Escherichia coli O157:H7
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Escherichia coli O157:H7 is an enterohemorrhagic strain of the bacterium Escherichia coli and an emerging cause of foodborne illness.<ref name=Karch_2005>Karch H, Tarr P, Bielaszewska M (2005). "Enterohaemorrhagic Escherichia coli in human medicine.". Int J Med Microbiol 295 (6-7): 405-18. PMID 16238016.</ref> An estimated 73,000 cases of infection and 61 deaths occur each year in the United States alone, although it is more common in less industrialized countries. Infection often leads to bloody diarrhea, and occasionally to kidney failure. Most illness has been associated with eating undercooked, contaminated ground beef, although it is also transmitted by person-to-person contact, produce, drinking unpasteurized milk or swimming in or drinking contaminated water. The 2006 United States E. coli outbreak was linked to a large quantity of bagged spinach contaminated with this strain.
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[edit] Biochemistry
E. coli serotype O157:H7 is a gram-negative rod-shaped bacterium. It is one of hundreds of strains of the bacterium Escherichia coli. Although most strains are harmless and live in the intestines of healthy humans and animals, this strain produces Shiga-like toxin(s) and can cause severe illness. It falls under the class of pathogenic E. coli known as the enterohemorrhagic Escherichia coli or EHEC. Alternative namings refer to the toxin producing capabilities, chiefly Verocytotoxin producing E. coli (VTEC) or less commonly Shiga-like Toxin producing E. coli (STEC).
The "O" (which is a capital O, not a zero) in the name refers to the somatic antigen number, whereas the "H" refers to the flagellar antigen. Other serotypes may cause (usually less severe) illness, but only those with the specific O157:H7 combination are reviewed here. Some (other) bacteria are classified by "K" or capsular antigens. (The "O" stands for the German phrase ohne Hauch; "H" for Hauch; and "K" for Kapsel.<ref>See Kauffmann-White-Schema in the German Wikipedia.</ref><ref>Dorlands Medical Dictionary, entry for "O antigen".</ref>)
E. coli O157:H7 was first recognized as a cause of illness in 1982 during an outbreak of severe bloody diarrhea; the outbreak was traced to contaminated hamburgers.<ref name=Riley_1983>Riley L, Remis R, Helgerson S, McGee H, Wells J, Davis B, Hebert R, Olcott E, Johnson L, Hargrett N, Blake P, Cohen M (1983). "Hemorrhagic colitis associated with a rare Escherichia coli serotype.". N Engl J Med 308 (12): 681-5. PMID 6338386.</ref> Since Shigella and E. coli are closely related bacteria capable of exchanging DNA, it is believed that such transfer of the gene for the toxin from Shigella gave rise to this E. coli strain, possibly not long before this first clinical observation.
[edit] Transmission
The major source of infection is undercooked ground beef; other sources include consumption of unpasteurized milk and juice, raw sprouts, lettuce, and salami, and contact with infected live animals. Waterborne transmission occurs through swimming in contaminated lakes, pools, or drinking inadequately chlorinated water. The organism is easily transmitted from person to person and has been difficult to control in child day-care centers.
E.coli O157:H7 is found on a small number of cattle farms and can live in the intestines of healthy cattle. The toxin requires highly specific receptors on the cells' surface in order to attach and enter the cell; species such as cattle, swine, and deer which do not carry these receptors may harbor toxigenic bacteria without any ill effect, shedding them in their feces from where they may be spread to humans. Meat can become contaminated during slaughter, and organisms can be thoroughly mixed into beef when it is ground into chopmeat. Bacteria present on the cow's udders or on equipment may get into raw milk. Although the number of organisms required to cause disease is not known, it is suspected to be very small.
Eating contaminated meat (especially ground meat) or produce that has not been cooked sufficiently to kill E. coli O157:H7 can cause infection. Contaminated foods look and smell normal.
[edit] Signs and symptoms
E. coli O157:H7 infection often causes severe, acute bloody diarrhea (although nonbloody diarrhea is also possible) and abdominal cramps. Usually little or no fever is present, and the illness resolves in 5 to 10 days. It can also be asymptomatic.
In some people, particularly children under 5 years of age and the elderly, the infection can cause hemolytic uremic syndrome, in which the red blood cells are destroyed and the kidneys fail. About 2%-7% of infections lead to this complication. In the United States, hemolytic uremic syndrome is the principal cause of acute kidney failure in children, and most cases of hemolytic uremic syndrome are caused by E. coli O157:H7.
[edit] Diagnosis
A stool culture can detect the bacterium, although it is not a routine test and so must be specifically requested. The sample is cultured on sorbitol-MacConkey (SMAC) agar, or the variant cefeximine potassium telluride sorbitol-MacConkey agar (CT-SMAC). However, like all cultures, diagnosis is slow using this method, and more rapid diagnosis is possible using PCR techniques. Newer technologies using fluorescent and antibody detection are also under development.
