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Diarrhea is an increase in the liquidity and/or frequency of the stools. It reflects an increase in stool water content due to impaired water absorption and/or active water secretion by the intestine. Although the advent of oral rehydration therapy has dramatically reduced the mortality rates in the past 50 years, diarrhea remains a major cause of morbidity and mortality in children, particularly among those in developing countries.1 Most episodes of diarrhea occur on the basis of 1 of 5 mechanisms: malabsorptive, secretory, osmotic, dysmotility, and inflammatory. Malabsorption is due to a decrease in absorptive surface area, as occurs after intestinal resection (short bowel syndrome) or with intestinal villous atrophy, as seen in celiac disease. Secretory diarrhea is caused by secretagogues such as bacterial toxins (eg, cholera), gut regulatory peptides (eg, vasoactive intestinal polypeptide), short-chain fatty acids, and bile salts, which can induce intestinal water secretion while inhibiting absorption. Secretory diarrhea characteristically persists even when the patient is in a fasting state. Osmotic diarrhea results from the intraluminal presence of malabsorbed solutes, such as lactose, which exert significant osmotic pressure that results in secretion of water into the intestines. Osmotic diarrhea characteristically decreases or stops completely during fasting. Dysmotility can lead to increased peristalsis, causing diarrhea due to rapid transit, or to decreased peristalsis, leading to bacterial overgrowth as the cause of diarrhea. Inflammatory disorders cause diarrhea by decreasing the mucosal absorptive capacity and/or through disturbances in motility. One or more of these mechanisms may be operative in an individual during an episode of diarrhea.

Acute diarrhea accounts for 2 to 3 million deaths per year with most occurring in young children in developing countries. In the United States, approximately 220,000 children under 5 years of age are hospitalized each year for acute diarrheal illnesses, accounting for 9% of all hospitalizations in this age group.2

Clinical Features and Differential Diagnosis

The common causes of acute diarrhea are listed in Table 385-1. Enteric infections account for most cases of acute diarrhea in children with diarrhea secondary to nonenteric infections, such as urinary tract, respiratory, or systemic infections, accounting for a smaller number of cases.3 Detailed information regarding each specific pathogen is found in Section 17. Viruses are responsible for the majority (60–80%) of enteric infections in children, especially those under the age of 2 years. Of these, rotaviruses are most prevalent and most likely to cause dehydrating diarrhea. Of all children requiring treatment for dehydration during the peak winter gastroenteritis season, 70% to 90% have rotavirus infection.4 Other winter-predominant viral agents, such as astroviruses and caliciviruses, tend to infect older children and cause illness of shorter duration. Viral gastroenteritis is typically transmitted by the fecal-oral route, but outbreaks in a community setting suggest the possibility of airborne spread as well, perhaps following episodes of vigorous emesis.5 The norovirus (or Norwalk agent), a calicivirus, is an important cause of foodborne outbreaks of gastroenteritis ...

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