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Foodborne viruses and fresh produce

A review of immediate interest to the food industry comes from the United Kingdom and deals specifically with fresh produce as a possible vehicle for viral infection. (Journal of Applied Microbiology 91 2001 759).

The authors, one from Campden & Chorleywood Food Research Association and one from the Public Health Laboratory Service, Central Public Health Laboratory, point out that, unlike bacteria, viruses do not grow or multiply in or on foods. However foods may become contaminated with human viruses and transmit infection.

Foods become contaminated either directly by infected people or through sewage pollution. The major foodborne viral pathogens are those that infect via the gastrointestinal tract. This includes Norwalk-like viruses (NLVs), rotavirus, astrovirus and hepatitis A virus.

Epidemiological features

Viruses are often transmitted directly from person to person but epidemiological investigations indicate that viral diseases can be transmitted by foods, particularly those that receive little or no processing, such as shellfish, fresh fruit and vegetables and salad items. The infective doses are not known but available evidence suggests that they are very low. It has been estimated that NLVs have an infective dose of between 10 and 100 virus particles.

Norwalk-like viruses In the published literature, Norwalk-like viruses have been associated with various items of fresh produce including washed salads, frozen raspberries, coleslaw, green salads, potato salad, and fresh cut fruits.

This group of viruses affect all age groups and from 1992 to 1997 accounted for one-third of all gastroenteritis outbreaks reported to the PHLS Communicable Disease Surveillance Centre (CDSC) in the UK. The number of outbreaks exceeded that attributed to Salmonella spp. but, unlike the salmonellosis outbreaks, only 6 per cent of the NLV outbreaks could definitely be attributed to food.

Hepatitis A virus As with viral gastroenteritis, transmission of this virus is by the faecal-oral route but the primary site of viral replication is in the liver. Epidemiological evidence linking hepatitis infection to food is sparse because of the long incubation period (3-6 weeks). From 1992-1999, the CDSC received over 19,000 laboratory confirmed reports of hepatitis A virus but only 155 were recorded as foodborne.

Outbreaks associated with fresh produce have been reported from several countries. Soft fruits, salads, strawberries and diced tomatoes have all been implicated. The reviewers consider that the year round global distribution of fruits and vegetables increases the risk of infection.

Contamination and survival

Fruits and vegetables may become contaminated with viruses in two ways. They may be contaminated in their growing area before harvest by coming into contact with inadequately treated sewage or sewage polluted water. Secondly, contamination can occur during processing, storage, distribution or final preparation either directly from infected people or from the environment. In most outbreaks of viral disease involving food produce, it is not known whether contamination occurred before, during or after harvest.

Mounting evidence suggests that viruses can survive long enough and in high enough numbers to cause human diseases through direct contact with polluted water or contaminated foods. Most of these studies have been carried out with viruses such as poliovirus which are easier to work with in the laboratory than are the gastrointestinal viruses. Survival appears to depend on a number of variables including the growing season, soil temperature, rainfall and soil type.

Other studies have focused on the survival of viruses on environmental surfaces such as stainless steel, glass and plastics. A range of enteric viruses including hepatitis A virus and rotavirus have been shown to persist for greater than 30 days on several types of porous and non-porous surfaces.

It is apparent from the lingering outbreaks that have occurred on cruise ships that NLVs survive well on environmental surfaces.

Survival of viruses on fruit and vegetables

The reviewers note that although a number of studies have been carried out on a range of commodities it is difficult to draw conclusions since experimental conditions and methods varied. However, importantly, most studies report viability in excess of the product shelf life. Chill storage temperatures (2-8°C) typically retard physiological damage and bacterial growth in minimally processed fruit and vegetables but may contribute to the survival and transmission of viruses to a human host.

Washing and sanitizing

Most fruit and vegetable washing systems are designed to remove gross contamination. However, their usefulness in removing microbial contaminants, as measured by a decrease in bacterial numbers, is seriously questioned (see, for example, Food Safety & Hygiene November 2001). Although there are no available data for viruses, this is likely to present a similar situation.

The reviewers were unable to find any information which related to the effect of chemical sanitizers on decontamination of enteric viruses on fruits and vegetables. The most commonly used sanitizers for treating fruits and vegetables are chlorine, chlorine dioxide, organic acids, surfactants and ozone. The mechanism of action of these chemicals on viruses and their interaction with the plant materials is poorly understood.

Chlorine based sanitizers are usually considered the most effective against enteric viruses but NLVs and hepatitis A virus appear to be relatively resistant to chlorine. In one series of experiments it was found that an infective dose (unquantified) of NLV was inactivated in water by 10mg/L of free chlorine with a contact time of 30 minutes. Five to six mg/L of free chlorine was still present in the test solution at the end of the 30 minute period (Applied and Environmental Microbiology 50 1985 261). Based on volunteer feeding trials, less severe treatments did not completely inactivate the virus.

In a separate study not involving volunteer feeding, hepatitis A virus appears to be rather more sensitive to chlorine than is noted above but shows greater resistance then Escherichia coli or Streptococcus faecalis (Applied and Environmental Biology 46 1983 619).

It is clear that the sort of sanitizing treatments given to fruit and vegetables commercially are unlikely to be as severe as the treatments reported in these experiments.

Despite some practical disadvantages, chlorine dioxide is gaining favour as a sanitizing treatment in this industry. Again there is no information specifically related to NLVs and hepatitis A virus although studies with bacteria (Food Microbiology 13 1996: 311) suggest that chlorine dioxide, when used in solution, will offer little advantage over chlorine.

Ozone should be at least as effective as chlorine as a sanitizer in this situation but there are no published studies to support this belief.

The authors conclude, with good reason, that studies are required to provide specific information on the efficiency of current washing and decontamination processes for the removal of viruses from fresh produce. This is particularly the case for NLVs and hepatitis A virus.

While controls exist in some countries over the standards of organic fertiliser and irrigation water which may be used in the cultivation of fruits and vegetables , this is not universal and produce may be contaminated in the field from time to time.

Meticulous attention to good food handling practices and education is essential if viral contamination is to be controlled. This should apply not only in the retail and catering industry but also on the farm.

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Prepared by Keith Richardson and Rachel Jackson
Food Science Australia
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