Listeria Food Poisoning: Risks, Symptoms and Prevention

Food poisoning caused by Listeria monocytogenes is relatively uncommon in the UK, but it can be extremely serious. The illness caused by the bacteria is called listeriosis.

Most healthy people will experience no symptoms or only a mild illness. However, listeriosis can cause severe and potentially life-threatening complications in vulnerable people.

Who Is Most at Risk from Listeria?

People at increased risk of serious listeriosis include:

• People who are pregnant
• Unborn and newborn babies
• Older people, with the risk increasing with age
• People with weakened immune systems
• People receiving treatment for cancer
• People who have received an organ transplant
• People taking medicines that suppress the immune system
• People with diabetes
• People with chronic liver or kidney disease
• People with certain other long-term health conditions

Food businesses serving hospitals, care homes, nurseries or other vulnerable groups must take particular care to control Listeria risks.

Why Is Listeria a Serious Food Safety Risk?

Listeria behaves differently from many other food-poisoning bacteria. Most harmful bacteria grow best in warm conditions, but Listeria can continue to grow slowly at refrigeration temperatures.

This means that refrigeration alone may not fully control the risk. The bacteria can gradually increase in chilled food, particularly when food:

• Has a long refrigerated shelf life
• Is stored above its recommended temperature
• Is kept beyond its use-by date
• Remains open for too long
• Becomes contaminated after cooking or production

Foods Commonly Associated with Listeria

Listeria is particularly associated with chilled ready-to-eat food that will not receive any further cooking before it is eaten.

Examples may include:

• Pre-packed sandwiches
• Prepared salads
• Cooked sliced meats
• Pâté, including vegetable pâté
• Cold-smoked fish, such as smoked salmon and smoked trout
• Cured fish, such as gravlax
• Soft mould-ripened cheeses
• Soft blue cheeses
• Unpasteurised milk and dairy products
• Some chilled desserts
• Some chilled convenience foods

Because these foods may be eaten without further cooking, any Listeria contamination may still be present when the food is consumed.

Listeria Advice for Vulnerable People

People who are pregnant or have a weakened immune system are advised to avoid ready-to-eat cold-smoked or cured fish unless it has been cooked until steaming hot throughout.

People who are pregnant should also avoid:

• All types of pâté, including vegetable pâté
• Unpasteurised milk and products made from it
• Soft mould-ripened cheeses, such as Brie and Camembert, unless cooked until steaming hot
• Soft blue cheeses, such as Danish Blue, Gorgonzola and Roquefort, unless cooked until steaming hot
• Undercooked food

Simply warming cold-smoked or cured fish may not be enough. It should be cooked until steaming hot throughout to destroy any Listeria that may be present.

What Are the Symptoms of Listeriosis?

Many people with listeriosis have no symptoms or experience only a mild illness lasting a few days.

Possible symptoms include:

• A high temperature
• Chills
• Muscle aches and pains
• Feeling sick
• Vomiting
• Diarrhoea
• Abdominal discomfort

People who are pregnant may also experience abdominal pain or notice that their baby is moving less than usual.

Serious Complications of Listeriosis

In vulnerable people, Listeria can spread beyond the digestive system and cause serious complications, including:

• Sepsis
• Meningitis
• Encephalitis
• Miscarriage
• Stillbirth
• Premature birth
• Serious infection in a newborn baby

Severe listeriosis requires urgent medical assessment and treatment.

When to Seek Medical Advice

Someone who is pregnant, immunocompromised or caring for a newborn baby should seek urgent medical advice if they believe they may have listeriosis.

They can contact:

• Their GP
• NHS 111
• Their midwife or maternity unit if pregnant
• Their specialist medical team

Call 999 or go to an accident and emergency department if somebody develops serious symptoms such as confusion, seizures, severe drowsiness, a stiff neck or difficulty looking at bright lights.

Controlling Listeria in Chilled Food

Because Listeria can grow at refrigeration temperatures, food businesses must combine several control measures.

