Listeriosis
Listeriosis
Listeriosis is a foodborne
infectious disease caused by a bacterium, Listeria monocytogenes. It's most
commonly caused by eating improperly processed meats and raw milk or food
products prepared with raw milk or unpasteurized milk like ice cream &
yogurt. It is a sporadic bacterial infection that can affect a wide range of
animals, birds, and humans. Healthy people rarely become ill from listeria
infection, but the disease can be fatal to unborn babies, newborns, and people
with weakened immune systems. Hence can be very serious for pregnant women,
people older than 65, and people with weakened immune systems. The most
frequently recognized form of clinical presentation is Encephalitis or
meningoencephalitis. Early diagnosis and prompt antibiotic treatment can help
to control the effects of listeria infection.
Etiology
Listeria monocytogenes is
a gram-positive, motile, non-spore-forming, extremely resistant, diphtheroid
coccobacillus. The exact range for the growth of Listeria is 39.2°F (4 °C)
to 98.6°F (37°C). It is a ubiquitous saprophyte, that lives in the plants or
soil environment. It has been isolated from 42 species of domestic and wild
mammals and other feedstuffs, including milk, cheese, meconium, and feces.
Epidemiology
Worldwide, more frequently
seen in temperate and colder climates. It is a relatively rare disease with 0.1
to 10 cases per 1 million people per year depending on the countries and
regions of the world. Although the number of cases of listeriosis is small, the
high rate of death (20%–30%) associated with this infection makes it a
significant public health concern.
A recent meta-analysis
conducted from the studies done in South East Asian countries found that: the
overall prevalence for L. monocytogenes was 16% (in food, animal, and
environmental sources) (95% [CI]: 10–23). The subgroup analysis revealed
ready-to-eat food of vegetable origin had the highest prevalence of 21% (CI:
6–41).
A Sri Lankan study
conducted in 1995 to identify L. monocytogenes contamination in food samples
taken from the market found that 38% of the samples contained bacteria. The
percentage of positive samples was highest among vegetables (49%), second in
chicken (34%), and lowest in dairy products (26%). Inactivation of L.
monocytogenes during the cooking of green leaves indicated that L.
monocytogenes was recovered from samples up to 8 min of cooking by which time
the temperature was 72OC.
The natural reservoir of
L. monocytogenes is found to be soil and mammalian GI tracts, which can
contaminate plants. Animals ingest the organism and further contaminate
vegetation and soil. Among animals, the transmission occurs via the feco-oral
route. From animals to humans, transmission occurs either through direct
contact with infected animals or via ingestion of contaminated food products
such as milk, cheese, meat, eggs, or vegetables. Outbreaks of this disease have
generally involved the ingestion of foods containing high doses of L.
monocytogenes. Epidemiologic investigation of several outbreaks of listeriosis
globally recognized that listeriosis is a foodborne disease.
The disease
Listeriosis affects all
ages and sexes. There are two main types of listeriosis: a non-invasive form
and an invasive form. Noninvasive listeriosis (febrile listerial gastroenteritis)
is a mild form of the disease affecting mainly otherwise healthy people.
Symptoms include diarrhea, fever, headache, and myalgia (muscle pain). Invasive
listeriosis is a more severe form of the disease and affects certain high-risk
groups of the population. It causes severe infections in humans. Human illness
due to L. monocytogenes usually occurs in the setting of immunosuppression,
pregnancy, or extremes of age. Although human exposure to L. monocytogenes is
not uncommon, Transmission of L. monocytogenes by food occurs following the
penetration of the organism through the intestine. Intracellular multiplication
can occur in various types of cells. Factors that may influence whether the
invasive disease will occur include the infecting organism's virulence, the
host's susceptibility, and the size of the inoculum.
Symptoms might begin a few
days after eating contaminated food. The incubation period is usually one to
two weeks but can vary between a few days and up to 90 days. It can be
presented with fever, chills, muscle aches, nausea, and diarrhea. If the
listeria infection spreads to your nervous system, signs and symptoms can
include headache, seizures, stiff neck, confusion or changes in alertness, loss
of balance, and convulsions. Nonpregnant adults with listeriosis are most often
diagnosed with meningitis, meningoencephalitis, or sepsis. Additional syndromes
include abscesses of the brain and spinal cord, endocarditis, endophthalmitis,
osteomyelitis, and septic arthritis.
