Food Safety Focus (228th Issue, July 2025) – Article 1

Norovirus in Ready-to-eat Food

Reported by Arthur YAU, Scientific Officer,
Risk Communication Section, Centre for Food Safety

Norovirus is a highly contagious pathogen, with as few as 10 to 100 norovirus particles to make someone ill. It is the leading cause of acute gastroenteritis worldwide. It causes the highest number of human outbreaks in the European Union in 2022 and about half of all outbreaks of food-related illnesses in the United States. It is notoriously difficult to eliminate. Norovirus can survive a wide range of temperatures from freezing point to 60˚C, and endure chlorine-treated water at levels up to 10 ppm. Outbreaks are more prevalent during colder months, and symptoms include abdominal pain, nausea, vomiting and diarrhoea. While most people will recover within a day or two, it can be severe for people of vulnerable groups, such as the elderly, young children, and those with weakened immune systems, sometimes resulting in death.

How is Norovirus Transmitted?

The only known host of norovirus is humans; it is spread primarily through the faecal-oral route. It can also be transmitted from person to person via droplets of vomit, which is increasingly affecting more people than foodborne outbreaks in recent years as reported in some countries. The key transmission routes of norovirus are:

1. Contaminated Food: Consuming food contaminated at its source is a major risk. Shellfish like oysters and other bivalves can concentrate norovirus from polluted water. Besides, cross-contamination of ready-to-eat food by food handlers may occur, if they do not wash their hands after handling raw food, mobile phones, money, rubbish or using the toilet, especially when they are symptomatic.
2. Environmental Contamination: Norovirus can survive on various surfaces like stainless steel, ceramic, countertops, etc. for a day or more, allowing transmission via contact with contaminated utensils or surfaces.
3. Dirty Hands: Infected individuals can shed norovirus in their faeces and vomit, contaminating their hands. If they fail to wash their hands properly after using the toilet, they can spread norovirus to food and surfaces. Notably, alcohol- based hand sanitizers cannot substitute hand washing as they are ineffective against norovirus.

Figure : Foods that are likely to be contaminated with norovirus, including shellfish, fruits (particularly berries), salads and sandwishes

Common Ready-to-eat Foods Contaminated with Norovirus

Numerous ready-to-eat foods have been linked to cases of norovirus outbreaks in many countries, including:

• Sandwiches: These often require extensive handling, increasing the chance of contamination since they are consumed without any further heating.
• Shellfish: Oysters and other bivalves, especially when consumed raw or undercooked, can accumulate norovirus from polluted waters.
• Salads: Fresh produce, especially leafy greens, may be contaminated by polluted irrigation water or during preparation.

• Fruits: Fresh fruits can become contaminated with unclean water or hands along the food chain. Frozen berries, such as raspberries, were predominant in past outbreaks, as freezing cannot kill norovirus.

Importance of Maintaining Good Environmental Hygiene

Good hygiene practices are crucial. Other than handwashing, surfaces in food preparation areas must be cleaned and disinfected regularly. Norovirus can become airborne during vomiting, making clean-up essential. When cleaning up vomitus or stools, use a 1:49 household bleach solution for sanitation:

1. Wear protective gears, including disposable gloves and face masks. Use disposable towels for cleaning up and avoid using mops.
2. Disinfect the contaminated area by applying the 1:49 bleach solution, leaving it for 15 to 30 minutes to effectively inactivate the virus.
3. Rinse with water and dry the area.
4. Remove disposable gloves and wash hands thoroughly afterwards.

Prevention Strategies

To prevent norovirus infections, it is vital to control norovirus at both farm and individual levels:

1. Cook Food Thoroughly: Ensure that shellfish are cooked to a higher core temperature of 90°C for 90 seconds, or is boiled at 100°C until shells open, continuing for 3 to 5 minutes.
2. Maintain Good Hand Hygiene: Wash hands thoroughly with liquid soap and water for at least 20 seconds and ensure that liquid soap and paper towels are readily available for staff to use.
3. Avoid Cross-Contamination: Use separate utensils and cutting boards for raw and ready-to-eat foods, and regularly clean and sanitise food preparation areas.
4. Educate and Monitor Food Handlers: Provide regular training on good hygiene practices and the risks associated with norovirus. Staff should understand the importance of adhering to good personal hygiene and proper food handling. Food handlers who exhibit symptoms of infection, such as vomiting or diarrhoea, should refrain from handling food until they have been symptom-free for at least 48 hours.In summary, norovirus poses a significant risk due to its ease of transmission. By preventing the spread of norovirus from farm to fork and from person to person, one can ensure food safety for all.