Institute of Food Science & Technology
The Voice of the Food Profession
Viruses require a host in order to multiply, and the original source of all foodborne viruses is the human intestine. They cannot grow in food. Contamination of food may occur either during preparation and serving by infected food handlers or by contact with sewage or sewage-polluted water.
The main food type associated with foodborne viruses is molluscan shellfish such as oysters, cockles and mussels, which are usually found in shallow coastal or estuarine waters, commonly near sewage outlets. These shellfish are filter feeders that can concentrate virus particles from the surrounding water. Molluscs are either eaten raw or after a mild heat process, which if poorly controlled may not inactivate virus particles present. Shellfish harvesting areas are classified according to the level of faecal indicator bacteria present in the shellfish flesh; if the levels exceed the specification for direct consumption, the shellfish must be relayed in cleaner water, receive an approved heat treatment or undergo a purification process (depuration) before sale. However, depuration cannot be guaranteed to remove viruses, and outbreaks of viral gastroenteritis have been attributed to depurated shellfish. Cultivation of molluscan shellfish in water protected from sewage contamination is therefore paramount in the control of viral infection.
Although molluscs are the most clearly implicated source of foodborne viral illness, they do not necessarily cause most illness.
Fruit and vegetables may act as vehicles of infection if fertilized with sewage sludge or irrigated with sewage-contaminated water. Guidelines issued by the World Health Organisation (WHO) state that fruit and vegetables intended to be eaten raw should not be fertilized with sewage or irrigated with contaminated water. Apart from an outbreak of Hepatitis A resulting from contaminated soft fruit, there are no proven outbreaks associated with contamination of these foods at source. Control of sewage sludge application to land is important to prevent viral (and other pathogens) being recycled to affect human and animal health. In the UK, the Sewage (Use in Agriculture) Regulations 1989 are designed to protect the environment and human and animal health where sewage sludge is used on agricultural land. A "Safe Sludge Matrix" (ADAS, 2001) recommends the minimum time periods between the application of sludges to land and its use for food production and includes a table of crop types, together with clear guidance on the minimum acceptable level of treatment for any sewage sludge (often referred to as biosolids).
Contamination of food by infected food handlers is an important cause of viral foodborne illness. Food items such as salads and dessert dishes that receive considerable handling during preparation and are not given any further heat treatment before consumption are often implicated in foodborne viral outbreaks. Consumption of contaminated water and ice, or their use in food preparation, has also caused viral illness.
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