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Transmission, management and features of norovirus infection

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VOL: 101, ISSUE: 05, PAGE NO: 28

Debbie Weston, BSc, RGN, is infection control adviser, East Kent Hospitals NHS Trust

Debbie Weston, BSc, RGN, is infection control adviser, East Kent Hospitals NHS Trust

Winter outbreaks of viral gastroenteritis due to small round structured viruses (SRSVs), resulting in ward and hospital closures, are widely reported in the media, (Hartley, 2005). Few areas of the UK remain completely unaffected, as circulation of these viruses is common in the community, with sporadic outbreaks and clusters throughout the year.

However, peak incidence occurs in winter, when community outbreaks tend to be at their height. This has a knock-on effect on the number of cases in hospitals and may cause a major hospital outbreak. Such outbreaks are problematic, causing major disruption to health services. They may result in increased risk of patient mortality among susceptible groups. The Health Protection Agency (HPA, 2005) estimates 600,000 to one million people are affected in the UK each year.

The virus
Noroviruses (genus Norovirus, family Caliciviridae) are a group of related, non-enveloped, single-stranded RNA viruses that cause acute gastroenteritis in humans (Centers for Disease Control and Prevention, 2005). They were named after the identification of a viral strain of gastroenteritis in a school in Norwalk, Ohio, US, in 1968. Norovirus is recognised as the official genus name for this group of viruses but they are commonly referred to as SRSVs as a result of their morphological features. According to the HPA, there are 130-250 outbreaks of SRSV- associated gastroenteritis in England and Wales each year. Peaks in incidence occurred with the emergence of a new strain in 1995-1996 and 2002. There are now at least four norovirus genogroups, divided into at least 20 genetic clusters (CDC, 2005).

Immunity to SRSV is thought to be strain specific and only lasts a few months. Therefore individuals are likely to be repeatedly affected throughout their lifetime.

SRSVs are highly contagious, and the inoculating dose may be only 10-100 virus particles (Caul, 1994). Box 1 details the clinical features of infection.

Transmission
Outbreaks can occur in a variety of settings, including schools, hospitals, nursing/residential homes, hotels and cruise ships. They have frequently been associated with eating cold foods, such as sandwiches and salads, bakery items, liquids (salad dressing and icing) and food contaminated at source, including oysters. Waterborne outbreaks in the community have been related to sewage contamination of wells and recreational water.

Outbreaks within health care settings tend to arise as a result of faecal-oral spread and person-to-person spread, through the widespread dissemination of virus particles generated through vomiting - the predominant feature of SRSV. This can result in environmental and fomite contamination, especially hard surfaces, equipment and furnishings, with the virus viable in the environment for up to 12 days (Cheesebrough et al, 1997). One recent study found contaminated fingers could transfer norovirus to up to seven clean surfaces (Barker et al, 2004).

Presymptomatic excretion of the virus in faeces begins a few hours before the onset of symptoms and can continue for up to two weeks (CDC, 2005). Although identification is often based on clinical features, stool specimens should ideally be obtained within 48-72 hours of onset of symptoms. However, less than 50 per cent of specimens tested will be positive for SRSV by electron microscopy. Although this is the first-line diagnostic method for SRSVs, it is not particularly sensitive, and other diagnostic tools such as polymerase chain reaction (PCR) may be used depending on the laboratory and local arrangements. Sending specimens needs to be discussed with the infection control team, as usually not all the specimens sent are tested.

Management of outbreaks
Staff awareness of SRSVs in general, and the importance of reporting suspected cases among patients/staff to the infection control team promptly, are perhaps the key issues in helping prevent outbreaks. Although there are many causes of diarrhoea and vomiting in patients, staff must take care not to jeopardise the management of patients with conditions where vomiting is a non-specific feature for fear of spreading SRSV. Box 2 details the criteria for suspecting an outbreak is due to SRSV.

