© 2008 Canadian Medical Association or its licensors
• OCTOBER 7, 2008 • 179(8)
iety about personal health and increase
the workloads of front-line physicians.
The drama further increases when the of-
fending microbe is not well known. This
primer on listeriosis hones in on the mi-
crobe, the incidence of infection, clinical
presentations, diagnosis, treatment and
prevention. The information is based on a
MEDLINE search on listeriosis and dis-
cussion with experts.
erious outbreaks of food-borne ill-
ness, whether regional or national,
dramatically raise community anx-
Listeria monocytogenes is a small, gram-
positive bacillus that can grow in anaero-
bic or aerobic conditions. It is found
widely in the environment in soil, decay-
ing vegetation and water and may be part
of the fecal flora of many mammals, in-
cluding healthy human adults.1
L. monocytogenes presents a particular
concern with respect to food handling be-
cause it can grow at refrigerator tempera-
tures (4°C to 10°C), temperatures com-
monly used to control pathogens in foods.
Freezing also has little detrimental effect
on the microbe. Although pasteurization is
sufficient to kill Listeria, failure to reach
the desired temperature in large packages can allow the organ-
ism to survive. Food can also be contaminated after processing
by the introduction of unpasteurized material, as happens dur-
ing the preparation of some cheeses. Listeria can also be
spread by contact with contaminated hands, equipment and
The centralized production of prepared ready-to-eat food
products in Canada increases the risk of higher levels of con-
tamination, since it requires that foods be stored for long peri-
ods at refrigerated temperatures that favour the growth of Lis-
teria. During the preparation, transportation and storage of
prepared foods, the organism can multiply to reach a thresh-
old needed to cause infection.
The approximate infective dose of L. monocytogenes is es-
timated to be 10–100 million colony forming units (CFU) in
healthy hosts, and only 0.1–10 million CFU in people at high
risk of infection (Box 1).2Foods such as raw vegetables, raw
(unpasteurized) milk and cheese, and meats (fresh and frozen)
may become contaminated with L. monocytogenes because of
where they come from and how they are processed. Ready-to-
eat foods such as cold cuts or deli meats, cheeses and other
dairy products are ideal sources for contamination.
Although L. monocytogenes was recognized as an animal
pathogen over 80 years ago,3the first outbreak confirming an
Listeriosis: a primer
Robert Bortolussi MD
Robert Bortolussi is with the Department of Pediatrics, IWK Health Centre
and Dalhousie University, Halifax, NS
Box 1: People at high risk of listeriosis
• The risk of listeriosis is about 20 times higher among pregnant women than
among nonpregnant healthy adults
• About one-third of listeriosis cases are diagnosed in pregnant women
• Pregnant women with listeriosis are at increased risk of spontaneous
abortion, preterm delivery or stillbirth
• Whereas pregnant women may get a mild form of the infection, newborns
are at greater risk of the serious effects of infection
• Newborns may present clinically with early-onset listeriosis (< 7 days) or late-
onset infection (≥ 7 days)
• Early-onset listeriosis is typically diagnosed within the first 24 hours of life
and is usually transmitted from the mother transplacentally
• Late-onset neonatal listeriosis is less common; the mode of acquisition is
poorly understood, but in most cases there is no history of maternal infection
• Listeriosis is not felt to be spread through breast milk
• Mortality among infected newborns is high (50%)
Elderly people (age ≥ ≥ ≥ ≥ 60 yr)
• About 50% of cases occur in this age group
• People with cancer, diabetes or kidney disease
• People with HIV/AIDS: listeriosis is up to 300 times more likely to occur in this
group than in healthy adults
• Patients receiving immunosuppressive drug therapy (e.g., high-dose
glucocorticosteroid, tumour necrosis factor inhibitor)
• Transplant patients receiving anti-rejection drug therapy
Published at www.cmaj.ca on Sept. 11, 2008.
CMAJ • OCTOBER 7, 2008 • 179(8)
indirect transmission from animals to humans was reported
only in 1983, in Canada’s Maritime provinces.4In that out-
break, cabbages, stored in the cold over the winter, were con-
taminated with Listeria through exposure to infected sheep
manure. A subsequent outbreak in California in 1985 con-
firmed the role of food in disseminating listeriosis. Since then
Listeria has been implicated in many outbreaks of food-borne
illness, most commonly from exposure to contaminated dairy
products and prepared meat products, including turkey and
deli meats, pâté, hot dogs and seafood and fish.5
Incidence of infection
The incidence of listeriosis is difficult to establish, since
symptoms may be mistaken for a flu-like illness or gastro-
enteritis and appropriate cultures not obtained. The Canadian
Listeriosis Reference Service was created in 2001 to actively
investigate cases and develop a molecular epidemiology data-
base of isolates as a resource for outbreak investigations. In
2006, listeriosis was named as a nationally notifiable disease
by the Public Health Agency of Canada. Between 1995 and
1999 (the most recent years for which data are available),
25–51 cases occurred annually in Canada.6However, passive
surveillance programs such as this may suffer from under-
reporting of cases.5Countries with surveillance programs
have reported rates of infection from 0.6 to 6.2 cases per mil-
lion, with countries having active surveillance programs re-
porting the highest incidence.5,7–11The case-fatality rates vary
from country to country, but invariably the highest mortality
is among newborns with infection acquired from their moth-
ers (25%–50%). Mortality among those over 60 years of age
is also high (10%–20%).
