Dr Roy Betts is head of microbiology at Campden BRI

Recently, we have become much more aware of viruses as foodborne pathogens. Their ability to cause large outbreaks of illness at very low doses is well known but where they come from and how to control them is less well understood. Viruses cannot grow on foods, they are carried on the surface until being eaten, where they can then infect us and cause a range of illnesses.

There are three viruses linked to foodborne illness: Norovirus, Hepatitis A and Hepatitis E. The latter is primarily a concern in meats, while the other two have been linked on a number of occasions to outbreaks associated with fresh produce.

The types of fresh produce implicated in viral outbreaks include lettuce, salad onions, pomegranate seeds, semi-dry tomatoes, and a wide range of fresh and frozen soft fruits. The size and distribution of the outbreaks can be large. In 2012 in Germany, Norovirus-contaminated frozen strawberries resulted in an outbreak involving 11,000 people, while in 2013 frozen blackberries and redcurrants caused an international outbreak of Hepatitis A, involving 10 European countries and nearly 1,500 people.

Surveys have attempted to estimate the prevalence of virus contamination in fresh produce, but have been tricky because the virus test methods are difficult to use. However, the results can be troubling. A survey of soft fruit in Belgium in 2009 indicated 34 per cent positive for Norovirus while a survey of leafy greens in Canada in the same year indicated 54 per cent Norovirus positive. One of the problems is that the methods used in surveys usually detect virus genetic material, and do not indicate if the virus found is able to infect. What we do know is that finding viral genetic material means infective virus has been present at some time on that product.

The final concern is a lack of a full understanding of how common food processes affect viruses. We understand how cooking, washing and disinfectants react with bacteria and that, if used correctly, it will kill them. With viruses our understanding is less well developed. We know freezing will preserve viruses, that some viruses appear more heat resistant than bacteria and certain disinfectants can be less effective against viruses than they are against bacteria.

So where does that leave fresh produce? Well, there have to be controls in place that help prevent contamination of the product right from the growing stage, for example, uncontaminated irrigation water, field hygiene, picker hygiene, and hygienically-maintained picking and transportation equipment. This, combined with good packhouse/process area hygienic measures, will limit the opportunity for virus contamination and lead to a safer fresh product.