Approach
The diagnosis is based on high clinical suspicion and the isolation of the pathogen from clinically sterile sites.[8][42]
A history of consuming contaminated food such as uncooked beef, pork, or poultry; unpasteurised milk and its products; soft cheeses; raw vegetables; and leftover food, especially by members of high-risk populations (older adults, pregnant women, those who are immunocompromised), should be considered.
Neonates born to infected mothers are at risk of vertical transmission either trans-placentally or through contact with an infected birth canal.[30][31]
Clinical evaluation
Invasive disease in immunocompetent people is rare. Asymptomatic stool carriers have a prevalence of up to 5% in the general population. When healthy adults are symptomatically infected, the clinical presentation mimics febrile gastroenteritis, which occurs sporadically or as an epidemic.[5][31][32][37]
In those at risk for systemic infection, symptoms and signs of bacteraemia and sepsis include fever, rigors, hypotension, and headache. These should prompt blood cultures prior to beginning antibiotic therapy. Heart murmur should raise suspicion of endocarditis. Signs or symptoms of meningitis, brain abscess, or meningoencephalitis may be present, with cranial nerve deficits, neck stiffness, altered mental status, focal neurological, or cerebellar signs. Serious brain stem encephalitis (rhombencephalitis) may occur with high frequency of cranial nerve damage and death. Seizures may also be seen in these conditions.
Neonates, adults over 45 to 50 years of age, and pregnant women may present with an atypical clinical picture, such as malaise.[1][2][9] Pregnant women may have flu-like symptoms (lethargy, fever, arthralgias, myalgias, rigors, fatigue, diarrhoea, vomiting, and abdominal pain).[2] Neonates may present with poor feeding. Indications of chorioamnionitis from Listeria (such as intra-partum fever) are a frequent finding in cases of neonatal infection.[2][4]
No testing is needed for an asymptomatic pregnant woman who reports consumption of a product that was recalled or implicated due to Listeria contamination. Asymptomatic pregnant women should be investigated only when they present with symptoms within 2 months after consumption of a contaminated food.[22]
Laboratory tests
General considerations:
Full blood count is helpful to detect leukocytosis or thrombocytopenia. Low platelets may indicate disseminated intravascular coagulation (DIC).
Coagulation studies and D-dimer may be performed subsequently if DIC is suspected.
A urine pregnancy test should be considered in any woman of childbearing age, as listeriosis may present in any trimester.
In cases of outbreaks or severe sporadic infections, suspected foods may be analysed.
In patients with gastroenteritis, stool cultures, serologies, or ova/parasites may be ordered according to clinical suspicion to exclude other pathogens. Stools for ova and parasites, enzyme-linked immunosorbent assay, or agglutination tests will typically be negative for other pathogens such as Campylobacter jejuni or Cryptosporidium.
Isolation of organism:
Initial diagnosis of listeriosis is made by isolation of the pathogen from normally sterile clinical sites (blood, cerebrospinal fluid [CSF], amniotic fluid, placenta, or fetal fluid).[1][5] Blood cultures typically have a sensitivity that varies from 0% in non-invasive disease to 75% in systemic infection.[1]
Success of isolation from non-sterile sites varies. Stool cultures may be positive in 5% of the healthy asymptomatic population and in up to 87% in outbreaks of febrile gastroenteritis due to Listeria.[1]
Samples from normally sterile sites such as CSF can be directly inoculated to tryptic soya agar that contains 5% sheep, horse, or rabbit blood. CSF cultures are positive in 20% to 40% of central nervous system (CNS) infection.[1][6][7][32]
Lumbar puncture:
CSF testing is done in patients presenting with signs and symptoms of CNS infection, generally following head imaging. Typically, the WBC count of the CSF is <5000 cells/mm³ and protein concentration <2 g/L (<200 mg/dL). During early CNS infection, CSF may be normal, and, if so, lumbar puncture should be repeated in 12-24 hours.
Gram stain of CSF is important for diagnosis in up to 75% of patients withListeria meningitis.[1][5] Due to the resemblance of the pathogen to diphtheroids or pneumococci and its tendency to become over-decolourised, the specificity of Gram stain is reduced.
Food testing:
Food samples containing more than 100 colony-forming units (CFU)/g of Listeriaspecies require investigation.[39] The level of food contamination can be assessed by direct enumeration of the organism on solid selective media. Listeria forms blue-green colonies in Listeria chromogenic isolation medium, due to its beta-glucosidase activity. Confirmation of colonies should be made by PCR.[39]
Perinatal listeriosis:
Confirmed by placental, amniotic fluid, stool, and cervical swab cultures.
Meconium Gram stain and culture may contribute to the diagnosis of neonatal listeriosis.[27][28][31]
Other laboratory studies:
Listeria serologies: low specificity due to antigenic cross-reactivity with other gram-positive bacteria; and low sensitivity, especially in early infection.[1][27][28] Ordered when an outbreak is suspected.
PCR blood: requires a specialised diagnostic centre but offers high sensitivity and specificity and can be used in cases where the diagnostic value of blood cultures is low due to prior antibiotic administration.
Imaging and physiological studies
Imaging in CNS infection includes computed tomography (CT) and magnetic resonance imaging (MRI) to exclude structural lesions.
CT of the brain is generally done prior to lumbar puncture. In meningitis, meningoencephalitis, and more specifically in brain stem infections, CT of the brain is inferior diagnostically compared with MRI.[6][7][32]
Where endocarditis is suspected, an echocardiogram should be considered to evaluate for valvular involvement. Electroencephalogram may be done when seizures are a presenting or complicating condition.
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