Initial symptoms of infection include nonspecific flu-like
symptoms, nausea, vomiting, cramps, diarrhea and fever.
There are few clinical features that are unique to listeriosis.
Therefore, clinicians must consider a variety of potential
causes for infection, including viral infections (influenza) and
other bacterial infections that may cause sepsis or meningitis.
Symptoms can develop at any time from 2 to 70 days after
eating contaminated food. Except for vertical mother–fetus
transmission, most cases of listeriosis begin with ingestion of
the organism from a food source.
Most healthy adults and children who consume contami-
nated food experience only mild to moderate symptoms. The
infection is usually self-limited, since, in healthy hosts, expo-
sure to Listeria stimulates the production of tumour necrosis
factor and other cytokines, which activate monocytes and
macrophages to eradicate the organism.12,13Few people with
normal immune function go on to have more severe, life-
threatening forms of listeriosis, characterized by septic shock,
meningitis and encephalitis.
In contrast, people with poor immune function are at much
higher risk of severe, life-threatening forms of listeriosis.
High-risk groups include pregnant women, newborns, elderly
people (≥ 60 years of age) and people with a weakened im-
mune system (Box 1). Individuals taking tumour necrosis fac-
tor inhibitors and transplant recipients taking anti-rejection
drugs are now included in this high-risk group.14
Pregnant women with listeriosis may have only mild symp-
toms of the infection. However, they are at increased risk of
having a spontaneous abortion during the first trimester or of
giving birth to a premature infant with acute sepsis if they are
exposed later in the pregnancy.
Newborns may present clinically with early- (< 7 days) or
late-onset forms of infection (≥ 7 days). Those with the early-
onset form are often diagnosed in the first 24 hours of life
with sepsis. Early-onset listeriosis is most often acquired from
the mother through transplacental transmission. Late-onset
neonatal listeriosis is less common than the early-onset form.
Clinical symptoms may be subtle and include irritability,
fever and poor feeding.13The mode of acquisition of late-
onset listeriosis is poorly understood, but acquisition of the
organism after birth is implicated, since there is usually no
maternal infection in such cases. The organism is not felt to
be spread through breast-feeding.
Common clinical features among patients with nonperina-
tal forms of listeriosis include meningitis (about 33% of such
cases) and septicemia (25%). Less commonly, patients may
have inflammatory gastroenteritis, endocarditis or joint
Diagnosis and management
Early diagnosis and treatment of listeriosis in high-risk pa-
tients is critical, since the outcome of untreated infection can
be devastating. This is especially true for pregnant women be-
cause of the increased risk of spontaneous abortion and
preterm delivery. Depending on the risk group, rates of death
from listeriosis range from 10% to 50%, with the highest rate
among newborns in the first week of life.
The Public Health Agency of Canada convened an expert
panel in August 2008 to provide information to health care
professionals and the general public on the diagnosis and
management of listeriosis during the recent outbreak. The fol-
lowing information is based on the panel’s discussion and ad-
dresses what should be done for patients who have eaten food
items that are suspected of being contaminated with Listeria
and who have symptoms of diarrhea with or without fever.
• For healthy adults and children with a normal immune sys-
tem, no Listeria-specific investigation is required. Gastro-
enteritis due to Listeria infection has a short duration and is
self-limited in this population. Culture of a stool specimen
for common bacterial enteric pathogens may be warranted if
indicated by the person’s history and clinical condition.
• For high-risk patients (Box 1), more aggressive investiga-
tion and management strategies should be followed. If the
patient has diarrhea only, culture of a stool specimen for
common bacterial enteric pathogens may be indicated. If
the patient has a fever, bacteremia must be suspected. Two
blood samples for aerobic culture should be drawn. Listeria
is only one of many bacteria that may cause infections in
such patients; thus, investigations and treatment should be
directed to cover common causes in addition to Listeria.
CMAJ • OCTOBER 7, 2008 • 179(8)
• For pregnant women with a fever or signs of sepsis, blood
and urine samples should be obtained for culture. Empiric
therapy, including ampicillin, should be started for cover-
age of Listeria.
• For infants born to women suspected of having listeriosis,
blood cultures should be taken and antibiotic therapy started.
Listeria is susceptible to ampicillin, which should be given
with gentamicin to patients with sepsis. If further advice on
the management of patients with listeriosis is needed, it
should be obtained in consultation with the appropriate spe-
cialist such as an obstetrician, a neonatologist or infectious
Prevention of infection
Given the characteristics of Listeria and the risk of infection,
what can be done to minimize and prevent infection? The
Canadian Food Inspection Agency and the US Centers for
Disease Control and Prevention have made recommendations
to protect against listeriosis in the home. People with a poor
immune system and pregnant women are advised to take even
more rigorous precautions by avoiding foods that may become
contaminated with Listeria (Box 2).5,7
Although food-borne outbreaks of listeriosis are uncommon,
they remain a major public health problem. In the United
States, about 20%–65% of deaths from all food-borne infec-
tions are due to Listeria.5Therefore, detecting an outbreak
early and identifying its source is a priority. Prompt reporting
of all cases of Listeria infection to public health authorities is
important to assist them in early detection of an outbreak.
