Abstract
In the general population, Listeria infection is relatively uncommon, with a prevalence of approximately 4 per million in Canada, 0.27 per million in the USA and between 0.1 and 11.3 per million population in different parts of Europe.
Pregnant women have a 12- to 18-fold increased risk in pregnancy (12/100 000), compared with the non-pregnant population.
Out of all infections with Listeria, 16–27 per cent occur in pregnant women.
The exact cause of its higher prevalence in pregnancy is not known. Immunosuppression associated with pregnancy results in suppression of cell-mediated immunity in the placenta, due to high concentration of maternal hormones and other unknown mechanisms.
Approximately 20 per cent of Listeria infections involve neonatal infection with potentially severe complications, therefore it is important to stop the outbreaks in pregnant woman and the unwanted consequences for their unborn fetuses.
Prevalence and Incidence
In the general population, Listeria infection is relatively uncommon, with a prevalence of approximately 4 per million in Canada, 0.27 per million in the USA and between 0.1 and 11.3 per million population in different parts of Europe.1
Pregnant women have a 12- to 18-fold increased risk in pregnancy (12/100 000), compared with the non-pregnant population.
Out of all infections with Listeria, 16–27 per cent occur in pregnant women.2
The exact cause of its higher prevalence in pregnancy is not known. Immunosuppression associated with pregnancy results in suppression of cell-mediated immunity in the placenta, due to high concentration of maternal hormones and other unknown mechanisms.
Approximately 20 per cent of Listeria infections involve neonatal infection with potentially severe complications, therefore it is important to stop the outbreaks in pregnant woman and the unwanted consequences for their unborn fetuses.
Microbiology, Organism and Epidemiology
Out of seven Listeria species, only Listeria monocytogenes is an important pathogenic for humans. Morphologically, they are indistinguishable from diphtheroids, and they are therefore mistaken for contaminants. They can mimic Streptococci, as coccoid forms are often seen in cultures or smears from infected tissue.
L. monocytogenes is a small, facultative anaerobic, gram-positive, flagellated, linear motile rod, which is non-spore-forming.3 It can survive for many months in soil and can be detected in the faeces of animals and humans.
It is mainly acquired by the ingestion of food products, such as meat (hot dogs, deli meat), dairy products, unwashed raw vegetables, seafood, soft cheeses, unpasteurised milk and dairy products, that have been contaminated.
Outbreaks usually occur due to contaminated food, often commercially produced, as the source of infection.4
Even refrigerated food can be contaminated, proving to be the source in around 11 per cent of cases.5
Many aspects of the epidemiology of listeriosis are unclear because of
Lack of a sensitive method for strain identification
Incubation period is very short for gastroenteritis (6 hours to 10 days) but longer for invasive listeriosis (11 to 70 days, with a mean of 28 days)
Mild or asymptomatic nature of infection
Variant sources of infection; infecting organism is found in dust, soil, water, sewage, decaying vegetation
Many species of wild and domestic mammals, avian species, crustaceans, pond trout, ticks and flies harbour Listeria
Milk products (particularly soft cheese) and uncooked vegetables, fish and shellfish, ready-to-eat meat products, ground beef and poultry have all been found to contain the organism
Up to 70 per cent of the human population harbour the organism in their gastrointestinal tract for short periods (6–16 per cent human faeces)
Listeria infection occurs primarily in elderly people, in pregnant women and their newborn infants, and in immunocompromised patients including patients with cancer, AIDS, organ transplant recipients or patients on corticosteroid therapy.6
Signs and Symptoms
The majority of people exposed to Listeria are either asymptomatic or have subclinical infection. Twenty-five per cent of culture-positive women report no symptoms. [EL 2]
Mild flu-like symptoms, fever.
Self-limiting febrile gastroenteritis. The fever is usually low-grade or may be significant, ranging from 36.5°C to 40.1°C.
Watery diarrhoea, nausea, vomiting.
Headache, backache and pain in joints and muscles.
Bacteraemia, meningitis and meningoencephalitis occur in 20, 40 and 39 per cent of cases, respectively.
Signs and symptoms usually lasts for two days followed by complete recovery.
In a series of 191 cases of listeriosis in pregnancy, 32 per cent suffered flu-like illness and 65 per cent had a fever. Other symptoms included backache (21.5 per cent) (which may be mistaken for a urinary tract infection), headache (10.5 per cent), vomiting/diarrhoea (7 per cent), muscle pains (4 per cent) and sore throat (4 per cent). Approximately 29 per cent of the women were asymptomatic.6, 7 [EL 2−]
Invasive infection is rare in healthy populations but tends to affect the immunocompromised, pregnant or older population.
Diagnosis 8
In the Pregnant Woman
Blood cultures are the standard and only reliable method for diagnosis in cases of fever and symptoms consistent with listeriosis
Gram stain: the presence of short, gram-positive rods found by Gram stain in maternal blood, urine or faecal matter
Fetal placental or cerebrospinal fluid (CSF) specimens support a presumed diagnosis of listeriosis, but the organism is easily confused with Streptococci, diphtheroids and even Haemophilus
Stool culture: the American College of Obstetricians and Gynecologists maintains the stool culture has not been validated as a screening tool and it is not recommended for the diagnosis of listeriosis as it may have low sensitivity8
Serologic tests are not diagnostically reliable
Blood cultures are mandatory in case of bacteraemia or sepsis, and CSF culture where meningitis is suspected
Vaginal and rectal cultures should also be obtained in symptomatic patients
In Perinatal Listeriosis
Culture can be obtained from
Blood
Urine
Meconium, amniotic fluid, placenta or fetal membranes
Pus from skin lesions
Tissue obtained at autopsy
Cerebrospinal fluid
Pleural fluid, respiratory tract and gastric aspirate
Histopathology and culture of rash
Culture of other infected tissues (joint, pericardial fluid)
CT scanning or MRI may be useful in detecting abscesses in the brain or liver.
