When epidemiological studies of disease frequency are pursued on a global basis, the dramatic variations in the incidence of many diseases are highlighted. For example, Segi's list of cancers in 40 countries showed a 12-fold difference in mortality from male oesophageal cancer between the lowest (Guatemala) and highest (Uruguay) countries.25 (Smith26 calculated that, if all countries could achieve the rate of the lowest observed incidence for all cancers, the present global incidence of cancer would be reduced by about 80%.) These observations supported the belief that specific environmental factors were strongly implicated in such diseases, and provided justification for the argument that a substantial proportion of cancer is preventable.
Although global analyses can show enormous variations in mortality and incidence, large variations can also be demonstrated within countries. At this geographical scale, mapping is the most effective measure of bringing the epidemiologist's attention to the geographical patterns of those diseases. For example, mapping can highlight those communities experiencing higher rates of mortality or incidence than those of their neighbours. In France, for example, the areas of Brittany and Normandy were found to have rates of oesophageal cancer three times higher than the rates in their neighbouring districts. The remarkable similarity between the maps showing male mortality from this cancer and those from cirrhosis of the liver led to the hypothesis that some form of alcohol might be causing the oesophageal cancer. The French produce a prodigious variety of alcohol: wine in many parts of the country; cider in the west; beer in the north and east; and spirits in the departments of the Nord and Paris. Analysis of the quantities of alcohol consumed by the residents of the various regions of France showed marked variations, with males in Brittany and Normandy having particularly high intakes of alcohol.27 A unique feature of this area of France is the widespread growing of apples and the production of a spirit called calvados, which is made from these apples. Traditionally, farmers were allowed to produce homemade calvados from private stills. In 1960 however, a law was passed limiting the amount of calvados produced by the farmers; and following the implementation of this law, a dramatic decline was seen in the incidence of cancer of the oesophagus. Variations in disease frequencies within continents or within countries have provided clues about causality in a wide range of diseases including lymphomas in Africa,28 adult T-cell leukaemia in Japan,29 and cancers of the oesophagus.30
Was this article helpful?