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Airborne transmission of food related viruses

Norwalk viruses cause acute gastroenteritis in humans world wide. The onset of vomiting and/or diarrhoea is sudden and often violent accompanied by headache, abdominal discomfort and low grade fever.

Identified sources of outbreaks include contaminated water, ice, shell fish and food contaminated by food handlers which is then eaten without any heating step, e.g. salads, fruit, sandwiches and bakery products. Outbreaks are often confined to closed communities such as schools, nursing homes, caravan parks, cruise ships or military installations (Grohmann in Foodborne microorganisms of public health significance, 5th ed. 1997).

Secondary spread of Norwalk and Norwalk like viruses is common and the faecal-oral route of transmission is important as is the possible contribution to secondary spread by airborne transmission. Aerosolised viruses could be inhaled and subsequently swallowed. This mode of transmission has been implicated in incidents before (Lancet 1989 2 p.961).

An unusual incident involving Norwalk virus has now been reported by workers in the UK. This incident involved 52 people who had eaten in the one restaurant on the one occasion and the authors believe it is the first detailed account of an outbreak of Norwalk like viral infection associated with airborne transmission arising from a diner who vomited during a meal (Epidemiology and Infection 2000 124 p.181).

The unfortunate diner was one of a party of 126 people seated at six separate tables in the function room of a large hotel. In addition a number of hotel residents and casual diners were dining in a restaurant on the premises but in a separate room.

The diner in question had not been ill prior to the meal and had little warning she was unwell prior to her vomiting onto the polished wooden floor. The floor was rapidly cleaned up by a waiter with mop and disinfectant and the meal continued. In the subsequent 36-48 hours, 52 other diners became ill with vomiting, diarrhoea and abdominal pain with symptoms lasting 24-48 hours in most of those affected.

The investigating team from local health authorities was able to establish from epidemiological and laboratory investigations that the illness of all victims was caused by the same strain of a Norwalk like virus. They were also able to establish to their satisfaction that the hotel kitchen and staff were not the source of infection. None of the foods served was significantly associated with illness and none of the diners in the separate restaurant became ill after eating that evening.

The investigators then studied the layout of the dining room and the arrangement of the tables used by the 126 diners and tried to correlate that with the incidence of illness. What they found was a highly significant relationship between the distance from the diner who became ill at her table and the risk of other diners subsequently developing illness. There was no significant deviation from this trend.

At the table on which the vomiting incident occurred, the subsequent illness rate of diners was 91 percent. At the table most isolated from the incident, the illness rate dropped to percent with other tables showing intermediate rates varying from 40 percent to 71 percent. The investigators concluded that the woman who was ill at the table brought the virus to the dining room and its airborne transmission after her vomiting attack led to infection of 52 other diners. The presence of ceiling fans and a ceiling mounted extractor fan above the incident table may have contributed to the spread of the virus.

The health officials warn that airborne transmission of enteric viruses may represent a significant risk in hospital wards and nursing homes and appropriate control measures to deal with airborne transmission of virus particles must be considered.

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Prepared by Keith Richardson and Beverley George
Food Science Australia
PO Box 52, North Ryde 1670. Tel +61 2 9490 8397 Fax +61 2 9490 8499
Email enquiries@csiro.au