Effect of Listeriosis on Pregnancy
During pregnancy, cellular immunity is reduced because of increased progesterone making pregnant women particularly susceptible to intracellular micro-organisms like L. monocytogenes. Vertical cell-to-cell transmission permits L. monocytogenes to infect the uterus and placenta.
Listeriosis is rarely serious for the pregnant woman herself and symptoms generally resolve after delivery with or without antibiotic therapy. Listeria infection in the pregnant woman rarely causes severe maternal complications such as
Meningitis
Adult respiratory distress syndrome
Endocarditis
Past infection does not offer immunity against reinfection.
Fetal Implications
During pregnancy, fetal infection (90 per cent) can occur via transplacental transmission. Vertical transmission can also occur via passage through an infected birth canal or ascending infection after ruptured amniotic membranes [EL 2]. Once the placenta is infected, it becomes a reservoir for reinfection,6 and placental micro-abscesses may be histologically evident.
First Trimester – miscarriage in 10–20 per cent of cases
Second Trimester – premature labour in 50 per cent of cases if infection occurs after 20 weeks
Stillbirth in almost all cases if infection occurs before 20 weeks
Third Trimester:
Stillbirth
Intrauterine death
Intrauterine growth retardation
Reduced fetal movements
Pathological cardiotocography (CTG)
Non-immune hydrops
Intracranial calcification
Chorioamnionitis9
In a retrospective case series study of 222 pregnant women, there were 11 Listeria infections. Nine of those 11 women developed the infection in their third trimester. There were six preterm deliveries (54 per cent) [EL 2]. All six had shown evidence of neonatal Listeria infections. Two of 11 pregnancies (18 per cent) resulted in fetal death because of spontaneous abortion and birth of a fetus that died immediately after birth.
The above figures indicate the serious neonatal risks of Listeria infections.
Ultrasound Imaging in Fetal Listeria Infections10
Ultrasound imaging may reveal mild fetal ascites, dilated gall bladder and small bowel loop dilatation, non-immune hydrops, intracranial calcifications, or intrauterine fetal demise.
Fetal surveillance is recommended according to the gestational age.
However, there is no study or data available to suggest the best plan for fetal surveillance and its frequency.
Neonatal Implications
Listeriosis in the neonate has two different presentations (early or late) depending on the timing of infection.
Neonatal infection occurs in almost 68 per cent of cases of maternal infection, with complete recovery in approximately 70 per cent of cases, and long-term sequelae in 12.7 per cent of cases.
Neonatal death can occur in up to 22 per cent of cases of maternal infection.
1 Early Onset
Early-onset disease occurs within the first seven days of life or soon after birth. The majority of affected infants have poor prognosis due to prematurity as a result of intrauterine infection (chorioamnionitis). Neonatal early-onset listeriosis is non-specific and difficult to distinguish from early-onset group B streptococcal or any other bacterial infection. However, signs of chorioamnionitis without rupture of the membranes should raise suspicion of Listeria infection and may prompt laboratory diagnosis.11
Clinical features of neonatal infections include:
Newborns generally pass meconium in utero and show fetal compromise on intrapartum fetal heart rate (FHR) tracings.
Respiratory distress or pneumonia in 38 per cent, with rapid breathing, and grunting which may include cyanotic episodes.
Poor feeding and fever are common presenting symptoms.
Septicaemia and meningitis are the most common clinical manifestation.
Severe infection causes skin lesions (slightly elevated, 1 to 2 mm pale patches on a bright erythematous base) mostly seen on the infant’s back and lumbar area called granulomatosis infantisepticum.
Disseminated granuloma abscesses involve the liver, spleen, adrenal glands, lungs, oesophagus, the posterior pharyngeal wall and placenta.
Neutropenia is more characteristic of severe infection. Thrombocytopenia sometimes occurs and anaemia is common, most likely secondary to the haemolysin listeriolysin.
On chest X-ray, non-specific pneumonia is common.
2 Late Onset
Late-onset neonatal infection occurs from one to eight weeks of life and typically affects full-term infants whose mothers had uncomplicated pregnancies.
The infants are usually healthy at birth and symptoms manifest at a mean of 14 days of life, strongly suggesting acquired horizontal postpartum infection at delivery from the maternal genital tract or through nosocomial infection (delivery rooms or nurseries).
Late-onset infection carries a better prognosis for the newborn (mortality rate of 0–20 per cent).
Laboratory features do not distinguish from other causes of bacterial meningitis.12
In more than 95 per cent of late-onset cases, babies do not appear seriously ill.
Severe and serious neonatal illness may appear as:
Fever and septicaemia
Meningitis presenting with fever and irritability
Micro-abscesses or granulomas granulomatosis infantisepticum similar to early-onset infection
Mortality is low, except in severe disease or when diagnosis and treatment are delayed. Morbidity and long-term sequelae are not common in treated cases.
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