Chapter 44

Geographic Variations of Nonmalignant Diseases in the Tropics

"To study the accurate geography of diseases ... is often essential to the complete determination of their natural history."
Dr. Gavin Milroy, F.R.C.P.
Suggestions for Consideration by the Council of the Royal College of Physicians, 1873.

It is easy to accept the fact that there is an infinite variety of diseases, but less easy to grasp that each disease may be infinitely variable, and that illnesses well known in medical school may behave quite differently in some other localities. Even more surprising, "common" diseases may not exist or may be rare in other places. Any author's description of any disease is valid for his or her own experience but may not reflect the experience of others in other countries. Well-known textbooks may be wrong - even this one! All of us who have practiced for any length of time in tropical countries will know that the variation in the pattern of disease is considerable, and any radiologist who leaves Europe or North America (or any other temperate climate) to practice in the tropics must grasp this fact. His or her colleagues from the tropics will also have to recognize these differences. It is not only that one needs to be knowledgeable about local parasites and unusual infections: almost all diseases have some geographic variation in both frequency and behavior, some well recognized, others unpublished yet known to all local medical practitioners, the licensed, unlicensed, and traditional.

Each part of the tropics has its own variations in diseases, often differing over surprisingly short distances. The pitfalls and difficulties of epidemiological studies are well described in an editorial in the British Medical Journal (November 1998) covering the use to tobacco in China: there the problems are compounded because the causes of death from tobacco use differ from those in the West and "there are widely different lung cancer risks in different Chinese cities." As a passing comment, if we only thought about the underlying reasons for such variety we might solve hitherto unsolved medical problems, but most of us are too busy to do more than wonder why, and many of us do not even find time to do that! All doctors, and radiologists in particular, are dependent on statistical likelihood to aid them with their final diagnosis, or even to provide a short list of differential possibilities; yet statistics are only valid for the population for which they are collected. For example, the common causes of intestinal obstruction seen in London differ somewhat from those of New York; neither list is applicable in Dakar, Bombay, or Caracas. Similarly, a patient with a high fever and consolidation of a lung segment seen in Europe or North America is most likely to have lobar pneumonia due to the pneumococcus or Klebsiella bacillus; in Africa it may well be tuberculosis. Klebsiella infections in the tropics more often cause bilateral bronchopneumonia and not lobar consolidation. A colonic stricture in an African is very rarely malignant. It is probably due to amebiasis, schistosomiasis, tuberculosis, or lymphogranuloma venereum, but it is almost certainly not cancer. Such examples are legion and some are briefly discussed in this chapter to stimulate the interest and awareness of the physician managing patients in the tropics, or, equally important, the other physicians caring for patients who have until recently lived all their lives in the tropics, but have now moved to a nontropical climate. No attempt is made to make the list comprehensive; no list could be, because the variations are too many and too local. Most of the examples quoted come from Africa but this reflects only personal experience and the wealth of such information in the medical journals of West, East, Central, and South Africa. The principle is correct for any continent or country, and unless the local disease pattern is known the reputation of the radiologist or any other physician will suffer ... rapidly.