Escherichia coli O157:H7
What is Escherichia coli ?
E. coli O157:H7, one of hundreds of strains of the bacterium Escherichia coli, is an emerging cause of foodborne illness. While most strains are harmless and live in the intestines of healthy humans and animals, this particular strain produces a powerful toxin that can cause severe illness. It was first identified as a cause of illness in 1982 during an outbreak of severe bloody diarrhea traced to contaminated hamburgers. (The combination of letters and numbers refers to specific markers found on the bacterium's surface that distinguish it from other E. coli, which have other O and H markers.)
How common is the infection?
No good national data are available because many laboratories do not routinely test for the organism. Data from one laboratory that does regularly test for E. coli O157:H7 suggest that an estimated 20,000 cases may occur in the United States annually. In some parts of the U.S., particularly the northernmost states, this infection is not rare. It may well be a global problem. Now common in Canada, the infection is being increasingly recognized in Europe, South Africa, the southern regions of South America, Austrailia and Japan.
In Illinois, 100-200 cases of E. coli O157:H7 are reported each year.
What sort of illness does it cause?
Many persons infected with the bacterium develop severe diarrhea and painful abdominal cramps, although some people show few or no symptoms. The diarrhea can be very bloody. Because there is usually little or no fever, a person may think some other condition is causing the bowel to bleed, and this infection may go unrecognized. The illness usually resolves in five to 10 days.
In some persons, particularly children younger than 5 years of age and the elderly, the infection can lead to destruction of red blood cells (hemolytic anemia) and acute kidney failure (also known as uremia). This complication, hemolytic uremic syndrome (HUS), can lead to stroke, seizures and death. About 2 percent to 7 percent of infections lead to HUS. In the United States, E. coli O157:H7 infection is the primary cause of HUS, which is the principal cause of acute kidney failure in children. Most children with HUS are hospitalized for about two weeks.
How is E. coli diagnosed?
There are many causes of bloody diarrhea and abdominal cramps. Specific laboratory tests can identify E. coli O157:H7 in the stool of an infected person. However, these tests often are not performed unless the laboratory is instructed to do them.
How is E. coli treated?
Most persons recover without antibiotics or other specific treatment in five to 10 days. Antidiarrheal agents, such as loperamide (Imodium®), should be avoided.
HUS, a life-threatening condition that is usually treated in an intensive care unit, often requires blood transfusions and kidney dialysis. With intensive care, the fatality rate for HUS is 3 percent to 5 percent.
Are there any long-term consequences?
Persons with diarrhea alone usually recover completely, although it may be several months before bowel habits are entirely normal.
Among those who develop HUS, about 8 percent have a poor outcome, such as chronic kidney failure, high blood pressure, stroke, paralysis, bowel resection, blindness or seizures. A decline in kidney function may appear years later in about one-third of those persons who have had HUS. Thus, this infection may be a preventable cause of chronic kidney failure.
How is E. coli O157:H7 spread?
Most cases of E. coli O157:H7 infection come from undercooked ground beef. Beef that is still pink, or has blood-tinged juices, has not been cooked enough to kill E. coli O157:H7. While the number of organisms required to cause disease is not known, it is suspected to be very small. Contaminated meat looks and smells normal. The infection also can result from drinking raw unpasteurized milk or drinking or swimming in sewage-contaminated water.
The bacterium is present in the stools of infected persons, and it can be passed from one person to another if hygiene and hand washing habits are inadequate. This is particularly likely to occur among toddlers who are not fully toilet trained. Family members and playmates of such children are at high risk of becoming infected. Bacteria are usually cleared from the stools within a week after the diarrhea resolves. However, in some cases, particularly in young children, the organism may persist in the stool for weeks after the diarrhea has resolved.
How does food become contaminated?
The organism can be found on a small number of cattle farms, where it can live in the intestines of healthy cattle. When the animal is slaughtered, the meat may be contaminated by intestinal contents. When this meat is ground, fecal organisms that were on the outside of the meat are then thoroughly mixed throughout the ground beef. These bacteria can survive unless the meat is thoroughly cooked.
Bacteria present on a cow's udders or on equipment may get into and contaminate raw milk.
What can the consumer do to prevent this illness?
What else can be done to prevent the infection?
E. coli O157:H7 will continue to be an important public health concern as long as it contaminates meat. It is conceivable that cattle could be vaccinated against the infection, but research into such prevention measures is just beginning.
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