Foodborne viral infections - Part 3

Part 3 — Foodborne viruses in the food factory

Also: Part 1 — Types of foodborne viruses
          Part 2 — Sources and detection of foodborne viruses

Virus survival characteristics

Foodborne viruses are hardy and may survive for prolonged periods in foods or the food handling environment as well as persisting in aquatic environments. They are highly resistant to chilling, freezing, preservatives and ionising radiation. Hepatitis A virus and norovirus have both been shown to survive 60°C for 10 minutes. However, inactivation of these viruses occurs at temperatures above 65°C at a rate proportional to the temperature but also depends on the composition of the medium. Both agents are killed by boiling. They are resistant to acidic conditions (pH 3) and can therefore survive on acid fruits such as strawberries and raspberries and in processes such as pickling in vinegar or yogurt production. They are also resistant to alcohol and high sugar concentrations.

Management of food handlers

Food handlers suffering from vomiting or diarrhoea should be excluded from work immediately. They should not return to work until at least 48 hours after cessation of symptoms. After the initial onset of symptoms, although infection may appear to be sufficiently mild to enable the food handler to continue working, this must be prevented since even very low numbers of norovirus can result in illness if transferred to the food.

Prevention of foodborne viral illness requires good staff supervision and food handlers should be encouraged to report symptoms of illness as soon as they occur. Staff should also be made aware that they could transfer viral contamination to food via hands and clothing following contact with an ill family member. There is no carrier state associated with norovirus although some patients, particularly children, may be asymptomatic.

Hepatitis A is mainly spread by person-to-person transmission but can be spread by food handlers. Cases of viral hepatitis are most infectious before jaundice is apparent, but exclusion of food handlers from work for one week after the onset of jaundice is recommended. There is no chronic carrier state. If food handlers are exposed to Hepatitis A, those shown not to be immune by antibody testing may be vaccinated or given prophylactic human normal immunoglobulin by injection. Children can show very mild or asymptomatic infection.

Control

Shellfish are an important cause of viral gastroenteritis. Cultivation in clean waters is very important, as depuration procedures may not be effective in eliminating viruses. Heat treatment to an internal temperature of 85–90°C, maintained for 90 seconds, is required to destroy viruses in molluscs but careful control is necessary to achieve this without toughening of the shellfish flesh. Consumption of uncooked molluscs such as oysters remains a risk, as is cross-contamination from contaminated shellfish to other food in the kitchen.

Contamination of foods other than shellfish will normally occur on the surface of the food, where viruses will be more susceptible to heat treatment. Heat processes commonly used in the food industry, such as heating to a core temperature of 70°C for two minutes, will significantly reduce the level of virus contamination but may not destroy all the viruses if the contamination level was very high.

The number of virus particles required to cause infection is very low. Contamination of food with norovirus by infected food handlers and person-to-person spread occurs easily due to the very large numbers of virus particles present in the stools and vomitus at the onset of symptoms, and to the survival characteristics of the viruses. Infection of personnel may take place by aerosol inhalation and subsequent ingestion as well as the hand-to-mouth route. Thorough hand washing with soap and warm running water and drying with disposable towels or air hand dryers are essential to minimise spread of contamination.

If vomiting has occurred in the factory, a rigorous and appropriate disinfection for viral decontamination of the environment must be implemented. This is best achieved by cleaning, using hot water and detergent, followed by disinfection with a chlorine-based disinfectant at a concentration of 500 ppm available chlorine. Food that may have been exposed to aerial contamination or handled by the ill person should be destroyed unless it is to be heated above a core temperature of 85°C following exposure. If contaminated food is not destroyed, cross-contamination and re-infection may occur. Any soiled clothing should be rinsed to remove gross contamination, preferably into the toilet bowl, and then laundered in a domestic or commercial washing machine with a hot cycle (above 85°C).

Implications for the food industry

The prevention of foodborne viral illness depends on the quality of raw materials, staff education and a high level of awareness in the manufacturing and catering industries. If payment is withheld during sickness, staff will be reluctant to cease working when symptomatic. However, the cost of sickness benefit is minimal compared with the potential costs incurred as a result of a foodborne outbreak.

Please see also Parts 1 and 2 of this article.

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