These include:

• Strict temperature control
• Effective cleaning and disinfection
• Preventing cross-contamination
• Maintaining the cold chain
• Following use-by dates
• Managing opened products correctly
• Effective stock rotation
• Controlling product shelf life

Safe Refrigeration Temperatures

Chilled ready-to-eat food should be kept at 5°C or below as good practice, particularly when it is intended for vulnerable people.

Food requiring chilled storage must generally be kept at 8°C or below in England, Wales and Northern Ireland. Operating refrigerators at 5°C or below provides a safety margin and helps control the growth of Listeria.

Refrigeration equipment should be:

• Checked regularly
• Kept clean and well maintained
• Fitted with a suitable temperature display or thermometer
• Loaded in a way that allows cold air to circulate
• Repaired promptly if faults are identified

Maintaining the Cold Chain

The cold chain must be maintained throughout the food’s journey, including:

• Delivery
• Refrigerated storage
• Preparation
• Transport
• Display
• Service

Chilled ready-to-eat food should remain outside refrigeration for the shortest possible time. Deliveries should be checked to confirm that food has arrived at an acceptable temperature.

Following Use-By Dates

A use-by date relates to food safety and must always be followed. Food must not be supplied, served or used after its use-by date.

Food businesses should:

• Check use-by dates every day
• Remove expired food immediately
• Follow the manufacturer’s storage instructions
• Keep food at the stated temperature
• Never extend a shelf life without suitable food safety evidence

Managing Opened Packs

Opening a packet can expose food to contamination and may reduce its safe storage life.

Once opened, chilled ready-to-eat food should be:

• Covered or placed in a suitable container
• Labelled with the opening date
• Stored at the correct temperature
• Protected from raw food
• Used within the manufacturer’s stated period
• Disposed of when its safe storage period has ended

The original use-by date must still be followed if it occurs before the recommended period for using the product after opening.

Using FIFO Stock Rotation

Effective stock rotation reduces the risk of chilled food remaining in storage for too long.

The first in, first out system, known as FIFO, ensures that older stock is used before newer deliveries.

Food handlers should:

• Place newer stock behind existing stock
• Check date labels during every stock rotation
• Label prepared and opened food clearly
• Remove expired or damaged products
• Keep accurate stock records where required

Preventing Listeria Cross-Contamination

Listeria can enter ready-to-eat food through contaminated hands, surfaces, equipment or utensils.

Food businesses should:

• Keep raw and ready-to-eat foods separate
• Use separate equipment where appropriate
• Clean and disinfect food-contact surfaces effectively
• Wash hands thoroughly before handling ready-to-eat food
• Maintain equipment in good condition
• Prevent condensation from dripping onto food
• Clean spills immediately
• Protect food during storage and preparation

Cleaning and Disinfecting for Listeria Control

Listeria can survive in food-production environments and may become established in damp, difficult-to-clean areas.

Particular attention should be given to:

• Refrigerator seals
• Drains
• Floor and wall joints
• Conveyor systems
• Slicers and cutting equipment
• Underneath and behind equipment
• Wheels and castors
• Damaged surfaces
• Areas affected by condensation

Cleaning schedules should state what must be cleaned, how it will be cleaned, how often it must be completed and who is responsible.

Surfaces must be cleaned before they are disinfected, and disinfectants must be used at the correct concentration for the required contact time.

Protecting Vulnerable Customers from Listeria

Food businesses serving vulnerable people should assess which foods present an increased Listeria risk and introduce appropriate controls.

This may include:

• Removing particularly high-risk foods from menus
• Cooking cold-smoked or cured fish until steaming hot
• Maintaining chilled ready-to-eat food at 5°C or below
• Using shorter shelf lives after opening
• Strengthening cleaning and monitoring procedures
• Training staff about vulnerable groups
• Obtaining advice from healthcare or food safety specialists

Reducing the Risk of Listeriosis

Although listeriosis is uncommon, its consequences can be severe. Controlling the risk requires careful food handling, effective cleaning, strict temperature control, good stock rotation and close attention to use-by dates.

These precautions are particularly important when preparing or serving food for people who may be at greater risk of serious illness.