[edit] Surveillance
E. coli O157:H7 infection is nationally reportable in the USA and Great Britain, and is reportable in most U.S. states. HUS (hemolytic-uremic syndrome) is also reportable in most US states.
[edit] Treatment
Most people recover without antibiotics or other specific treatment in 5-10 days. There is no evidence that antibiotics improve the course of disease, and it is thought that treatment with some antibiotics may precipitate kidney complications.[citation needed] Antidiarrheal agents, such as loperamide (imodium), should also be avoided.
Hemolytic uremic syndrome is a life-threatening condition usually treated in an intensive care unit. Blood transfusions and kidney dialysis are often required. With intensive care, the death rate for hemolytic uremic syndrome is 3%-5%.
[edit] Prognosis
The majority of infections resolve completely. Those who develop hemolytic uremic syndrome suffer more long-term consequences. 3-5% of those with HUS die, causing about 61 deaths annually in the USA. One third of this group have abnormal kidney function many years later, and a few require long-term dialysis. Another 8% of this group develop other lifelong complications, such as high blood pressure, seizures, blindness, paralysis, and, if surgery is required to remove part of the bowel, additional procedure-related side-effects.
There are currently long term studies continuing in Walkerton, Ontario looking at the long term effects of E. coli O157:H7 after approximately 2500 people were infected through the municipal water system in May 2000.
[edit] Costs
The pathogen results in an estimated 2,100 hospitalizations annually in the United States. The illness is often misdiagnosed; therefore, expensive and invasive diagnostic procedures may be performed. Patients who develop HUS often require prolonged hospitalization, dialysis, and long-term follow-up.
[edit] Relationship to industrial agriculture
Several authors and scientists have pointed out the link between E.coli contaminations in food and industrial scale meat and dairy farms. The most recent E. coli outbreak has once again demonstrated this link because the source of this E. coli was traced back to "a large ranch in the Salinas Valley that has a beef cattle operation" about a half-mile from the spinach fields where spinach became contaminated.<ref>Sander, Libby. "Source of Deadly E. Coli Is Found", New York Times, 2006-10-13. Retrieved on 2006-10-13.</ref>
The E. coli samples were found in a water sample in a creek, in the gastrointestinal tract of a wild boar on the property, and from cattle fecal specimens. None of the nine positive matches came from a nearby spinach field that was the source of the contaminated produce. That leaves one significant unanswered question: How did the E. coli get from the ranch to the spinach field? The answer may be the wild boars. "Animals, wildlife and water were in close proximity to the field," Reilly said. "We have evidence for fences torn down, wildlife going into the actual spinach fields themselves. That's where the investigation is centered right now. There's clear evidence that the pig population has access and goes onto the fields. http://www.texasgrassfedbeef.com/e__coli_o154_h7_and_spinach.htm
There are several variants of E. coli and they can be found in a healthy human gut, but the deadly strain, O157:H7 was virtually unheard of until the 1980's. It is believed that this strain evolved in the digestive system of grain fed cattle on large industrial farms.<ref>Pollan, Michael. "The Vegetable-Industrial Complex", New York Times, 2006-10-17. Retrieved on 2006-10-17.</ref> On these farms, grain is used as cattle feed because it is nutrient-packed and increases efficiency. A side effect of feeding grain to cattle is that it increases the acidity of their stomach - and it is in this acidic gut that the deadly O157:H7 thrives.
Another reason why the strain was not known before the 1980's was that only then better technology became available to identify bacteria. http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=130684 As a single gram of rich, undisturbed soil may contain as many as 5,000 different species of bacteria (more bacterial species, even, than all those that have been described by science), it is not surprising that most of these have not been categorized yet.
