Infection by Escherichia coli O157:H7 and Other Enterohemorrhagic E. coli (EHEC)

Enterohemorrhagic E. coli (EHEC) include > 100 serotypes that produce Shiga and Shiga-like toxins (Shiga toxin–producing E. coli [STEC]; also known as verotoxin-producing E. coli [VTEC]). However, only a small number of serotypes are linked to human disease.

E. coli O157:H7 is the most common STEC in North America. However, non-O157 STEC serotypes (particularly O26, O45, O91, O103, O111, O113, O121, O128, and O145) may also cause enterohemorrhagic illness, particularly outside the United States. In 2011, serotype O104:H4 caused a significant, multinational outbreak in Europe.

In some parts of the United States and Canada, E. coli O157:H7 infection may be a more common cause of bloody diarrhea than shigellosis or salmonellosis. E. coli O157:H7 infection can occur in people of all ages, although severe infection is most common among children and older adults.

E. coli O157:H7 and other STEC have a bovine reservoir. Infection can be transmitted via food or water contaminated with cow manure, as in the outbreaks and sporadic cases that typically occur after ingestion of undercooked beef (especially ground beef, eg, hamburgers) or unpasteurized milk. In the 2011 European O104:H4 outbreak, infection was transmitted by contaminated raw bean sprouts (1). The organism can also be transmitted by the fecal-oral route, especially among infants in diapers (eg, via inadequately chlorinated children’s wading pools).

After ingestion, E. coli O157:H7 and similar STEC serotypes produce high levels of various toxins in the large intestine; these toxins are closely related to the potent cytotoxins produced by Shigella dysenteriae type 1. These toxins appear to directly damage mucosal cells and vascular endothelial cells in the gut wall. If absorbed, they exert toxic effects on other vascular endothelia (eg, renal).

Up to 22% of cases (mostly children < 5 years and adults > 60 years) are complicated by hemolytic-uremic syndrome (1), which typically develops in the second week of illness. Death may occur, especially in older adults, with or without this complication.

EHEC infection typically begins acutely with severe abdominal cramps and watery diarrhea that may become grossly bloody within 24 hours. Some patients report diarrhea as being “all blood and no stool,” which has given rise to the term hemorrhagic colitis. Fever, usually absent or low grade, occasionally reaches 39° C. Diarrhea may last 1 to 8 days in uncomplicated infections.

Chronic colonization with E. coli O157:H7 can occur after resolution of symptoms and may result in reinfection as well as transmission to others.

Hemolytic-uremic syndrome causes a rapid fall in hematocrit and platelet count, elevated serum creatinine, hypertension, and possibly signs of fluid overload, bleeding diathesis, and neurologic symptoms and signs.

• Stool cultures

• Rapid stool assay for Shiga toxin or molecular testing

E. coli O157:H7 and other STEC infections should be distinguished from other infectious diarrheas by isolating the organism from stool cultures. Culture of EHEC infections requires special media (sorbitol MacConkey agar). Identifying the specific serotype helps identify the origin of an outbreak. Often, the clinician must specifically ask the laboratory to test for the organism.

Because bloody diarrhea and severe abdominal pain without fever suggest various noninfectious etiologies, EHEC infection should be considered in suspected cases of ischemic colitis, intussusception, and inflammatory bowel disease. Characteristically, no inflammatory cells are found in the stool fluid. A rapid stool assay for Shiga toxin or molecular testing for the gene that encodes the toxin can facilitate an immediate diagnosis (1), but positive results must be interpreted in the appropriate clinical context.

Patients at risk of noninfectious diarrheas may need sigmoidoscopy. If done, sigmoidoscopy may reveal erythema and edema; barium enema or plain abdominal radiographs typically show evidence of edema with thumbprinting.

• Supportive care

The mainstay of treatment for EHEC infection is supportive. Although E. coli is sensitive to most commonly used antibiotics, antibiotics have not been shown to alleviate symptoms, reduce carriage of the organism, or prevent hemolytic-uremic syndrome. Fluoroquinolones are suspected of increasing release of enterotoxins and the risk of hemolytic-uremic syndrome and should be avoided. Likewise, antiperistaltic agents should not be given because they increase the risk of systemic complications.

In the first 2 weeks after infection, patients at high risk of developing hemolytic-uremic syndrome (eg, children < 5 years, older adults) should be observed for early clinical symptoms and signs of complications, such as hemolytic anemia, thrombocytopenia, proteinuria, hematuria, red cell casts, and rising serum creatinine. Edema and hypertension develop later. Patients who develop complications are likely to require intensive care, including dialysis and other specific therapies, at a tertiary medical center.

Improved meat processing procedures in the United States have helped reduce the rate of meat contamination.

Correct disposal of the stool of infected people, good hygiene, and careful hand washing with soap and running water limit spread of infection.

Preventive measures that may be effective in the day care setting include separating children known to be infected with STEC from children who are not infected (contact isolation) or requiring 2 negative stool cultures before allowing infected children to attend.

Pasteurization of milk and thorough cooking of beef prevent food-borne transmission.

Reporting outbreaks of bloody diarrhea to public health authorities is important because intervention can prevent additional infections.