Gastroenteritis is inflammation of the lining of the stomach and small and large intestines. Most cases are infectious,
although gastroenteritis may follow ingestion of drugs and chemical toxins (eg, metals, plant substances). Symptoms
include anorexia, nausea, vomiting, diarrhea, and abdominal discomfort. an estimated 3 to 6 million children die each
year from infectious gastroenteritis. Infectious gastroenteritis may be caused by viruses, bacteria, or parasites, Four
categories of viruses cause most gastroenteritis: rotavirus, calicivirus (which includes norovirus [formerly Norwalk
virus]), astrovirus, and enteric adenovirus.

The character and severity of symptoms vary. Generally, onset is sudden, with anorexia, nausea, vomiting, borborygmi,
abdominal cramps, and diarrhea (with or without blood and mucus). Malaise, myalgias, and prostration may occur. The
abdomen may be distended and mildly tender; in severe cases, muscle guarding may be present. Gas-distended
intestinal loops may be palpable. Borborygmi are present even without diarrhea (an important differential feature from
paralytic ileus). Persistent vomiting and diarrhea can result in intravascular fluid depletion with hypotension and
tachycardia. In severe cases, shock, with vascular collapse and oliguric renal failure, occurs.

Bacterial gastroenteritis is less common than viral. Bacteria cause gastroenteritis by several mechanisms. Certain
species (eg, Vibrio cholerae, enterotoxigenic strains of Escherichia coli) adhere to intestinal mucosa without invading
and produce enterotoxins. These toxins impair intestinal absorption and cause secretion of electrolytes and water by
stimulating adenylate cyclase, resulting in watery diarrhea. Clostridium difficile produces a similar toxin when overgrowth
follows antibiotic use

Some bacteria (eg, Staphylococcus aureus, Bacillus cereus, Clostridium perfringens) produce an exotoxin that is
ingested in contaminated food. The exotoxin can cause gastroenteritis without bacterial infection. These toxins generally
cause acute nausea, vomiting, and diarrhea

Other bacteria (eg, Shigella, Salmonella, Campylobacter, some E. coli strains) invade the mucosa of the small bowel or
colon and produce microscopic ulceration, bleeding, exudation of protein-rich fluid, and secretion of electrolytes and
water. The invasive process and its results can occur whether or not the organism produces an enterotoxin. The
resulting diarrhea contains WBCs and RBCs and sometimes gross blood.

Several different subtypes of E. coli cause diarrhea. Enterohemorrhagic E. coli is the most clinically significant subtype
in the US. It produces Shiga toxin, which causes bloody diarrhea. E. coli O157:H7 is the most common strain of this
subtype in the US. Hemolytic-uremic syndrome is a serious complication that develops in 2 to 7% of cases, most
commonly in the young and old, Enterotoxigenic E. coli produces two toxins (one similar to cholera toxin) that cause
watery diarrhea

Yersinia enterocolitica can cause gastroenteritis or a syndrome that mimics appendicitis.

Giardia lamblia (see Intestinal Protozoa: Giardiasis), adhere to or invade the intestinal mucosa, causing nausea,
vomiting, diarrhea, and general malaise. The infection can become chronic and cause a malabsorption syndrome.

Cryptosporidium parvum causes watery diarrhea sometimes accompanied by abdominal cramps, nausea, and vomiting.

Other parasites that can produce symptoms similar to those of cryptosporidiosis, especially in immunocompromised
hosts, include Cyclospora cayetanensis, Isospora belli, and a collection of organisms referred to as microsporidia (eg,
Enterocytozoon bieneusi, Encephalitozoon intestinalis). Entamoeba histolytica (amebiasis) is a common cause of
subacute bloody diarrhea in the developing world and occasionally occurs in the US.