more or less independent of internal socioeconomic development: the control of epidemics is more closely linked with organised healthcare and disease control programmes. Advances in public health policies have ensured a rapid decline in mortality rates in developing countries in this century.
Air, water and... The consolidation of epidemiology as a scientific field of enquiry has resulted in systematic improvements in healthcare facilities and public health policies, but its evolution has been very gradual. It was Hippocrates in the 5th century BC who first attempted to establish a systematic relationship between human diseases and the environment in his book Air, Waters, and Man. The book remained the only theoretical basis for understanding endemic ("persisting in a particular locality") and epidemic diseases ("affecting a number of people within a relatively short period"), until the new sciences of bacteriology and immunology emerged in the 19th century.
The devastating spread of the Black Death (bubonic plague) in Europe in the 15th and 16th centuries stirred public health officials to create a system of sanitary control to combat infectious diseases. This was the period when observation stations, isolation wards and disinfection procedures were established.
In 1862, John Gruant, a medical practitioner, published a book of statistics that enumerated deaths and sometimes suggested their causes -- the first modern effort towards epidemiology. When extensive sanitary surveys established the relationship between communicable diseases and environmental filth, it became apparent that safeguarding public health is the province of the engineer as much as it is of the physician, thus making public health policy a part of medicine.
In modern times, epidemiology has become an important tool to study human nutrition. It has been particularly useful in isolating dietary habits associated with some chronic degenerative diseases like dental caries, coronary heart diseases and high blood pressure. A more recent development has been to employ epidemiology to study widespread environmental contamination that causes new epidemics. The increasing recognition of health hazards from products introduced into the environment by humans has led to a greater need for monitoring disease frequencies and linkages between the environment and the appearances of unusual clusters of disease. In a world where population is growing by reckless leaps and bounds, epidemiology is the one safe statistical method to anticipate, and fight, the scourge of mass pestilence.