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Listeriosis is a systemic infection
caused by
foodborne
pathogen
Listeria monocytogenes, a small, motile, nonsporulating,
hemolytic, gram-positive coccobacillus.
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On cultures the organism is
easily confused with streptococci or corynebacteria.
Predisposing
factors:
Listeriosis is the leading food-borne fatal infection affecting most
often the immunocompromised, the elderly, pregnant women and neonates.
Newborns can
acquire the infection in utero or during passage through the birth
canal, and are acutely ill with multiple organ involvement and
symptoms of either pneumonia or meningitis.
Listeriosis is a
severe disease with high hospitalization and case fatality rates.
It is
usually sporadic but may also be epidemic.
It is usually
asymptomatic or very mild in other (immunologically normal)
individuals.
Listeria
monocytogenes can survive and grow over a wide range of environmental
conditions such as refrigeration temperatures, low pH and high salt
concentration. This allows the pathogen to overcome food preservation
and safety barriers, and pose a potential risk to human health.
Outbreaks of listeriosis
have been caused by
unpasteurized
milk,
contaminated cheeses and other dairy
products.
Raw vegetables can also transmit the disease.
Distribution:
L. monocytogenes has been
isolated worldwide from surface water, soil, vegetation, the faeces of
healthy people, from many species of domestic and wild mammals, and
from several species of birds.
Inspite of this wide distribution, the
spread of infection from animals to humans is rare.
Most human
infections are in urban rather than rural environments and occur
during July and August, while the peak months of infections in animals
are January through May.
Mode of infection:
The
organism initially colonizes the intestinal epithelium, then enters
the macrophages and spreads hematogenously. It enters cells by induced
phagocytosis, then escapes the phagosome by means of the virulence
factor, listeriolysin, and grows in the cytoplasm. Using the cell's
actin transport system, it spreads directly from cell to cell.
Most infections fall into one or
two groups.
1. Listeriosis of pregnancy, includes prenatal and postnatal
infections.
2. Listeriosis of the adult population, is
characterized by meningo-encephalitis and septicemia.
Chronic alcoholics, patients with
carcinoma, leukemia, or lymphomas and those receiving
immunosuppressive therapy are all susceptible.
Maternal infection early in pregnancy
leads to abortion or prematurity.
Infected live-born, premature
infants show signs of infection within a few hours of birth.
These
include respiratory distress, pneumonia, hepatosplenomegaly, papular cutaneous and mucosal nodules, leukopenia, and thrombocytopenia.
In
neonatal listeriosis acquired during delivery, the onset is 3 days to
2 weeks after delivery.
Infections before birth involve many
organs and tissues.
The amniotic fluid, the placenta, and cord are all
heavily infected.
The cutaneous lesions are raised and necrotic with
red margins.
Abscesses are found in the liver, spleen, lymph nodes,
adrenals, lungs, pleura, esophagus, posterior pharynx and tonsils.
Microscopically, the visceral lesions are foci of necrosis and
suppuration that contain many bacteria.
Older lesions contain histiocytes and occasionally epithelioid cells and lymphocytes, but not
giant cells.
Still older lesions have a fibrous wall.
Neurologic
sequelae are common, and the mortality is high even when therapy is
prompt.
Meningitis is the most common form of
listeriosis in adults, and resembles other bacterial meningitides.
Microscopically, the leptomeninges are infiltrated with lymphocytes,
plasma cells, macrophages, and neutrophils.
The inflammation extends
into the brain along the Virchow-Robin spaces.
Septicemic listeriosis in adults is
most common in immunodeficient patients, and causes severe illness
with high fever and prostration.
Shock and disseminated intravascular
coagulation may lead to an erroneous diagnosis of gram-negative
sepsis.
Listerial endocarditis resembles endocarditis caused by other
bacteria.
Septicemia may seed the brain with military abscesses and
cause a suppurative leptomeningitis.
Diagnosis
: The diagnosis is made by isolating L. monocytogeses in culture or demonstrating the typical gram-positive
bacilli in tissue sections.
Treatment
:
Prolonged treatment with antimicrobials is
usually required, because patients tend to relapse if treatment is
less than 3 weeks.
Penicillin or ampicillin is the treatment of
choice.
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