Introduction

Articles are frequently published in reputable radiological journals describing diseases such as Echinococcosis (Hydatid disease ) as "rare", or expressing surprise that a worm (Ascaris) has been demonstrated in the common bile duct by ultrasonography. Case reports show that plain radiography will demonstrate "unusual" calcification in the bladder of a patient with schistosomiasis.

The truth is very different, because about 40 million people suffer from hydatids, at least one quarter of the world's people harbour ascaris, 200 million or more have schistosomiasis and untold millions have a bewildering variety of "tropical diseases". Very few tropical diseases are rare.

Which is why this chapter is important, even if the title is not really accurate: most of the diseases described in this chapter are not confined to the tropics, but will be seen all over the world in immigrants or brought home by travellers. Some have adapted and now flourish in distinctly non-tropical climates, presenting in ways which differ from the same disease in the tropics. Nevertheless, it is in the tropics that these diseases occur most frequently. Individually and together they are the most numerous of all afflictions which cause ill health in people of all ages.

It is very important to remember that the natural history of these diseases will vary, depending on whether there is an acute onset in the traveller who has never previously been exposed, or a recurrent chronic infection which is almost part of the normal life of someone who lives in the tropics. It is equally important to be aware of their geographical distribution. Chaga's disease, for example, occurs only in the central and southern parts of the Americas, and will not occur in patients who do not live there, or who have never visited that part of the world. When reviewing the diagnostic images of someone who has visited or lived in the tropics, it is essential to know not only where they come from, but also how long they were there. Most of the tropical diseases are due to infections or parasites and there is a variable delay before they become clinically obvious. The pattern also depends on the stage of the disease and the general health of the patient.

The radiologist should have as much information as possible about the results of the laboratory tests for the individual patient, particularly in travellers who have returned home after visiting the tropics. There are too many specific diagnostic tests for each infection for them all to be included here, but there is one which is useful for screening when a parasitic infection is suspected. In the acute stage, the invasion or multiplication of so many parasites often causes a marked peripheral eosinophilia. Any patient with a strange infection or an unusual abnormality on a radiograph or ultrasound scan, who has just been to the tropics and who has a raised eosinophil count, is not likely to have one of the more common "western diseases". Unfortunately, this is not absolute and the differential cell count may be normal. As is necessary for every diagnostic interpretation, the whole clinical picture must be taken into account. What also makes it more difficult is that many patients who live or have live d in the tropics will suffer from multiple diseases. Parasitic infection may almost be a normal state, and the illness which brings them to their doctor may be added onto, or be an acute exacerbation superimposed on several other less obvious infections. And, to complicate the diagnosis even further, laboratory findings may be misleading, because normal levels are not the same in every part of the world, while high immunological titres may indicate a past infection not relevant to the present illness.

If the patients bring images with them, extract as much information from them as possible, even if the quality is not ideal. Imaging facilities vary enormously throughout the world, but there is usually some useful information even in a poor film or scan. Do not put poor quality images aside as unhelpful: compare them with the current findings. You and your patient may need all the help you can get!

In a global textbook such as this, it is not possible to provide a detailed account of the epidemiology, and life history of the many tropical diseases, nor can they all be included. Some have no radiological interest, others can only be mentioned briefly. When in doubt, or curious, refer to a more comprehensive tropical radiology text. This chapter describes only the most common findings in the most common diseases, not their many possible permutations.

Philip E.S. Palmer, with Stanley P. Bohrer, Carlos Bruguera, Xing-Rong Chen, Mahmoud R. EImeligi, Hassen A. Gharbi, S.B. Lagundoye, M. W. Wachira