The Public Health Laboratory Service Viral Gastroenteritis Working Party published a report on managing hospital outbreaks of gastroenteritis due to SRSVs (Chadwick et al, 2000). It makes a series of recommendations:

- Wards should be closed to prevent the introduction of further susceptible patients;

- Symptomatic patients should be isolated or cohorted, and a stool specimen obtained for microbiology/virology;

- Enteric precautions must be taken, and gloves and aprons must be worn for contact with an affected patient or contaminated equipment. Bedpan washers and macerators should be fully operational;

- Effective hand hygiene remains essential to preventing spread. Although alcohol handrubs/gels have broad-spectrum activity against bacteria and viruses, they are ineffective in the presence of organic material where liquid soap and water are required;

- Any exposed food, such as fruit, should be removed as this can become contaminated and may result in patients becoming symptomatic through ingestion;

- Transfers to other parts of the hospital should be avoided to stop spread to unaffected areas. If transfer is urgent on medical grounds, the infection control team should be contacted for advice and the receiving ward/department notified beforehand so appropriate infection control measures can be taken;

- Discharges to nursing/residential homes should not take place until 72 hours after the last documented cases, unless the patient was affected and has since recovered;

- Non-urgent investigations should be cancelled or postponed. Clinically urgent investigations should go ahead;

- Non-essential staff should not visit the ward. Essential staff should still visit the ward to treat patients but should visit the affected ward last if possible;

- Visiting should be restricted for non-essential visitors or discouraged altogether, especially if they are children;

- Symptomatic members of staff should be excluded from work until asymptomatic for 48 hours. Staff shortages as a result of SRSV are a major resource issue with implications for patient care. Issues around staff movement and use of agency/bank staff require consultation between the infection control team, hospital management and the agency/bank coordinators;

- Staff must be aware of the clinical features of SRSV and report symptoms promptly;

- Vomit and faeces should be cleaned up promptly using 1,000ppm (0.1 per cent) hypochlorite solution;

- Contaminated linen and curtains should be managed according to guidelines for infected linen;

- 0.1 per cent hypochlorite solution should be used for disinfecting all horizontal hard surfaces after cleaning, with particular attention to bathroom fixtures and fittings, including taps and door handles. Vacuum cleaning carpeted areas and buffing floors is not recommended as these may recirculate SRSVs. Soft furnishings and carpets can be steam cleaned if heat resistant. Deep cleaning, including changing curtains, should not start until 72 hours after the last new case, and 72 hours after any vomiting or diarrhoea has contaminated the environment.

Risk management
Major hospital outbreaks of SRSVs are a logistical nightmare. They have a major impact on trust activities, with potential financial implications. Trusts are under increasing pressure to meet government targets in general. Outbreaks of SRSV, particularly in winter when bed occupancy is often at its highest and patient turnover is generally slower, present a major challenge for infection control teams and hospital managers. The recommendation to close a ward or hospital is therefore always controversial. Closing wards affects the rest of the hospital, particularly A&E. Closing a hospital - a decision that must be approved by the strategic health authority - will have resource implications for neighbouring trusts.

All of this must be balanced by assessing the risk to patients of acquiring SRSV infection against the risks of not being admitted and not receiving medical attention because the ward or hospital is closed.

The latest National Audit Office report discusses outbreaks of norovirus in three acute NHS trusts in the same region of the UK between April 2002 and March 2003. Of the 227 unit/ward outbreaks reported, 158 were closed. A total of 2,154 patients were affected. The cost of these outbreaks was £1.97m per year, or £657,000 per trust, in terms of lost bed days and staff sickness. This figure does not take into account other costs incurred such as additional cleaning, bedblocking due to delayed discharge and impact on waiting lists and government targets.

However, closing wards is recognised as an effective way of controlling the duration of an outbreak. In these cases, areas closed within the first three days of an outbreak contained it significantly faster than areas that were not closed or were closed after the fourth day (NA0, 2004). Therefore, although controversial, the recommendation to close wards can limit the duration of an outbreak and expedite a return to normality for patients and staff.

- This article has been double-blind peer-reviewed.

For related articles on this subject and links to relevant websites see www.nursingtimes.net

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