Foodborne viral infections — Part 2

Part 2 — Sources and detection of foodborne viruses

Also: Part 1 — Types of foodborne viruses
          Part 3 — Foodborne viruses in the food factory

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.

Shellfish

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

Fruit and vegetables may act as vehicles of infection if fertilised 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 fertilised 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.

Food handlers

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.

Detection

Detection of viruses in food has not been considered possible in a routine laboratory because of their requirement for a living host or animal tissue for growth. In addition, the level of virus particles in a contaminated food is usually very low. Specialist laboratories may achieve detection using cell culture and complex extraction methods but techniques previously available are not suitable for routine application and recovery rates remain poor. The use of the polymerase chain reaction (PCR) is being developed for detection of norovirus in foodstuffs implicated as the source of outbreaks. A recent method, said to give results in under 4 hours, involves recirculating-IMS linked to PCR (paper on inter-laboratory trials awaiting publication).

Methods currently used for routine detection of NLVs in faeces are based on immunoassays and PCR. Ideally, samples should be collected within 48 hours of the onset of symptoms since these contain the greatest levels of the virus. Use of this PCR test enables the detection of more than 90% of NLV infections and also allows characterisation of the strains.

Detection of Hepatitis A virus in faeces is not usually possible because, by the time jaundice occurs, the peak of excretion of virus particles has passed. Diagnosis is based on detection of specific IgM and IgG antibodies in the blood serum or saliva.

Please see also Parts 1 and 3 of this article.

The Institute of Food Science & Technology authorised this Information Statement dated April 2008, which replaces the version issued in March 2002.