Listeriosis during pregnancy
Pregnant women are about
20 times more likely to contract listeriosis than other healthy adults.
Listeriosis may develop at any time during pregnancy, although most infections
are detected in the third trimester. Although listeria is a mild illness in pregnant
women, the consequences for their babies can be serious. Miscarriage,
Stillbirth, Premature birth, or a potentially fatal infection can occur after
birth.
Various detection methods,
including polymerase chain reaction (PCR), are available for diagnosing
listeriosis in humans. The initial diagnosis of listeriosis is made based on
clinical symptoms and detection of the bacteria in a smear from blood,
cerebrospinal fluid (CSF), meconium of newborns (or the fetus in abortion
cases), as well as from feces, vomitus, foods or animal feed.
The spinal fluid
examination may show pleocytosis with predominantly polymorphonuclear
leukocytes; Gram stain may show gram-positive bacilli but is more often
unrevealing; the protein level is elevated; and the glucose level usually is
within normal limits. However, many other patterns have been observed.
Diagnosis is made by culture of L. monocytogenes from spinal fluid, blood, or
some other usually sterile site.
Prevention
Listeriosis is a serious,
but preventable and treatable disease. Due to the long incubation period, it is
challenging to identify the food which was the actual source of the infection.
The controlling L. monocytogenes are challenged due to its ubiquitous nature,
high resistance to common preservative methods, such as the use of salt, smoke,
or acidic condition in the food, and its ability to survive and grow at
refrigeration temperatures (around 5 °C). The increased number of
immunosuppressed populations at present has led them to an increased risk of listeriosis.
More concern was gained as a clinical entity due to high morbidity and
mortality and also an increase in safety measures of the food industry.
Vulnerable people with a weakened immune system should avoid unsafe food. L.
monocytogenes is not especially heat resistant and is killed at normal
pasteurization temperature. Pasteurization heats milk to a high enough
temperature for a long enough time to kill germs. Heat the milk to 63°C (150°F)
for at least 30 minutes or 72°C (162°F) for at least 15 seconds and thoroughly
cooking the meat product to 165ºF/74ºC will kill the bacteria. Therefore
listeriosis due to cooked poultry is linked either to inadequate cooking or
recontamination after cooking. However, the Sri Lankan practice of thoroughly
cooking meat in the form of curry probably reduces the risk of listeriosis
through poultry meat. In the current context, fast food outlets serving various
preparations of poultry meat may pose a threat if inadequately cooked.
L. monocytogenes can
easily propagate in nature and can maintain a non-zoonotic life cycle in soil,
water, and vegetation. Therefore raw vegetables are considered an important
vehicle of bacteria. Since green leaves are grown in low-lying areas with
plenty of water the possible contamination with L. monocytogenes is high. The
practice of eating green leaves either raw or partially cooked as “mallun” is
common in Sri Lanka. Therefore proper cleaning before preparation and as
evidenced by studies shows that cooking time and temperature should exceed 8
min and 72OC respectively in order to inactivate a heavy inoculum of L.
monocytogenes. Lettuce and cabbage too were found to be contaminated and
adequate precautions should be taken when preparing salads. It has been shown
that when vegetables are held at 4OC for 4 d the population of L. monocytogenes
can increase. The use of refrigeration in supermarkets and homes does not
reduce the risk due to the ability of the organism to multiplicate during
refrigerated storage.
In conclusion, it can be
said that those engaged in the food industry as well as at household levels
should practice good food hygienic practices in order to eliminate the organism
during processing, avoid post-processing contamination and check sample
products for contamination.
References
https://www.who.int/news-room/fact-sheets/detail/listeriosis
https://foodsafetytech.com/column/planning-is-key-component-of-listeria-prevention/
Gunasena, D., Kodikara,
C., Ganepola, K., & Widanapathirana, S. (1995). Occurrence of Listeria
monocytogenes in food in Sri Lanka. Journal of the National Science Foundation
of Sri Lanka, 23(3), 107. https://doi.org/10.4038/jnsfsr.v23i3.5848
Lanka, S. (2021).
Epidemiological Bulletin Listeriosis in Ruminants, 14(03).
Schuchat, A., Swaminathan, B., & Broome, C. V. (1991). Epidemiology of human listeriosis. Clinical Microbiology Reviews, 4(2), 169–183. https://doi.org/10.1128/CMR.4.2.169
