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Listeria monocytogenes
is a Gram-positive rod-shaped bacterium. It is the agent of
listeriosis,
a serious infection caused by eating food contaminated with the bacteria.
Listeriosis has recently been recognized as an important public health problem.
The disease affects primarily pregnant women, newborns, and adults with weakened
immune systems. Listeriosis is a serious disease for humans; the overt form
of the disease has a mortality greater than 25 percent. The two main clinical
manifestations are sepsis and meningitis. Meningitis is often complicated by
encephalitis, a pathology that is unusual for bacterial infections.
Researchers to have isolated L. monocytogenes from
soil, leaf litter, sewage, silage, dust and water. The organism often moves
through the animal and human intestinal tract without causing illness, and has
been found in many domestic and wild animals, including birds and fish. Because
L. monocytogenes is widely present in the environment, it would be
impossible to prevent animals from coming in contact with the bacteria. However,
farmers, animal producers, food processors and food handlers can all take steps
to reduce contamination and keep food safe from L. monocytogenes.
L.
monocytogenes is not "new." Since 1911, scientists
have known it infects animals, and in 1929 the first case of human infection was
detected. In earlier time, many
believed farm animals transmitted L. monocytogenes to farm workers. But
when listeriosis appeared in city dwellers, public health authorities realized
that animal contact was not always the source of disease transmission.
It has been only in
the past decade that researchers have recognized L. monocytogenes as an
agent of foodborne illness. Fecal contamination is one way the organism is
spread to raw agricultural products. For example, farm animals may pick it up
from consuming improperly fermented silage, and then vegetables may become
contaminated when animal manure carrying the organism is used for fertilizer.
Animals in a herd also may pick up L. monocytogenes from other animals or
manure containing the organism.
L. monocytogenes
is a remarkable tough organism. It resists heat, salt, nitrite and acidity much
better than many organisms. The bacteria survive on cold surfaces and also can
multiply slowly at -4°C
24°F, defeating one traditional food safety
defense--refrigeration. (Refrigeration at 4°C
40°F or below stops the multiplication of many
other foodborne bacteria. Refrigeration does not kill most bacteria.) Commercial
freezer temperatures of -18°C
0°F, however, will stop L. monocytogenes from multiplying.
Healthy
people do not often develop noticeable listeriosis symptoms after eating food
containing L. monocytogenes. However, some people are very susceptible to
the disease.
The highest
incidence of listeriosis has been in persons over 60 years old and newborns. One
third of infections occur during pregnancy and may lead to spontaneous abortions
or serious illness in newborns. Others most at risk include patients with immune
systems compromised by cancer, AIDS, or immunosuppressive medications such as
steroids; and patients suffering from cirrhosis, diabetes and ulcerative
colitis. The disease symptoms are variable and depend on the individual's
susceptibility. Symptoms may be limited to fever, fatigue, nausea, vomiting and
diarrhea. However, these symptoms can precede a more serious illness.
The more serious forms of
listeriosis can result in meningitis (brain infections) and septicemia (bacteria
in the bloodstream). Pregnant women may contract flu-like symptoms of
listeriosis; complications can result in miscarriage, stillbirth, or septicemia
or meningitis in the newborn. In older children and adults, complications
usually involve the central nervous system and blood stream, but may include
pneumonia and endocarditis (inflammation of the lining of the heart and valves).
Skin contact with L. monocytogenes can cause localized abscesses or skin
lesions. It takes from one to six weeks for a serious case of listeriosis
to develop, although flu-like symptoms may occur 12 hours after eating L.
monocytogenes-contaminated food. Onset time probably depends on the health
of the patient, the strain of L. monocytogenes and the dose--or amount of
bacteria-- ingested.
Listeriosis can be
positively diagnosed, using clinical laboratory techniques, only by culturing
the organism from blood or cerebrospinal fluid. Listeriosis can be treated with
antibiotic drugs such as penicillin or ampicillin.
Hazard
Analysis and Critical Control Point (HACCP) system as the most
effective strategy for controlling the presence of L. monocytogenes and
other pathogenic bacteria on food products. In addition to encouraging adoption
of this strategy by all who handle food, from farm worker to plant processor to
consumer, the agencies are working with industry to design strong HACCP
programs. Most of the food industry supports HACCP.
In a HACCP, points at which food risks are
more likely to be introduced are identified, and interventions are introduced
where control is possible to reduce the potential for consumption of unsafe
products. For instance, insufficient cooking or raw meat, poultry or milk may
allow the survival of pathogenic bacteria and present a hazard. Therefore, the
agencies require adequate cooking temperatures to destroy the bacteria.
Areas of concern in food processing plants
include plant design and layout, equipment design, process control, personnel
practices, cleaning and sanitizing procedures, and verification of pathogen
control.
Listeria bacteria do not change the
taste or smell of a food. As a final check, food handlers--in homes, restaurants
and institutional kitchens--must follow basic food safety procedures for
destroying any potentially harmful bacteria, thereby avoiding any foodborne
illness.
Recommendations for all Individuals:
Although most people are at very low risk for
listeriosis, the risk of listeriosis and other foodborne illnesses can be
reduced by following these tips:
 | Avoid raw/unpasteurized milk.
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| Keep raw and cooked foods separate when
shopping, preparing, cooking and storing foods. Otherwise, bacteria in juices
from raw meat, poultry or fish might contaminate a cooked food. For instance,
transfer cooked meat, poultry or fish to a clean platter -- never to the dish
that held the raw food of animal origin.
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| Wash hands, knives, and cutting boards after
handling uncooked foods.
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| Wash raw vegetables thoroughly before eating.
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| Thoroughly cook all food of animal origin,
including eggs. Cook raw meat to an internal temperature of
71°C 160°F, raw
poultry to 82°C 180°F,
and raw fish to 71°C 160°F
or until it is white and flaky. Reheat leftovers thoroughly.
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| Read and follow label instructions to "keep
refrigerated" and "use by" a certain date.
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| Keep hot foods to (above
60°C 140°F). Do not keep them out for
longer than two hours at room temperature - at which L. monocytogenes
can thrive - before eating.
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| Keep cold foods cold (at or below
4°C 40°F). Do not
keep them out for longer than two hours at room temperature before eating.
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| Divide leftovers into small, shallow covered
containers before refrigerating, so that they chill rapidly and evenly.
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| Keep your refrigerator clean and keep
the temperature at 1-4°C
34-40°F.
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Recommendations to High-Risk Individuals:
Persons at increased risk for listeriosis such
as pregnant women, the elderly, and those with immunosuppressive conditions can
decrease the risk by:
| Avoiding soft cheese such as Mexican style,
Feta, Brie, Camembert and blue cheese. Mexican-style cheeses are soft, white,
ethnic (Hispanic-Latin American) cheeses such as Queso Blanco and Queso
Fresco. There is no need to avoid hard cheese, processed slices, cottage
cheese or yogurt.
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| Reheating leftover foods or ready-to-eat foods
such as hot dogs thoroughly until steaming hot before eating.
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| Although the risk of listeriosis associated
with foods from delicatessen counters is relatively low, pregnant women and
immunosuppressed persons may choose to avoid these foods or to thoroughly
reheat cold cuts before eating. |
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