Winter vomiting

The current outbreaks of winter vomiting reported from Glasgow and elsewhere in Scotland have affected as many as 20 hospitals and resulted in the cancellation of many elective and emergency admissions. Winter vomiting is due to infection with subtypes of the genus Norwalk-like virus—a term that is now preferred to small round structured virus.¹ Infections with Norwalk-like viruses are reported even less often in national surveillance in the United Kingdom than other gastrointestinal pathogens such as salmonella and campylobacter (see table), although they probably account for as much clinical disease in the population as any other pathogen. The undoubtedly enormous cost of control measures, cancelled operations, and other disruptions in hospitals has not been estimated in the United Kingdom.

Between 1993 and 2001, laboratory reports of identifications of the small round structured virus to the Scottish Centre for Infection and Environmental Health rose from 85 to 328.²,³ Reports of general outbreaks of infectious intestinal disease due to small round structured virus infections increased from 17 in 1996 (the year surveillance started) to 38 in 2001 (unpublished data). It is impossible to estimate what proportion of these increases is due to improved laboratory technology, improved reporting, or a true increase in infection as in the same period the Communicable Disease Surveillance Centre of the Public Health Laboratory Service received between 1500 and 2500 laboratory reports a year, with no particular trend.⁴,⁵

The taxonomy of the family Caliciviridae, in which the small round structured virus has recently been placed,⁶ is complicated and not fully resolved.⁷ The family consists of four genera, one of which is the Norwalk-like viruses. Although not routinely sub typed, the Norwalk-like viruses can be divided into species named after the place where they were first identified. In the United States these are Hawaii, Snow Mountain, and Norwalk—the town in Ohio where the prototype virus was identified in 1972.⁸ In the United Kingdom the species are named after Southampton, Taunton, Bristol, and Lordsdale—the commonest species in the United Kingdom and worldwide (O Caul, personal communication). From further afield is the Desert Storm species.

Humans are the only known hosts of Norwalk-like viruses. Infection can be acquired through consuming contaminated food or water, or more commonly from an infected person via the faeco-oral route, through fomites, or by aerosol spread, particularly when a patient vomits. Food borne transmission is often due to poor hygiene in the kitchen, but filter feeding bivalve molluscs harvested from water polluted by sewage also pose a threat.

In England over 1500 times more cases occur in the community than are recorded.⁹ This represents an enormous burden on general practitioners. In the population as a whole (including hospitals and residential homes) infections with Norwalk-like viruses are over 300 times commoner than national surveillance suggests. The equivalent multiples for salmonella and campylobacter in the whole population are 3 and 10 respectively. Outbreaks are also under-reported, and the increase in the United Kingdom following events at the Victoria Infirmary in Glasgow is probably at least in part due to improved reporting.

Although the incidence of infections due to Norwalk-like viruses is higher in winter, cases and outbreaks occur year round. Most reported outbreaks occur where people live in close proximity to each other. Of the 38 general outbreaks of infectious intestinal disease in Scotland in 2001 in which a Norwalk-like virus was confirmed 18 were in residential institutions, 17 in hospitals, two in schools, and one in a restaurant (unpublished data from the Scottish Centre for Infection and Environmental Health). Internationally outbreaks have also been reported in hotels, military barracks, cruise ships, and social functions.

The incubation period varies from 10 to 70 hours but is usually 24-48 hours. Symptoms include nausea, vomiting (often projectile), diarrhoea, and stomach cramps. Vomiting and diarrhoea can occur with little or no warning. Patients may also suffer headache, fever, chills, and muscle aches. The illness usually resolves within 24-48 hours, but may last a week or longer. There is no specific treatment and no long term effects. Patients with diarrhoea or vomiting should drink plenty of liquids to prevent dehydration.

Spread by food or water can be controlled by scrupulous personal hygiene and appropriate sewage disposal. All food handlers with diarrhoea should stay off work until 48 hours after their symptoms have resolved. If their personal hygiene can be relied on—a condition that should apply to all food handlers—they can then return to work without virological examinations.¹⁰,¹¹

Controlling spread of Norwalk-like virus infection in hospitals*

• Ensure strict personal hygiene by all staff, patients, and visitors, including scrupulous hand washing

• Restrict movement of patients from affected wards and of staff between affected and unaffected wards

• Stop admission of new patients to affected wards

• Exclude symptomatic staff, including food handlers, and isolate affected patients, until 48 hours after symptoms have resolved

• Ensure immediate cleaning and disinfection of areas contaminated by vomitus and faeces

• Clean and disinfect the environment, frequently in affected wards and particularly in wards reopening to new admissions

• Conduct surveillance of the course of the outbreak

*Control measures in the community essentially depend on maintaining personal hygiene

The small infectious dose for Norwalk-like viruses—10 to 100 organisms—makes control of person to person spread more difficult.¹² Frequent hand washing, prompt disinfection of contaminated surfaces, and washing of soiled articles of clothing should reduce the likelihood of cross infection via the faeco-oral route. However, aerosol spread after vomiting poses an intractable problem, especially as the absence of a prodrome excludes the option of isolating patients likely to vomit. When outbreaks occur in hospitals, staff working on affected wards should be restricted to that ward to prevent cross infection. This is very difficult to achieve in practice. The closure of wards to new admissions is often necessary (see box).

To control the problem requires appropriate surveillance systems. Surveillance of cases and outbreaks in the United Kingdom massively underestimates the size of the problem of infections with Norwalk-like viruses. It is obviously inappropriate to ask general practitioners to send faecal samples from every mild case of diarrhoea that presents to them, but a sentinel system would improve our knowledge of infection with these viruses in the population. Outbreak surveillance data would be improved if more were reported to the outbreak surveillance systems that operate in Scotland and England and Wales.

Table.

Laboratory reports to Communicable Disease Surveillance Centre (for England and Wales) and Scottish Centre for Infection and Environmental Health

Organism	Reports for England and Wales 2000	Reports for Scotland 20015
Salmonella	16 423    (unpublished data)	1576
Campylobacter	56 42013	5429
Norwalk-like virus	1 60413	328

Figure.

CDC/SPL

Norwalk-like viruses. Coloured transmission electron micrograph of a section through two groups of Norwalk-like viruses