Chapter 132. Infections Caused by Listeria monocytogenes (Part 2) docx

Chapter 132. Infections Caused by
Listeria monocytogenes
(Part 2)

Immune Response
The innate and acquired immune responses to L. monocytogenes have been
studied extensively in mice. Shortly after IV injection, most bacteria are found in
Kupffer cells in the liver, with some organisms in splenic macrophages. Listeriae
that survive the bactericidal activity of initially infected macrophages grow in the
cytosol and spread from cell to cell. In the liver, the result is infection of
hepatocytes. Neutrophils are crucial to host defense during the first 24 h of
infection, while influx of activated macrophages from the bone marrow is critical
subsequently. Mice that survive sublethal infection clear the infection within a
week, with consequent sterile immunity. Knockout mice have been used to show
that interferon γand tumor necrosis factor (TNF) are essential in controlling
infection. While innate immunity is sufficient to control infection, the acquired
immune response is required for sterile immunity. Immunity is cell-mediated;
antibody plays no measurable role. The critical effector cells are cytotoxic (CD8+)
T cells that recognize and lyse infected cells. The bacteria grow and spread from
cell to cell. The host recognizes and lyses infected cells, and extracellular bacteria
are killed by circulating activated phagocytes. A hallmark of the L. monocytogenes
model is that killed vaccines do not provide protective immunity. The explanation
for this fundamental observation is multifactorial, involving the generation of
appropriate cytokines and the compartmentalization of bacterial proteins for
antigen processing and presentation.
Epidemiology
L. monocytogenes usually enters the body via the gastrointestinal tract in
foods. Listeriosis is most often sporadic, although outbreaks do occur. Recent
annual incidences in the United States range from 2 to 9 cases per 1 million
population. No epidemiologic or clinical evidence supports human-to-human
transmission (other than vertical transmission from mother to fetus) or waterborne

infection. In line with its survival and multiplication at refrigeration temperatures,
L. monocytogenes is commonly found in processed and unprocessed foods of
animal and plant origin, especially soft cheeses, delicatessen meats, hot dogs,
milk, and cold salads. Because food supplies are increasingly centralized and
normal hosts tolerate the organism well, outbreaks may not be immediately
apparent; pulsed-field gel electrophoresis has proved useful in linking cases to
specific foods. FoodNet, an active U.S. surveillance program, has demonstrated
decreases in listeriosis incidence, although recent data from some European
countries show a stable or increased number of cases, perhaps because of
enhanced active surveillance. The U.S. Food and Drug Administration has a zero-
tolerance policy for L. monocytogenes in ready-to-eat foods.
Diagnosis
Symptoms of listerial infection overlap greatly with those of other
infectious diseases. Timely diagnosis requires that the illness be considered in
groups at risk: pregnant women; elderly persons; neonates; individuals
immunocompromised by organ transplants, cancer, or treatment with TNF
antagonists or glucocorticoids; and patients with a variety of chronic medical
conditions, including alcoholism, diabetes, renal disease, rheumatologic illness,
and iron overload. Meningitis in older adults (especially with parenchymal brain
involvement or subcortical brain abscess) or a local outbreak of culture-negative
febrile gastroenteritis should trigger consideration of L. monocytogenes infection.
Listeriosis occasionally affects healthy, young, nonpregnant individuals. HIV-
infected patients are at risk; however, listeriosis seems to be prevented by
trimethoprim-sulfamethoxazole (TMP-SMX) prophylaxis targeting other AIDS-
related infections. The diagnosis is typically made by culture of blood,
cerebrospinal fluid (CSF), or amniotic fluid. L. monocytogenes may be confused
with "diphtheroids" or pneumococci in gram-stained CSF or may be gram-variable
and confused with Haemophilus spp. Serologic tests and polymerase chain
reaction assays are not clinically useful diagnostic tools at present.
Clinical Manifestations

Listerial infections present as several clinical syndromes, of which
meningitis and septicemia are most common. Monocytosis is seen in infected
rabbits but is not a hallmark of human infection.
Gastroenteritis
Appreciated only since the outbreaks of the late 1980s, listerial
gastroenteritis typically develops within 48 h of ingestion of a large inoculum of
bacteria in contaminated foods such as milk, deli meats, and salads. Attack rates
are high (50–100%). L. monocytogenes is neither sought nor found in routine fecal
cultures, but its involvement should be considered in outbreaks when cultures for
other likely pathogens are negative. Manifestations include fever, diarrhea,
headache, and constitutional symptoms. The largest reported outbreak occurred in
an Italian school system and included 1566 individuals; ~20% of patients were
hospitalized, but only one person had a positive blood culture. Isolated
gastrointestinal illness does not require antibiotic treatment. Surveillance studies
show that 0.1–5% of healthy asymptomatic adults may have stool cultures positive
for the organism.
Bacteremia
L. monocytogenes septicemia presents with fever, chills, and
myalgias/arthralgias and cannot be differentiated from septicemia involving other
organisms. Meningeal symptoms, focal neurologic findings, or mental status
changes may suggest the diagnosis. Bacteremia is documented in 70–90% of
cancer patients with listeriosis. A nonspecific flulike illness with fever is a
common presentation in pregnant women. Endocarditis of prosthetic and native
valves is an uncommon complication, with reported fatality rates of 35–50% in
case series. A lumbar puncture is often prudent, although not necessary, in
pregnant women without central nervous system (CNS) symptoms.