During an outbreak of listeriosis, vigilance in the prepara-
tion of food is especially important for immunocompromised
patients and pregnant women. Initial manifestations of infec-
tion are nonspecific and include flu-like symptoms. Such
symptoms should be thoroughly investigated if they develop
in patients at high risk of listeriosis (Box 1). In contrast, in pa-
tients with normal immune function, symptoms are usually
milder and the infection self-limited; such patients do not
need tests or treatment. Proper preparation, storage and han-
dling of food by industry and in the home can minimize the
risk of a widespread outbreak.
1. Schlech WF III, Schlech WF IV, Haldane H, et al. Does sporadic Listeria gastro-
enteritis exist? A 2-year population-based survey. Clin Infect Dis 2005;41:778-84.
2. Farber JM, Ross WH, Harwig J. Health risk assessment of Listeria monocytogenes
in Canada. Int J Food Microbiol 1996;30:145-56.
3.Murray EGD, Webb RA, Swann MBR. A disease of rabbits characterized by a
large mononuclear leucocytosis, caused by a hitherto undescribed Bacillus: Bac-
terium monocytogenes (n. sp.). J Pathol Bacteriol 1926;29:407.
4. Schlech WF III, Lavign PM, Bortolussi R, et al. Epidemic listeriosis — evidence
for transmission by food. N Engl J Med 1983;308:203-6.
5.Lynch M, Painter J, Woodruff R., et al. Surveillance for foodborne-disease out-
breaks — United States, 1998–2002. MMWR Surveill Summ 2006;55(SS10):1-34.
Available: www.cdc.gov/mmwr/preview/mmwrhtml/ss5510a1.htm (accessed 2008
6. Public Health Agency of Canada. Notifiable Diseases On-Line. Notifiable disease
incidence by age group — listeriosis, 2006. Ottawa: The Agency. Available:
http://dsol-smed.hc-sc.gc.ca/dsol-smed/ndis/c_age3_e.html (accessed 2008 Sept 6).
7.Food Safety Network. Listeria fact sheet — updated. Guelph (ON): University of
Guelph; 2003. Available: www.foodsafetynetwork.ca/en/article-details.php?a=
3&c=14&sc=112&id=439 (accessed 2008 Aug 25).
8. Koch J, Stark K. Significant increase of listeriosis in Germany — epidemiological
patterns 2001–2005. Euro Surveill 2006;11:85-8.
9. Siegman-Igra Y, Levin R, Weinberger M, et al. Listeria monocytogenes infection in
Israel and review of cases worldwide. Emerg Infect Dis2002;8:305-10.
10.Okutani A, Okada Y, Yamamoto S, et al. Nationwide survey of human Listeria
monocytogenes infection in Japan. Epidemiol Infect 2004;132:769-72.
11. Doorduyn Y, de Jager CM, van der Zwaluw WK, et al. Invasive Listeria monocy-
togenes infections in the Netherlands, 1995–2003. Eur J Clin Microbiol Infect Dis
12. Cresence V, Dharsana K, Lekshmi M, et al. Listeria — review of epidemiology
and pathogenesis. J Microbiol Immunol Infect 2007;40:4-13.
13. Bortolussi R, Mailman T. Listeriosis. In: Feigin R, Cherry J, Demmler-Harrison G,
et al, editors. Textbook of Pediatric Infectious Diseases. 5th ed. Philadelphia (PA):
WB Saunders; 2004. p. 1330-6.
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ment. Proc Am Thorac Soc 2005;2:456-60.
Competing interests: None declared.
Box 2: Recommendations for reducing the risk
• Thoroughly cook raw food from animal sources, such as
beef, pork and poultry
• Wash raw vegetables thoroughly before eating
• Keep uncooked meats separate from vegetables and
from cooked foods and ready-to-eat foods
• Avoid raw (unpasteurized) milk or foods made from
• Wash hands, knives and cutting boards after handling
• Consume perishable and ready-to-eat foods as soon as
Additional recommendations for high-risk groups
• Do not eat hot dogs or ready-to-eat foods such as deli
meats unless they are reheated until steaming hot
• Wash hands after handling hot dogs and ready-to-eat
• Do not eat soft cheeses (e.g., feta, brie and camembert)
or blue-veined cheeses unless they have labels that
clearly state they are made from pasteurized milk
• Do not eat refrigerated pâtés or meat spreads; canned
or “shelf-stable” (pasteurized) pâtés and meat spreads
may be eaten
• Do not eat refrigerated smoked seafood unless it is
contained in a cooked dish, such as a casserole; canned
or shelf-stable smoked seafood may be eaten
Correspondence to: Dr. Robert Bortolussi, Department of Pediatrics,
IWK Health Centre, Goldbloom Pavilion, 5890University Ave., Halifax
NS B3K 6R8; fax 902 470-7232; email@example.com