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Etiology and Pathology

The etiology of tropical sprue is unknown and still speculative. Many diseases that produce diarrhea and malabsorption have been incorrectly called "sprue," and when they occurred in the tropics or subtropics they have been wrongly labeled "tropical sprue." There are still some who consider tropical sprue, nontropical sprue and celiac disease to be varied manifestations of the same basic disorder. There are important fundamental differences, however. The nontropical diseases almost never have a megaloblastic bone marrow and are difficult to treat, whereas the tropical form is almost always megaloblastic and responds to antibiotic, folic acid and vitamin therapy. Tropical sprue rarely shows the tetany and hypoprothrombinemia seen in its nontropical counterpart. Although these other diseases can occur in both tropical and nontropical regions, tropical sprue is distinctly uncommon outside of the tropics.

There are many theories concerning the etiology of tropical sprue. These include infection, allergy, genetic, geographic and nutritional factors, failure of intestinal mucosal maturation, folate and vitamin B12 deficiency or malabsorption, alteration of bacterial flora of the gut, vascular compromise and neurovascular dysfunction. Thus far none are fully accepted as the cause, but there is mounting evidence supporting an infectious etiology in many patients. Most investigators now believe that tropical sprue is, at least in large part, infectious in etiology, although no definitive organism has been identified. The exact nature of the infection, whether initiated and/or perpetuated by enterotoxigenic coliform bacteria, viruses or a combination of these, is unclear (Haghighi and Wolf, 1997). Thus, this thesis remains unproven, but the often dramatic response of tropical sprue to antibiotic therapy does serve to heighten the likelihood of an infectious etiology or component.

The various etiological postulates and their "pros" and "cons" are summarized in Table 16.1.

In tropical sprue there is alteration of the epithelium of the gastrointestinal tract from the mouth to the anus, but the greatest changes occur in the jejunum. An atrophic process predominates in the jejunal and ileal mucosa and, in decreasing order of severity, the tongue, buccal mucosa, stomach and colon are affected. In the jejunum, the normal finger-like villous processes become shortened and broadened, producing a leaf-like or ridge-like appearance, indicating partial villous atrophy (unlike celiac disease which causes total villous atrophy).

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