Escherichia coli O157:H7 is a leading
cause of foodborne illness. Based on a 1999
estimate, 73,000 cases of infection and 61 deaths
occur in the United States each year. In the ten
CDC Foodborne Diseases Active Surveillance Network
(FoodNet) sites (which represent 15% of the US
population), there was a 29% decline in E.
coli O157:H7 infection since 1996-98 (see FoodNet
Reports).
Infection with E. coli often leads to bloody
diarrhea, and occasionally to kidney failure.
People can become infected with E.coli
O157:H7 in a variety of ways. Though most illness
has been associated with eating undercooked,
contaminated ground beef, people have also become
ill from eating contaminated bean sprouts or fresh
leafy vegetables such as lettuce and spinach.
Person-to-person contact in families and child
care centers is also a known mode of transmission.
In addition, infection can occur after drinking
raw milk and after swimming in or drinking
sewage-contaminated water.
Consumers can prevent E. coli O157:H7
infection by thoroughly cooking ground beef,
avoiding unpasteurized milk, and by washing hands
carefully before preparing or eating food. Fruits
and vegetables should be washed well, but washing
may not remove all contamination. Public service
announcements on television, radio, or in the
newspapers will advise you which foods to avoid in
the event of an outbreak.
Because the organism lives in the intestines of
healthy cattle, preventive measures on cattle
farms, during meat processing, and during the
growth, harvest and processing of produce are
being investigated.
What
is Escherichia coli O157:H7?
E. coli O157:H7 is one of hundreds of
strains of the bacterium Escherichia coli.
Although most strains are harmless, this strain
produces a powerful toxin that can cause severe
illness. E. coli O157:H7 has been found
in the intestines of healthy cattle, deer, goats,
and sheep.
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. Since then, more
infections in the United States have been caused
by eating undercooked ground beef than by any
other food.
The combination of letters and numbers in the
name of the bacterium refers to the specific
markers found on its surface and distinguishes it
from other types of E. coli.
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How
is E. coli O157:H7 spread?
The organism can be found on most cattle farms,
and it is commonly found in petting zoos and can
live in the intestines of healthy cattle, deer,
goats, and sheep. Meat can become contaminated
during slaughter, and organisms can be
accidentally mixed into meat when it is ground.
Bacteria present on the cow's udders or on
equipment may get into raw milk. In a petting zoo,
E.coli O157:H7 can contaminate the
ground, railings, feed bins, and fur of the
animals.
Eating meat, especially ground beef, that has
not been cooked sufficiently to kill E. coli
O157:H7 can cause infection. Contaminated meat
looks and smells normal. The number of organisms
required to cause disease is very small.
Among other known sources of infection are
consumption of sprouts, lettuce, spinach, salami,
unpasteurized milk and juice, and by swimming in
or drinking sewage-contaminated water.
Bacteria in loose stool of infected persons can
be passed from one person to another if hygiene or
hand washing habits are inadequate. This is
particularly likely among toddlers who are not
toilet trained. Family members and playmates of
these children are at high risk of becoming
infected.
Young children typically shed the organism in
their feces for a week or two after their illness
resolves. Older children and adults rarely carry
the organism without symptoms.
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What
illness does E. coli O157:H7 cause?
People generally become ill from E. coli
O157:H7 two to eight days (average of 3-4) after
being exposed to the bacteria. Escherichia
coli O157:H7 infection often causes severe
bloody diarrhea and abdominal cramps. Sometimes
the infection causes non-bloody diarrhea or no
symptoms. Usually little or no fever is present,
and the illness resolves in 5 to 10 days.
In some persons, particularly children under 5
years of age and the elderly, the infection can
also cause a complication called hemolytic uremic
syndrome (HUS), in which the red blood cells are
destroyed and the kidneys fail. About 8% of
persons whose diarrheal illness is severe enough
that they seek medical care develop this
complication. In the United States, HUS is the
principal cause of acute kidney failure in
children, and most cases of HUS are caused by E.
coli O157:H7.
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How
is E. coli O157:H7 infection diagnosed?
Infection with E. coli O157:H7 is
diagnosed by detecting the bacterium in the stool.
About one-third of laboratories that culture stool
still do not test for E. coli O157:H7, so
it is important to request that the stool specimen
be tested on sorbitol-MacConkey (SMAC) agar for
this organism. All persons who suddenly have
diarrhea with blood should get their stool tested
for E. coli O157:H7.
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How
is the illness treated?
Most people recover without antibiotics or
other specific treatment within 5 to 10 days.
Antibiotics should not be used to treat this
infection. There is no evidence that antibiotics
improve the course of disease, and it is thought
that treatment with some antibiotics could lead to
kidney complications. Antidiarrheal agents, such
as loperamide (Imodium®), should also be avoided.
In some people, E. coli O157:H7
infection can cause a complication called
hemolytic uremic syndrome (HUS), a
life-threatening condition that is 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%.
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What
are the long-term consequences of infection?
Persons who only have diarrhea usually recover
completely.
A small proportion of persons with hemolytic
uremic syndrome (HUS) have immediate complications
with lifelong implications, such as blindness,
paralysis, persistent kidney failure, and the
effects of having part of their bowel removed.
Many persons with hemolytic uremic syndrome have
mild abnormalities in kidney function many years
later.
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What
can be done to prevent the infection?
Cattle are the principal source of E. coli O157
infection; they carry E. coli O157 in
their intestines. Changes in the preparation of
animals for slaughter and in slaughter and
processing methods could decrease the
contamination of carcasses with E. coli O157
and the subsequent contamination of meat. Testing
ground beef for E. coli O157 and
withholding it from the market until the test is
negative, as many meat producers began doing in
2002, is probably partly responsible for the
subsequent decrease in illnesses.
Cattle manure is an important source of E.
coli O157. Manure can contaminate the
environment, including streams that flow through
produce fields and are used for irrigation,
pesticide application, or washing. Collaborative
efforts are needed to decrease environmental
contamination and improve the safety of produce.
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What
can you do to prevent E. coli O157:H7
infection?
