EHJ March 2005, pages 22-24

Staff and customers of a Suffolk pub along with an investigating EHP were all struck down during a Norovirus outbreak. Mark Sims, Hamid Mahgoub, Torbjorn Sundkvist and Augustine Pereira explain how they stemmed the highly contagious outbreak, step-by-step

You know there are long days ahead when you get complaints alleging that two groups of people suffered gastrointestinal illness after eating at the same pub. Officers from Suffolk Coastal DC food and safety team received just such a complaint on a Monday last August. The first complainant alleged that a party of 12 who had lunch at the pub the previous Thursday had fallen ill. A second complainant alleged that a separate party had lunch on the Saturday and some of them also fell ill. The level of vomiting and the incubation time pointed to Norovirus, although at this stage bacterial food poisoning could not be ruled out. Norovirus, or small round structured virus, can spread from person-to-person via the faecal oral route or from vomiting. Virus particles are spread on aerosols caused by vomiting, travelling on air currents from open windows or air conditioning. The result is often indirect person-to-person spread via the environment.

Norovirus, sometimes colloquially called the winter vomiting bug, may have in this instance hit in mid-summer at a busy public house frequented by tourists during some of the hottest days of the year. The Health Protection Agency's Suffolk Health Protection Unit was immediately informed and an outbreak control team (OCT) meeting was called for the following day.

EHPs started their investigation later that morning by contacting members of the two parties, obtaining food histories, detailing symptoms and organising faecal samples from cases with diarrhoea. Meanwhile, two officers visited the pub where they checked food safety standards, sampled food, questioned staff present and arranged for faecal samples from them.

Five people made ill in the Saturday party had eaten cold crab claws, a potential source of infection for this group. Food histories from the Thursday party found no common food factor. Most patients reported vomiting while a smaller number also developed diarrhoea, some with a fever but all seemed to recover quickly.

An inspection of the premises revealed poor hygiene around toilets, used by customers and staff. At the time, the pub was serving up to 130 meals most days with up to 200 being served at Sunday lunchtimes. No food was left over from the two parties to sample. Crab claws were served cold, but it was unclear whether these were prepared using bare hands or if the food handler was wearing disposable gloves. Four samples of lobsters and prawns were taken for examination.

On Tuesday morning, EHPs returned to check on improvements to hygiene and to interview the remaining members of staff. Twelve of the 17 staff at the pub were found to have suffered gastrointestinal illnesses in the days prior to the outbreak. The first staff member had fallen ill on the previous Tuesday. Some could have been working on the days they fell ill while others returned to work before the recommended 48-hour symptom-free exclusion period. Staff did not eat meals at the pub but admitted to picking at food, mainly chips, in the kitchen.

The findings of the investigation were reported to the first meeting of the OCT on the Tuesday afternoon. Viral gastroenteritis was suspected of being the cause, most probably Norovirus. This meant that contaminated surfaces such as roller towels rather than just food could present an ongoing viral source. The OCT agreed a case definition: "A suspected case was any person who had reported with gastrointestinal symptoms including at least any one of the following - diarrhoea, vomiting, abdominal pain, nausea; and anyone who had eaten from or was working at the pub within 72 hours of falling ill on or after the 10 August. Confirmed cases were suspected cases with positive microbiological results." The next meeting of the OCT was arranged for the Friday of that week. A third visit was made to the pub on Tuesday evening by one of the officers to check that the 48-hour symptom-free exclusion period was being followed and to deliver written guidance on precautions to control Norovirus-type infections. Table 1 summarises the advice given to the pub. Further questioning revealed that two pub staff had cleared vomit from the male customer toilets the weekend before the two parties ate at the public house. Both staff tested positive for Norovirus but only one of them reported symptoms. On the Wednesday one of the investigating EHPs fell ill with nausea and diarrhoea and was later found to be positive for Norovirus.

The second OCT meeting was held on the Thursday where an epidemic curve (figure 1) suggested some form of continuous infection, with an incubation period of approximately two days. Analysis of the available data from the Saturday party (table 2) showed a strong suggestion that crab claws might have been implicated. Statistical analysis revealed a p-value of 0.05357 thus approaching significance.

The virus was isolated from four of the 14 stool samples sent for microbiological investigation from the diarrhoea cases. This included the investigating officer. Environmental contamination in the premises was suspected as was food contamination by infected staff. One of the staff tested positive for Campylobacter despite being asymptomatic, but this was considered to be an incidental finding. No bacterial pathogens were isolated from the stool samples obtained from cases with diarrhoea or any other symptoms.

This was an outbreak of gastrointestinal illness affecting 27 out of a possible 40 people who had either eaten or worked at the pub in the eight-day period after the first staff member became ill. Analysis of the data collected suggested a continuous infection during that time period with a possible incubation period of approximately two days. Symptoms described by the cases were mostly short-lived and relatively mild with vomiting predominating over diarrhoea. These features suggest Norovirus (Kaplan criteria). [1]

Norovirus is notoriously difficult to clean from the environment. This was therefore likely to be how the virus was being passed on. Particularly suspect was the shared toilets between staff and customers. The crab claws served at the 14 August party were also a possible vector as they had a p-value of 0.05357, which approaches significance. It is suspected that as few as 10 to 100 virus particles can cause infection[2], so it is extremely easy to spread from environmental contamination. A study has shown that where fingers come into contact with virus-contaminated material, Norovirus can be consistently transferred via the fingers to melamine surfaces and from there to other typical hand-contact surfaces, such as taps, door handles and telephone receivers.

It was found that contaminated fingers could, sequentially transfer virus to up to seven clean surfaces.[3]Norovirus can survive for up to several days in the environment and still cause infection.[4]

The EHP who became ill did not eat food or drink at the premises and did not use the toilets. This suggests the virus was extremely contagious and existed in the environment. High levels of hygiene are needed to stem the spread of Norovirus. Bleach-based disinfectants play an effective role in controlling the spread of such a virus. A role that disinfectants with only bactericidal properties cannot fill.

Advice to investigators to protect their own health when involved in such outbreaks is that they are at risk from infection. Precautions to minimise the risk of acquiring infection are summarised in table 3

References

1. The frequency of a Norwalk-like pattern of illness in outbreaks of acute gastroenteritis. Am J Public Health 1982; 72:1329-1332.
2. Laboratory diagnosis of Norovirus, which method is the best? Intervirology 2003; 46:232-238.
3. Effects of cleaning and disinfection in reducing the spread of Norovirus contamination via environmental surfaces. Journal of Hospital Infections (2004) 58, 42-49.
4. Norwalk and other human caliciviruses, molecular characterisation, epidemiology and pathogenesis. In Microbial Food borne Diseases, pp 460-493.

Mark Sims is principal EHO at Suffolk Coastal DC. Dr. Augustine Pereira is specialist registrar in public health at Norfolk, Suffolk and Cambridgeshire Strategic Health Authority, Torbjorn Sundkvist is a consultant in communicable disease control and Hamid Mahgoub is a health protection medical specialist for the Health Protection Agency.