Introduction
Gastroenteritis describes a group of conditions usually caused by infection with a microorganism or ingestion of chemical toxins.
Gastroenteritis usually consists of mild to severe diarrhea that may be accompanied by loss of appetite, nausea, vomiting, cramps, and discomfort in the abdomen. Although gastroenteritis usually is not serious in a healthy adult, causing only discomfort and inconvenience, it can cause life-threatening dehydration (see Section 12, Chapter 158) and electrolyte imbalance (see Section 12, Chapter 155) in the very ill, the very young, and the very old.
Causes
Infections that produce gastroenteritis can be transmitted from person to person, especially if someone with diarrhea does not thoroughly wash his hands after a bowel movement. A person, and sometimes large numbers of people (in which case an outbreak of illness is called an epidemic), can also become infected by eating food or drinking water that has been contaminated by infected stool. Most foods can be contaminated with bacteria and cause gastroenteritis if not cooked thoroughly or pasteurized. Contaminated water is sometimes ingested in unexpected ways, such as when swimming in a pond contaminated by stool from an animal or in a swimming pool contaminated by stool from another person. In some cases, gastroenteritis is acquired through contact with animals that carry the infectious microorganism.
Certain bacteria produce toxins that cause the cells in the intestinal wall to secrete electrolytes and water. One such toxin, produced by the bacterium Vibrio cholerae, is responsible for the watery diarrhea that is the main symptom of cholera; other Vibrio species, often present in raw shellfish, produce a similar but less severe gastroenteritis. A toxin produced by the common bacterium Escherichia coli (E. coli) may cause traveler's diarrhea and some outbreaks of diarrhea in hospital nurseries.
Some bacteria (such as certain strains of E. coli, Campylobacter, Shigella, and Salmonella) invade the lining of the intestine. There, they damage cells, causing tiny sores (ulcerations) that bleed and allow a considerable leakage of fluid containing proteins, electrolytes, and water.
Besides bacteria, several types of viruses, such as rotaviruses and the Norwalk virus, cause gastroenteritis. During the winter in temperate climates, rotaviruses cause most cases of diarrhea that are serious enough to send infants and toddlers to the hospital.
Certain intestinal parasites, particularly Giardia lamblia, stick to or invade the lining of the intestine and cause nausea, vomiting, diarrhea, and a general sick feeling. The resulting infection, called giardiasis, is more common in cold climates but occurs in every region of the United States and throughout the world. If the disease becomes persistent (chronic), it can keep the body from absorbing nutrients, a condition known as a malabsorption syndrome. Another intestinal parasite, called Cryptosporidium, causes watery diarrhea that is sometimes accompanied by abdominal cramps, nausea, and vomiting. The resulting infection, called cryptosporidiosis, is usually mild in otherwise healthy people, but it may be severe or even fatal in people with a weakened immune system. Both Giardia and Cryptosporidium are most commonly acquired by drinking contaminated water.
Gastroenteritis may result from eating chemical toxins. These toxins are usually produced by a plant, such as poisonous mushrooms, or by certain kinds of exotic seafood and thus are not the product of an infection. Gastroenteritis due to chemical toxicity can also occur after ingesting water or food contaminated by chemicals such as arsenic, lead, mercury, or cadmium. Eating large amounts of acidic foods, such as citrus fruits and tomatoes, gives some people a chemical-induced gastroenteritis.
 See the table Microorganisms That Cause Gastroenteritis.
 See the sidebar Gastroenteritis as a Side Effect of Drugs.
Symptoms
The type and severity of the symptoms depend on the type and quantity of microorganism or toxin ingested. Symptoms also vary according to the person's resistance. Symptoms often begin suddenly--sometimes dramatically--with a loss of appetite, nausea, or vomiting. Audible rumbling of the intestine and abdominal cramping may occur. Diarrhea is the most common symptom and may be accompanied by visible blood and mucus. Loops of intestine may be painfully swollen (distended) with gas. The person may have a fever, feel generally sick, and experience aching muscles and extreme exhaustion.
Severe vomiting and diarrhea can lead to marked dehydration (see Section 12, Chapter 158). Symptoms of dehydration include weakness, decreased frequency of urination, dry mouth, and, in infants, lack of tears when crying. Excessive vomiting or diarrhea can result in low levels of potassium in the blood (hypokalemia). Low levels of sodium in the blood (hyponatremia) also may develop, particularly if the person replaces lost fluids by drinking fluids that contain little or no salt, such as water and tea. Water and electrolyte imbalances are potentially serious, especially in the young, the old, and people with chronic diseases.
Diagnosis
The diagnosis of gastroenteritis is usually obvious from the symptoms alone, but the cause often is not. Sometimes other family members or coworkers have recently been ill with similar symptoms. Other times, gastroenteritis can be traced to inadequately cooked, spoiled, or contaminated food, such as raw seafood or mayonnaise left out of the refrigerator too long. Recent travel, especially to certain foreign countries, may give clues as well.
If the symptoms are severe or last for more than 48 hours, stool samples may be examined in a laboratory for white blood cells and bacteria, viruses, or parasites. In rare cases, laboratory analysis of vomit, food, or blood may help identify the cause.
If the symptoms persist beyond a few days, a doctor may need to examine the large intestine with a colonoscope (a flexible viewing tube) to determine whether the person has a disease such as ulcerative colitis.
Prevention and Treatment
Because most infections that cause gastroenteritis are transmitted by person-to-person contact, particularly through direct or indirect contact with infected stool, good hand washing with soap and water after a bowel movement is the most effective means of prevention. To prevent food-borne infections, meat and eggs should be cooked thoroughly, and leftovers should be refrigerated promptly after cooking. Only pasteurized dairy products and pasteurized apple juice should be used. For infants, a simple and effective way to prevent gastroenteritis is breastfeeding.
Usually the only treatment needed for gastroenteritis is to drink an adequate amount of fluids. Even a person who is vomiting should drink as much as can be tolerated, taking small frequent sips. If vomiting or diarrhea is prolonged or the person becomes severely dehydrated, intravenous fluids and electrolytes may be needed. Because children can become dehydrated more quickly, they should be given fluids with the appropriate mix of salts and sugars. Any of the commercially available solutions designed to replace lost fluids and electrolytes (rehydration solutions) are satisfactory. Carbonated beverages, teas, sports drinks, beverages containing caffeine, and fruit juices are not appropriate. For adults, a doctor may give a drug, either as an injection or as a suppository, to control severe vomiting. These drugs usually are not given to young children.
As the symptoms subside, the person may gradually add bland foods--such as cooked cereals, bananas, rice, applesauce, and toast--to the diet. If the diarrhea continues after following the bland diet for 12 to 24 hours and there is no blood in the stool to indicate a more serious bacterial infection, the doctor may prescribe a drug such as diphenoxylate or instruct the person to use an over-the-counter drug such as loperamide or bismuth subsalicylate. Again, these drugs usually are not given to young children.
Because antibiotics can cause diarrhea and may encourage the growth of organisms resistant to antibiotics, they are rarely appropriate, even when a known bacterium is causing gastroenteritis. Antibiotics may be used, however, when certain bacteria, such as Campylobacter, Shigella, and Vibrio, are the cause.
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