Listeria spp. are Gram-positive non-sporing rods which are ubiquitous in the environment and found worldwide.¹ Listeria monocytogenes is the major pathogen, although occasional human infections with L. ivanovii and L. seeligeri have been reported.

Listeria spp. are not very pathogenic to healthy adults, who are likely to experience only mild infection, causing flu-like symptoms or gastroenteritis. However, listeriosis can occasionally lead to septicaemia or meningitis. Pregnant women, the elderly and people with weakened immune systems (including those treated with oral corticosteroids) are more susceptible to listeriosis. Unborn babies and neonates are at particular risk of severe illness and listeriosis in pregnancy can cause miscarriage, premature delivery or severe illness in a newborn child.¹

Epidemiology

• The total number of cases of L. monocytogenes reported to the Health Protection Agency in 2009 was 213 (179 non-pregnancy-related and 34 pregnancy-related).¹
• L. monocytogenes is common in wild animals, domesticated animals and in soil and water. It causes disease in many animals and is a common cause of miscarriage and stillbirth in domestic animals.
• Most cases of infection in humans are thought to be foodborne.² Some cases occur by direct contact with animals (particularly during calving, lambing, and postmortem examinations). Spread from mother to fetus may occur in utero or during birth.
• Listeria spp. have a peculiar property of being able to grow at low temperatures, ie. on contaminated refrigerated food, and are an important foodborne pathogen. Soft cheeses and meat-based paté have been implicated in outbreaks.
• The bacterium has been isolated from a range of raw foods including vegetables, uncooked meats and processed foods. Eating cooked food that has then been refrigerated and recooked is a particular risk for causing infection. Usually killed by cooking or pasteurisation, it can survive some forms of pasteurisation, particularly if the bacterial count is high.
• L. monocytogenes is often carried in the human bowel (1-10% carrier rate).

Presentation

• Incubation time can vary from 3-70 days in adults, and a few days to a few weeks in infants.¹
• Infection in healthy children and adults, including maternal infections, may be asymptomatic.
• L. monocytogenes most often causes an influenza-like illness. More severe infection in risk groups may lead to stillbirth, septicaemia or meningoencephalitis.
• Infection of a pregnant woman early in pregnancy often leads to miscarriage. Maternal infection during pregnancy may be asymptomatic or include fever, myalgia, headache, sore throat, cough, vomiting, diarrhoea and vaginitis.
• The organism may be transmitted across the placenta. Infections in late pregnancy may lead to stillbirth, or death of the infant within a few hours of birth. About half of infected infants at or near term will die.
• Infants with listeriosis may present in the first few days of life with poor feeding, lethargy, jaundice, vomiting, respiratory distress, skin rash and shock. Infants usually have pneumonia. The death rate is very high.
• Infants presenting at age 5 days or older often present with meningitis.
• In adults, the disease is usually asymptomatic or causes mild illness such as an influenza-type illness, conjunctivitis, skin lesions or gastroenteritis. More severe infection, particularly in immunocompromised adults, may cause meningitis, pneumonia, septicaemia and endocarditis.
• Vets and farmers may develop cutaneous listeriosis, presenting as papular or pustular lesions on the arms or hands, following contact with infected animals.

Investigations³

• Cultures of amniotic fluid, blood, urine and cerebrospinal fluid (CSF); stool cultures are not sensitive or specific.
• Serological testing is unreliable.
• CXR.
• MRI is superior to CT scan for demonstrating central nervous system disease, especially in the brainstem.
• Transoesophageal echocardiography should be performed if endocarditis is suspected.

Management

• Meningitis caused by listeriosis: intravenous amoxicillin/ampicillin plus gentamicin for 14 days.⁴ Intravenous amoxicillin/ampicillin alone should then be continued for a further 2-4 weeks for meningoencephalitis and a further 4-6 weeks for endocarditis.⁵
• Whenever listeriosis is a clinical possibility, e.g. acute pyogenic meningitis, and the organism is unknown, intravenous amoxicillin/ampicillin should always be part of the regimen.⁶
• Listeria spp. are resistant to cephalosporins.
• Gentamicin should be avoided in pregnancy and amoxicillin/ampicillin is then used alone.
• Erythromycin is used instead of amoxicillin/ampicillin if the patient is allergic to penicillin.
• Intravenous co-trimoxazole is the best second-line treatment for listerial meningoencephalitis.⁷

Complications

Infants who survive listeriosis may suffer long-term neurological damage and delayed development.

Prognosis¹

• Infection of the fetus with L. monocytogenes results in a poor outcome with approximately a 50% death rate.
• The late-onset infant form also has a high death rate.
• Healthy older children and adults have a lower death rate but there is a 20-50% mortality from septicaemia and meningoencephalitis, with significant long-term morbidity in survivors.

Prevention

• Pregnant women should avoid contact with wild and domestic animals.
• Pregnant women should avoid consumption of soft cheeses, delicatessen meats, patés, spreads, refrigerated smoked seafood, and cold salads from salad bars.
• Non-pasteurised soft cheeses have also been implicated in outbreaks of listeriosis.
• Food should always be adequately cooked, or thoroughly reheated.

Document references

1. Listeria, Health Protection Agency
2. Schlech WF 3rd; Foodborne listeriosis. Clin Infect Dis. 2000 Sep;31(3):770-5. Epub 2000 Sep 26. [abstract]
3. Weinstein KB et al; Listeria monocytogenes, eMedicine, Jun 2008
4. British National Formulary; 60th Edition (September 2010) British Medical Association and Royal Pharmaceutical Society of Great Britain, London (link to current BNF)
5. McLauchlin J, Low JC; Primary cutaneous listeriosis in adults: an occupational disease of veterinarians and farmers. Vet Rec. 1994 Dec 24-31;135(26):615-7. [abstract]
6. Safdar A, Armstrong D; Antimicrobial activities against 84 Listeria monocytogenes isolates from patients with systemic listeriosis at a comprehensive cancer center (1955-1997). J Clin Microbiol. 2003 Jan;41(1):483-5. [abstract]
7. Jones EM, MacGowan AP; Antimicrobial chemotherapy of human infection due to Listeria monocytogenes. Eur J Clin Microbiol Infect Dis. 1995 Mar;14(3):165-75. [abstract]

Acknowledgements