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The etiology of acute diarrheal diseases can be divided into three categories: viral, bacterial, and parasitic. In developed nations, such as the United States, rotaviruses and Norwalk-like viruses (10%-27%) are frequent causes of diarrhea (Guerrant et al., 1985). In contrast, in the tropics enterotoxigenic Escherichia coli (E. coli) (21%-23%), Shigella, (8%-11%), and Campylobacter (7%-14%) predominate as causative agents of acute diarrhea. Similarly, enterotoxigenic E. coli (47%), Shigella (1%-22%), and Salmonella (4%-7%) are the principal causative agents in travelers' diarrhea. It is anticipated that troops rapidly deployed from the continental United States to tropical areas will be at risk for travelers ' diarrhea. Enteric infections can be divided into three types: luminal, mucosal, and systemic (Guerrant et al., 1985). Luminal enteric infections are caused by organisms such as E. coli, Vibrio cholera, (V. cholera) Staphylococcus, and Giardia. Diarrhea is caused by enterotoxins that interfere with absorption in the small bowel. In luminal enteric infections, diarrheal stools are watery and there are no fecal white blood cells. Mucosal enteric infections can be caused by Shigella, Campylobacter jejuni, Salmonella, and Clostridium difficile. There is colonic mucosal invasion by bacteria, causing an inflammatory dysentery with the presence of fecal polymorphonuclear leukocytes. In systemic enteric infections the ileum is involved with the potential invasion of the blood stream and enteric fever. Examples of causative agents are Salmonella typhi, Yersinia, and Campylobacter fetus.

What is the distribution of diarrheal diseases worldwide? Travelers ' diarrhea is found in Mexico, Central and South America, the Caribbean, Africa, the littoral Mediterranean, and Asia (Steffen, 1986). The incidence of travelers' diarrhea ranges as high as 50% in these regions. The incidence of dysentery (diarrhea with fever or blood in stools) among travelers to Mexico, Central and South America, Africa, and Asia ranges upwards of 9%-11%.

Signs and symptoms of travelers' diarrhea include gas (79%), fatigue (74%), cramps (68%), nausea (61%), fever (56%), abdominal pain (55%), anorexia (53%), headaches (39%), chills (30%), and vomiting (29%) (Gorbach and Hoskins, 1980). In small bowel diarrhea caused by E. coli, Giardia, V. cholera, or reovirus, the location of pain is in the midabdomen, with large volumes of watery diarrhea causing dehydration (Gorbach and Hoskins, 1980). Proctoscopy is normal. With large bowel diarrhea caused by Shigella, invasive E. coli, or amebiasis, the pain is in the lower abdomen and in the rectum. The stool volume is usually small and may be mucoid (dysenteric). Blood and leukocytes are very common in the stool. Proctoscopic findings include a friable and hemorrhagic mucosa and mucosal ulcers. What is the efficacy of prophylactic treatment of acute diarrheal diseases? For Peace Corps volunteers, doxycycline, 100 mg twice weekly,

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