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VIRAL FOODBORNE ILLNESS - AN OVERVIEW

An overview of viral foodborne illness that introduces the viruses which cause food poisoning, explaining how such viruses are spread by food handlers and contaminate food, and giving examples of foods typically implicated in food poisoning outbreaks.

Introduction

Viruses are very small and highly infectious microorganisms that are increasingly recognised as a significant cause of foodborne illness. Although the true incidence of foodborne viral food infection is undetermined, it is probably grossly under-reported.

Viruses get into food primarily by contamination with faecal material from infected persons, although they can also be spread in vomit. However, unlike bacteria, viruses do not multiply or produce toxins in food. Food therefore merely acts as a vehicle for the passive transfer of viruses into the human host.

Although almost any food could, in theory, be involved in transmitting a virus the most commonly associated foods are those that are either not cooked or are only lightly cooked prior to consumption, e.g. shellfish and raw fruit and salad vegetables.

By far the most significant foodborne viruses are:

Noroviruses (NoV) (which used to be called Small Round Structured Viruses - SRSVs or Norwalk-Like Viruses - NLVs) - in terms of the number of infections caused
hepatitis A virus (HAV) - in terms of the severity of the infection.

Although gastroenteritis can be caused by other viruses such as rotavirus and astrovirus, these are rarely implicated in foodborne outbreaks.

Spread by Food Handlers

Central to the spread of foodborne viral infections are food handlers. These may be:

infected food handlers with symptoms - shedding of virus occurs during the period of illness
infected food handlers that have recovered from illness - where the shedding of NoV, for example, may last for up to 3 weeks after recovery.
infected food handlers without symptoms i.e. asymptomatic - common for all foodborne viruses. For example, carriers of hepatitis A typically shed high quantities of the virus 10 - 14 days before any symptoms may arise.
food handlers having contact with sick people (e.g. children or other relatives) in the household, a proven factor in the spread of viral foodborne disease.

The two most important means of intestinal virus spread by food handlers are:

poor personal hygiene i.e. inadequate hand washing by infected food handlers/persons after visiting the toilet. This leads to fingers/hands being contaminated with virus particles present in the stools (faeces) which in turn can then be passed onto food or food contact surfaces. 10% of HAV particles, for example, can be easily transferred in this way.
by particles shed in vomit / aerosols which either directly contaminate food, or contaminate surfaces that come into contact with food and that are not adequately cleaned and disinfected after a (projectile) vomiting incident.

Viral Foodborne Illness - Some Specific Features

Some general features of foodborne viral infections and important differences from foodborne bacterial infections are:

only a few particles are needed to cause illness
high numbers of viral particles are shed in the stools (faeces) of infected persons (up to 10¹¹ (one hundred thousand million) particles per gram stool reported)
viruses need specific living cells in order to replicate and therefore cannot do so in food or water
foodborne viruses are typically quite stable, being able to persist in the environment. Most foodborne viruses are more resistant to heat, disinfection and pH than are most vegetative (growing) bacteria.

Foods Associated with Outbreaks

Outbreaks have been associated with many different food items e.g. filter-feeding shellfish (oysters, clams, mussels), berries, fresh or frozen fruits, salad vegetables, delicatessen meats, sandwiches.
Food products at highest risk are:

foods requiring intensive handling/manipulation
foods handled under poor hygienic conditions
foods manually handled/prepared close to the point of consumption ('close-to-fork')
food that has not been cooked or has been only lightly cooked
shellfish grown an

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