Pathophysiology

The portion of the gastrointestinal tract affected by food- and water-borne illnesses depends on the pathogen. Staphylococcal food poisoning is caused by a toxin that has no effect on the intestinal mucosa. Instead, once absorbed, the toxin acts directly on nausea centers in the brain and thus causes severe nausea and vomiting.

Infections of the small intestine may disrupt ionic exchange and result in increased chloride secretion and sodium retention within the bowel lumen. Water follows, thereby overwhelming absorption capacity, and diarrhea ensues. Viruses create diarrhea by distorting the epithelium and interfering with absorptive capabilities. This process results in loss of fluid, electrolytes, and, in some cases, fats and sugars. Because of the high secretory capabilities of the small intestine, infection of this region can lead to large-volume, watery diarrhea. In patients with large fluid losses, significant electrolyte disturbances and acidosis may occur, more commonly with infections of the small intestine.² Given the predominant location of the small intestine, cramping and discomfort are localized more often to the upper abdomen.

Invasive organisms primarily affect the distal ileum and large intestine. Common invasive organisms include Campylobacter, Salmonella, Shigella, Yersinia, and Entamoeba histolytica. These pathogens penetrate the intestinal lining and create an intense inflammatory response. This process results in losses of fluid and, in some cases, varying amounts of blood. The large intestine has less secretory function. Therefore, infections of this region are more likely to cause smaller, frequent episodes of diarrhea that is more apt to contain mucus or blood when compared with small intestine infections. Severe alterations in electrolytes and acid-base balance are less common. Symptoms of large intestine infections tend to be felt in the lower quadrants, and in some cases they may mimic appendicitis, with focal right lower quadrant pain.

Small bowel and large bowel are both susceptible to toxin-producing organisms that induce diarrhea through direct damage to the epithelium, thereby disrupting the regulation of fluid balance. Because the lactase enzyme responsible for breakdown of lactose is present primarily in the epithelial surface, any infection that causes significant epithelial damage may lead to temporary lactose intolerance. Whereas bacterial toxin-induced diarrhea tends to be isotonic, virally induced diarrhea causes losses primarily of sodium, potassium, and bicarbonate.

Presenting Signs and Symptoms

Presenting signs and symptoms vary widely, depending on the pathogen causing the disease. Many parasites that infect the intestines cause no discernible symptoms. Asymptomatic chronic bacterial carrier states can occur in otherwise healthy individuals. When symptoms do occur, they are limited in most patients to vomiting and diarrhea of varying degrees. Rarely, patients have more systemic symptoms. Listeria monocytogenes infections, more common in pregnant women, cause fever, myalgias, headache, and neck stiffness. Scombroid poisoning from fish can cause symptoms of a severe allergic reaction, and ciguatera poisoning, also from fish, can cause neurologic symptoms including severe paresthesias and dysesthesias.

Although food containing preformed toxins can cause illness within 1 to 6 hours, most acute gastroenteritis has at least a 12- to 48-hour incubation period (Table 183.1). Typically, the onset is gradual and is often noticed as mild dyspepsia after eating a meal. This is the meal often suspected by the patient to be the cause of the illness. However, the pathogen has usually been incubating for the last 1 to 2 days and is just starting to cause symptoms.

Table 183.1 Symptom Onset Time

Within 1 to 2 hours after the initial dyspepsia, nonbloody vomiting often begins. If bleeding does occur, the most common cause is a Mallory-Weiss tear. This results when forceful ejection of gastric contents creates a tear in the mucosa at the junction of the esophagus and the stomach. It rarely occurs during the first episode of vomiting, and treatment is almost always supportive. An initial presence of blood, or a persistent predominance of gross blood, should prompt a search for noninfectious causes of vomiting such as a bleeding peptic ulcer or esophageal varices.

Diarrhea can occur simultaneously with vomiting, or it may be delayed for up to 48 hours after vomiting begins. In some cases, diarrhea may be the only component of clinical illness. Gross diarrheal blood is more common than hematemesis and varies by pathogen (Box 183.1). Dysentery, defined as a diarrheal stool containing gross blood, can be accompanied by fever, abdominal pain, and tenesmus.

Differential Diagnosis

Vomiting and diarrhea can result from other pathologic states, or they may be iatrogenic, such as from a medication side effect. Vomiting and diarrhea can herald an endocrine emergency such as in adrenal crisis, or they may represent a completely benign event, such as diarrhea after long-distance running. Maintaining a level of suspicion for other causes is essential, to minimize the chance of missing a more dangerous disorder or to reassure a patient when the cause is benign. The following subsections describe additional diagnoses to consider.