Introduction

Definition and characteristics of norovirus

Noroviruses were first called as “Norwalk virus” because they were found in Norwalk, Ohio, during the outbreak of stomach flu (gastroenteritis). Norovirus typically causes acute gastrointestinal illness and is most common in children. Typical symptoms after infection were nausea, vomiting and diarrhoea, which could lead to severe loss of water in the body and require staying in hospitals.¹

Impact of Norovirus outbreaks

Norovirus outbreaks have significant public health implications, as the virus is highly contagious and spreads easily through contaminated food, water, and surfaces. Recent data shows a sharp rise of around 77% in cases across England, particularly affecting vulnerable populations like the elderly aged over 65 years and young children aged under 5 years. The virus can cause severe dehydration, leading to hospitalisations, especially in high-risk groups. Healthcare and educational facilities are frequently impacted by outbreaks, underlining the importance of proper hygiene and infection control measures to mitigate the spread.²

Norovirus can easily be transmitted from person to person by contact with contaminated surfaces. Food, water and surfaces may have viruses from splashing of stool or air transmission of viral particles.¹

Key sources of norovirus outbreaks

Contaminated food

Improperly handled or prepared food

Noroviruses are a key reason for outbreaks in food settings such as restaurants. Sometimes, this is because staff may touch ready-to-eat foods such as raw fruits or cold foods with their bare hands, which can make these foods contaminated with viruses.³ A study demonstrated that in all food types, shellfish were involved in the greatest number of foodborne outbreaks.⁴

Cross-contamination in kitchens and food production

Most food vehicle related to Norovirus was seafood, which was found in around 60%, and almost 90% of seafood vehicles were oysters. Apart from oysters, some raw foods are also found to be contaminated with Norovirus, such as clams, lettuce, mussels, raspberries and shellfish.⁴ The most common food categories associated with viral contamination are salads and vegetables, then seafood dishes, consequently.

Role of infected food handlers in transmission

Food handlers can lead to outbreaks of Norovirus associated with a wide range of food substances and settings, such as wedding events.⁴ One study demonstrated that the most common settings where outbreaks of norovirus occurred were restaurants. Around 45% of the food-handling staff were found to be contaminated with the virus, which was mostly sampled from staff in the kitchen.⁴ However, risk factors for food-handler outbreaks depend on the time or place of exposure. Other settings of food handlers include food handlers based in hospitals or non-hospitals, such as nursing homes and retirement homes.⁴

Contaminated water

Drinking water contaminated by sewage

Water is also one of the sources of norovirus outbreaks. Contamination occurs particularly by sewage or wastewater from homes, businesses and other buildings in a community, which contains the virus from the stool of patients.⁵ Most norovirus contaminations were typically found in groundwater.⁵ Therefore, if drinking water does not have enough disinfection processes, people might receive a waterborne viral infection.

Recreational water 

Finland has over 1,300 water treatment plants, many of which still rely on surface water from lakes or rivers, making them vulnerable to contamination. Inadequate disinfection is a common cause of waterborne outbreaks, especially during spring when snow melts, causing floods and surface runoffs. These conditions, along with sewer line breaks near wells, have led to several large outbreaks. In areas without proper sewage disposal, smaller waterborne outbreaks have occurred in private homes and rental cottages.⁵

Person-to-Person contact, contaminated surfaces and objects

Not only contamination in water and food but also person-to-person contact can transfer norovirus to others. As the virus can spread simply by close or direct contact with someone who carries the virus or surfaces contaminated with the virus, people who touch that surface or carrier can easily get an infection. Moreover, eating food, drinking liquid, and touching surfaces in public areas like schools or cruise ships can spread the virus.⁶

High-risk settings for norovirus outbreaks

Healthcare facilities

Hospitals and nursing homes

Healthcare settings such as hospitals, daycare centres or nursing homes are associated with infection due to their semi-enclosed environment, which allows particles or viruses to transmit rapidly. Moreover, the immune system that is developed by our bodies to fight norovirus is short-lived, and there are no effective vaccines at the moment.⁷

