Foodborne virus research
Pathogenic viruses are an emerging problem for the food industry. Although they cannot grow on or in foods, they are carried by foods. They tend to have low infective doses, and general knowledge of how they react to common microbiological controls, such as heat, sanitisers, low pH and low water activity, is quite sparse. However, ongoing research into the control of viruses in food production is helping to fill some of these gaps.
Virus detection methods are based on genetic analysis; genes may remain detectable after the virus is rendered non-infective, making interpretation of inactivation data difficult. Much work is done with “surrogate” viruses. These are viruses that are considered to be similar to the foodborne viruses of concern, but differ in that they can be cultured in the laboratory, and we can therefore assess their ability to infect after treatment.
We have investigated the persistence of the bacteriophages MS2 (RNA virus) and øX174 (DNA virus) at a range of pH values in a broth system. Both viruses were capable of surviving at pH values of 3-7 for at least 60 days. Both had lower resistance to pH 2, with øX174 inactivated after 4 days and MS2 after 18 days. One of the key preservative strategies employed in foods is acidic conditions to control microbial growth. These results indicate that MS2 and øX174 are capable of persisting at low pH under chilled conditions for at least 4 days. Another interesting point of note is that acid survival was not influenced by the acid type used.
Sources of infection
A recent report from the UK Advisory Committee on the Microbiological Safety of Food suggested that the most important viruses associated with foodborne infection are norovirus, hepatitis A and hepatitis E.
Noroviruses are highly contagious and infected food handlers can spread contamination to food they handle. Transmission has also been noted via contaminated shellfish and fresh produce.
Hepatitis A incubation period can be long, in some cases up to 5 weeks. This can make tracing the source of an outbreak very difficult. Again, shellfish, fresh produce and infected food handlers have been reported to be the cause of outbreaks.
Hepatitis E is a relatively new addition to the list of food pathogens. There are increasing numbers of human cases, some of which could be linked to the consumption of both raw and ready-to-eat pork products. A recent UK survey found that, of over 600 pigs tested, almost 93% were seropositive for Hepatitis E and nearly 6% carried the RNA of the virus (i.e. were infected at the time of testing). Survey work found evidence of Hepatitis E RNA in pigs' livers at the slaughterhouse, on surfaces both at processing plants and at the point of sale, and in pork sausages at the point of sale.
These viruses are fairly resistant to a number of control measures. Some sanitisers have only a limited effect, and heat may have to be applied for some time to deliver an effective kill. We can offer practical advice on how to minimise the risk of viral contamination and on how to determine the effectiveness of such treatment. We have validated virus detection methods and are now working on establishing effective control measures.
Project: Control of viruses in food production