All you need to know about Listeria. St Emlyn’s.

Before you start reading this blog summary, I have a small confession to make: I do not think I have ever treated an adult patient with confirmed Listeria infection. Maybe this is because of the nature of my work (I work as an adult and children’s emergency doc in a busy city centre ED so I rarely have the opportunity to follow up my patients’ results) or maybe it is because it is a rare disease but more likely it is the combination of both…

It is exactly because it is a relatively rare disease (again depending on where you work!) and because of sporadic breaking news involving some local shop/company calling back food products after an outbreak that I have decided to write this quick summary to help fellow (mostly emergency) healthcare providers in their daily work.

What is Listeriosis?

Listeriosis is mostly a foodborne disease caused by a non–spore-forming, gram-positive bacillus that has a Terminator-like name: Listeria monocytogenes. These bacilli are widely distributed in the environment and infection usually occurs after consumption of contaminated raw, chilled, or ready-to-eat foods, and can cause sporadic infection and outbreaks of disease. It is easy to see why outbreaks are frequent in our fast-paced world where we rely so much on cold/chilled food as a convenience! There is however no need to panic as the germs are easily killed by heating/re-heating food at high temperatures or by simply avoiding eating products that are beyond their expiry period.

What is the incubation period?

This can be tricky to answer as there is variability in the onset of symptoms. It will obviously depend on the amount of germs the patient has ingested and the level of their individual immunity. It is however reported that the incubation period can vary from one day to up to 70 days. This long incubation period can obviously cause some issues for both clinician and patients during history taking as the patient might not remember that cold sandwich that was lingering in the fridge they had for lunch a few days ago. As a general rule, food-induced listeriosis should however manifest itself within a few days.

Who is at risk?

Listeriosis is usually a benign and self-limiting disease. It can however cause severe disease in vulnerable groups, including the elderly, pregnant women, unborn and newborn babies, and people with impaired immunity. You therefore need to be careful with this sub-group of patients.

What are the symptoms?

In most people with intact immune system, listeriosis will result in no symptoms or only causes mild symptoms for a few days, such as fever and diarrhoea ( an almost constant feature but also similar to other foodborne infections) which may last a few days and resolve without treatment.

Symptoms of a generalised infection are related to sepsis, and include fever, confusion, myalgias, collapse, and/or diarrhoea. This can be difficult to clinically diagnose if you missed the usually present clinical symptom of diarrhoea (think about your paediatric patients!). Infection during pregnancy may result in foetal loss through miscarriage or stillbirth, neonatal meningitis or bacteraemia. Neonatal infection can range from mild illness to neonatal death, usually within 10 days of delivery. It is essential to keep this in mind as a 2017 Public Health England document states that among pregnancy associated cases, where known, 73.9% of women had live births and 26.1% resulted in still birth or miscarriage. The overall mortality rate of L monocytogenes infection is 15%-20%. These are truly worrying numbers!

How do I diagnose Listeriosis?

As always, take a thorough history but the patient might not remember that sandwich or cheese which might well have smelled and looked fine when consumed but was over its expiry date.

Listeriosis is usually diagnosed from peripheral blood cultures. These results might take a short while to come back so make sure you have up to date patient’s contact details if you decide not to admit your patient: you might well need to call them back for intravenous antibiotics.

Cerebrospinal fluid can also help in the diagnosis but this is useful obviously in the generalised infection or in the very little patients in whom you might be doing a lumbar puncture as part of your septic screen.

The infection usually does not get transmitted between humans (apart vertically between mother and child) but you should observe the universal precautions of strict hand washing and barrier protection when attending the suspected cases.

As with every infectious disease blog, I would not like to specify which antibiotics are recommended but would rather encourage you to consult your local microbiology guidelines.

In conclusion:

Listeriosis remains a rare disease. There had been only four outbreaks (comprising 2 or more linked cases) of listeriosis investigated in England in 2017.

You therefore need to remain vigilant – especially in the high risk groups as generalised infection has significant mortality and morbidity.

Janos Baombe

@baombejp on Twitter

Cite this article as: Janos Baombe, "All you need to know about Listeria. St Emlyn’s.," in St.Emlyn's, July 12, 2019,

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