Therefore, norovirus is the most common gastroenteritis outbreak in healthcare settings. Normally, norovirus infection is mild and cured with symptomatic treatments; however, it poses a significant concern for vulnerable populations such as the elderly or immunocompromised individuals. This is because these populations, after infection, can experience severe symptoms and affect healthcare services, which further impact economic and societal costs.⁸

Food service environments

Food service environments such as restaurants, cafeterias or catering services play a key role in norovirus outbreaks. Over 90% of restaurants reporting norovirus outbreaks were involved in contamination by ill workers. These resulted from poor food handling practices and inadequate sanitation.⁸

Schools and daycare centres

Schools and children's care centres are one of the common norovirus outbreak settings. In childcare centres, children have been faced with many shared surfaces, which quickly spread the virus. As children themselves are carriers of infection, for example, they are ill. The mode of transmission can include person-to-person, food, environment, animal contact and water. Moreover, it can be spread through shared surfaces and toys during school periods.⁹

Factors contributing to outbreaks

Poor hygiene and sanitation practices

Inadequate handwashing and personal hygiene

Norovirus is primarily transmitted through the faecal-oral route, making inadequate handwashing a significant risk factor. Individuals who do not wash their hands thoroughly after using the bathroom or before handling food can easily spread the virus.¹⁰

Lack of proper sanitation in food handling:

Contamination can occur if proper hygiene practices are not followed in food preparation areas. For example, cross-contamination in kitchens due to improper cleaning can result in large-scale outbreaks, particularly in food service environments.¹⁰

Overcrowded or confined spaces

Spread in close-contact settings

Environments with a high population density, such as schools, nursing homes, and cruise ships, are especially vulnerable to norovirus outbreaks due to close proximity and frequent contact between individuals.¹⁰

Limited access to proper sanitation

In crowded settings where access to proper handwashing facilities is limited, the virus can spread rapidly, as people are less likely to maintain proper hygiene.¹⁰

Asymptomatic carriers

Silent spreaders

Asymptomatic carriers of norovirus can unknowingly spread the virus, complicating efforts to identify and isolate infectious individuals.¹¹

Challenges in controlling transmission

Since asymptomatic carriers show no signs of illness, it is difficult to control outbreaks, as they can continue transmitting the virus without being detected.¹¹

Prevention and control of norovirus outbreaks

Food safety practices

In the restaurant setting, there should be some policies. For example, food workers should not directly contact ready-to-eat food, and staff who are infected should be encouraged to take leave. Restaurants should also have proper hand hygiene, such as hand washing and using gloves or utensils, to prevent direct contact with ready-to-eat foods. Apart from personal hygiene, fresh produce should be washed. Meals containing oysters or other shellfish should be cooked at a temperature of at least 145°F. All ready-to-eat foods should be separated from raw oysters because of their high survival temperatures.¹²

Hygiene and sanitation

People should be educated on the importance of handwashing with soap and water and hand hygiene. Contaminated surfaces that contact food should be cleaned and properly disinfected, as contaminated surfaces are important in reducing norovirus. The virus is persistent and can be transferred to food preparation surfaces.¹²

To prevent the infection, you should wash your hands with soap for at least 20 seconds, in particular after using the restroom and changing diapers, before eating meals and after contacting someone or something. Additionally, using hand sanitiser alone cannot kill the virus.¹²

Public health interventions

After detecting norovirus infection, all healthcare professionals, such as nurses or pharmacists, should educate patients and their families about the importance of hand washing. Food handlers should be told about hand hygiene and stay up to date on hygienic codes, and also should keep the food storage environment clean and disinfected. Patients who are diagnosed with norovirus should be self-quarantined. Pharmacists should tell patients to drink ample fluids and stay hydrated, coupled with ensuring them that antibiotics are not effective against the viral infection.¹

Summary

Norovirus is a common stomach flu outbreak that causes vomiting and diarrhoea. In normal people, the symptoms may last for 2 days, but in vulnerable populations, the symptoms can be more severe and lead to hospitalisation. These result from contaminated foods, water and person-to-person contact. Prevention and control can be performed by strictly adhering to hygiene, such as regularly washing hands or implementing food safety practices.