In 2003, an article in the Journal of Dairy Science found that between 30 and 80 percent of cattle carry E. coli O157:H7.<ref>Callaway, T. R., Elder, R.O.; Keen J.E.; Anderson, R.C.; Nisbet, D.J. (2003). "Forage Feeding to Reduce Preharvest Escherichia coli Populations in Cattle, a Review". Journal of Dairy Science 86: 852-860. Retrieved on 2006-09-22.</ref> In that same journal article, a quick fix was pointed out: Cows that are switched from a grain diet to a forage diet saw, within 5 days, a 1,000 fold decrease in the abundance of strain O157. But until changes like this are made, the source of many E. coli outbreaks will continue to be high-yield meat and dairy farms.<ref>Plank, Nina. "Leafy Green Sewage", New York Times, 2006-09-21. Retrieved on 2006-09-21.</ref>
This is debatable. The presence of the bacterium in the livestock is much lower than reported. A 2002 USDA NAHMS study found that 38.5% of dairy farms had at least one cow that was culture [+] when sampled, but only 4.3 % of individual cows were shedding the organism. The article indicating a 1000 fold difference seems unreliable as the opposite experiment was not done (hence it can just be due to the change). More statistically convincing results are presented in papers stating the opposite, finding half as little of the strain in manure from animals fed by corn instead of barley. (FEMS Microbiol Lett. 2005 Nov 1;252(1):25-33/ J Food Prot. 2000 Nov;63(11):1467-74/ J Food Prot. 2004 Apr;67(4):666-71)
More likely, rather than change the way cattle are fed or raised on industrial farms there will instead be pressure to find technolgical solutions like food irradiation, plans for HACCP, or simply cooking burgers longer. Suggestions like this have led some experts, like Professor of Science and Environmental Journalism at UC Berkeley, Michael Pollan, to suggest that "All of these solutions treat E. coli O157:H7 as an unavoidable fact of life rather than what it is: a fact of industrial agriculture."<ref>Pollan, Michael. "The Vegetable-Industrial Complex", New York Times, 2006-10-17. Retrieved on 2006-10-17.</ref>
Advocates such as Howard Lyman and groups such as People for the Ethical Treatment of Animals have promoted vegetarianism in response to cases of E.coli infection.
[edit] Prevention
[edit] Agricultural
Beef processing is the most common point of contamination, when during the slaughtering process the contents of a cow's intestines mix with the meat and bacteria flourish in the warm, moist conditions. If the infected parts are then ground, the bacteria goes from the surface of the cut to the interior of the ground mass. Thus, ground beef is more likely to be a source of infection than steak. In steak only the surface area of a cut is exposed during rendering, and cooking the outside affects the entire exposed portion. In ground beef, however, bacteria is mixed throughout the meat mass, requiring the entire mass to be heated thoroughly to eliminate the pathogen. Additionally, in the production of ground beef, meat from multiple cows is often ground together, enabling contamination from a single cow to infect an entire lot of ground beef.
Accordingly, elimination of infection is unlikely until preventative measures either reduce the number of cattle that carry E.coli O157:H7 or reduce the contamination of meat during slaughter and grinding.
[edit] Culinary and dietary
Cooking all ground beef and hamburger thoroughly, using a digital instant-read meat thermometer, will eliminate the organism. Ground beef should be cooked until a thermometer inserted into several parts of the patty, including the thickest part, reads at least 72°C (160°F).
When preparing meat, it should be kept separate from other foodstuffs, and all surfaces and utensils which come into contact with raw meat should be washed thoroughly before being used again. Hand washing is similarly important. Placing cooked hamburgers or ground beef on an unwashed plate that held raw patties can transmit infection.
Avoid unpasteurized milk, juice, and cider. Commercial juice is almost always pasteurized, and juice concentrates are also heated sufficiently to kill pathogens.
Fruits and vegetables should be washed thoroughly, especially those that will not be cooked. Children under 5 years of age, immunocompromised persons, and the elderly should avoid eating alfalfa sprouts until their safety can be assured. Methods to decontaminate alfalfa seeds and sprouts are being investigated.
Contaminated water should be boiled at a rolling boil for at least one minute (longer at higher altitudes) before consumption. Care while swimming to avoid ingestion of potentially contaminated water can reduce the chances of infection.
Proper hand washing after using the lavatory or changing a diaper, especially among children or those with diarrhoea, will reduce the risk of transmission. Anyone with a diarrheal illness should avoid swimming in public pools or lakes, sharing baths with others, and preparing food for others.
[edit] Opportunities
Learning more about the ecology of this organism in cattle and other ruminants may help in devising methods to decrease its prevalence in food animals. Learning how this pathogen contaminates produce items could lead to measures that would increase their safety. Decreasing the incidence of these infections would decrease HUS, the major cause of kidney failure in children in the United States. Transmission in day care centers highlights need for better infection-control practices.
(adapted from two public domain sources[1], [2])
[edit] See also
[edit] References
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[edit] External links
- For more information about reducing your risk of foodborne illness, visit the US Department of Agriculture's Food Safety and Inspection Service website at: http://www.fsis.usda.gov or the Partnership for Food Safety Education at:
- For more advice on cooking ground beef, visit the U.S. Department of Agriculture web site at: http://www.fsis.usda.gov/OA/topics/gb.htm
- briandeer.com - report from the Sunday Times of London on a UK outbreak, May 17 1998
- [3] - CBS5 report on september 2006 outbreakes:Escherichia coli O157:H7
fr:Escherichia coli O157:H7 ja:O157 no:Escherichia coli O157:H7 pl:Escherichia coli O157:H7
