The incident, reported recently in the Lancet, adds a new and worrying dimension to an infection increasingly recognised as a serious public health problem. The Snowdonia outbreak was almost certainly caused by a microbe known as a small, round-structured virus (SRSV).
Most forms of gastroenteritis are transmitted through food, water or by person-to-person contact. But the virus that caused this one may have been airborne. That possibility indicates the need for improvements in the measures used to prevent intestinal infections.
Health officers investigating the Snowdonia incident asked each of those who became ill what they had eaten, and when. But they were unable to pinpoint any item of food as the likely cause. However, the nature of the illness did suggest that it could be attributable to an SRSV. Specimens of faeces or vomit obtained from 20 victims, were studied by electron microscopy and other techniques, and in eight cases (seven of them coach party members) the investigators found an SRSV.
In their report, Dr Paul Chadwick and his colleagues suggest that, as with other epidemics in the past few years, the virus was carried through the air from the initial case to other passengers. They speculate the coach's air-conditioning system may have helped to disseminate the microbe. This, in turn, could explain why such an unusually high proportion of the tourists on board were affected. (The victims in the hotel were probably infected by person-to-person contact.) These findings are not conclusive, but they are compelling - particularly since the elderly woman was ill not in the lavatory, where other passengers could have picked up the virus, but in her seat.
The type of infection SRSVs cause was first described in 1929 as 'epidemic winter vomiting disease'. In 1972, US microbiologists identified the microbe responsible for a typical outbreak, in a primary school in Norwalk, Ohio. Since then, several viruses of very similar appearance have been identified and named after the geographical region in which they were first reported, such as Southampton, Taunton and Hawaii viruses. SRSV is the collective epithet they have been given in more recent years.
One feature of SRSVs has hampered their investigation and the development of tests to diagnose individual cases and chart the spread of epidemics. Unlike most disease-causing microbes, they cannot be grown in the laboratory (either in tissue culture or in animals). They must be identified by studying their shape and general appearance under an electron microscope. In addition, samples of faeces and vomit from infected patients typically contain only tiny quantities of an SRSV.
There was, however, a big step forward last year when researchers in Southampton and Bristol determined the entire sequence of 7,696 sub- units composing the single strand of ribonucleic acid in Southampton virus. This will allow a suitable test to be developed, and facilitate an understanding of the relationship between it and other types of SRSV.
Meanwhile, the likelihood of SRSVs being airborne requires urgent clarification. The viruses can certainly be transmitted by the sorts of route commonplace in the case of food-poisoning bacteria, such as salmonellae. Two particular hazards have come to light - shellfish harvested from contaminated waters and then inadequately purified, and food handlers infected with the virus. But dissemination of SRSVs through the air has been strongly suspected, not only in Snowdonia but also in at least two other recent incidents. Each of those outbreaks occurred in a setting likely to include elderly, debilitated or chronically ill individuals. .
Practical implications include the extension of precautions used to prevent transmission of gastroenteritis in hospital patients, from faecal contamination to airborne carriage, and enforced sick leave for employees suspected of harbouring an SRSV. But on the last point the omens are not promising. In 1990 the Public Health Laboratory Service recommended food handlers not return to work for at least 48 hours after recovering from an attack. The Food Industry Medical Officers' Working Group has challenged that suggestion.
'Power Unseen - How Microbes Rule the World', by Bernard Dixon, is published by W H Freeman/Spektrum.