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Food Focus February 2008

Norovirus outbreak scuppered

Early intervention through hand-washing and isolation controlled a potential norovirus outbreak at a scout Jamboree. This was remarkable especially in a ‘tent city’ with over 7000 participants

When scouts arrived at a jamboree early in the New Year, some showed symptoms of vomiting and diarrhoea. Immediate steps were taken to implement hand-washing and sanitisation actions between tents and before touching food. Patients were kept isolated and in some cases their travel home was delayed to prevent the illness spreading to fellow passengers. This early action meant only 55 people succumbed to the disease.

Although not started by food in this case, norovirus is highly contagious through person-to-person contact – airborne particles and dirty hands can quickly contaminate food and surfaces. It is probably the leading cause of diarrhoea and vomiting in this country because it spreads so easily on to food and surfaces. Outbreaks are prevalent in institutes where people live in close confinement, eg, rest homes or hospitals. Contaminated salads, water, ice, bakery items and berries have all caused outbreaks.

Any food can become contaminated from infected food handlers or contaminated water. A relatively common source of norovirus is from shellfish harvested from sewage-contaminated water. Filter feeders, such as oysters, collect small food particles, including pathogens. Because shellfish are often eaten uncooked or lightly steamed, the virus may not be destroyed and can infect people.

The virus survives freezing, drying and pasteurisation. However, cooking foods such as shellfish at high temperatures (90°C for 90 seconds) reduces the risk of becoming infected.

Sudden onset – prolonged infection

Symptoms of norovirus infection can begin quite suddenly, within 12 hours of exposure. Symptoms include nausea, violent vomiting, diarrhoea and stomach cramps, and victims may also have low-grade fever, chills, headache, muscle aches and general tiredness.

Patients usually recover within 48 hours without needing medical treatment. However, they may still be infectious and shed virus particles for prolonged periods of time. Therefore food-handling businesses and care-providers must have strict staff-sickness policies that require staff who are ill (or have been within 48 hours) to stay away from work for at least 48 hours after their symptoms have cleared.

How to avoid it

Wash your hands frequently. Wash your hands especially after going to the toilet, changing nappies or before food preparation or eating. Most viruses and other pathogens are washed off your hands with proper washing. Follow the 20+20 rule – lather your hands with soap then wash for 20 seconds with warm water, then dry your hands for 20 seconds with a dry, clean towel or paper towel.

Don’t prepare food if you’re showing symptoms of a norovirus infection, or have been sick within 48 hours. You can be contagious for prolonged periods after the symptoms have disappeared. It’s particularly important to practise scrupulous hand hygiene at these times.

Make sure your own hygiene is meticulous if you are caring for someone who is sick. Keep the toilet area and all bathroom surfaces (toilet, taps, handles etc) really clean with household disinfectant or bleach. These are significant sources of infection.

Remove and wash any furnishings, including bedding, that may be contaminated, using soap and hot water (if possible). Viral particles can survive for at least 12 days on some surfaces. (Workmen at a rest home whose occupants had suffered a bout of norovirus fell ill with the disease themselves. Viral particles were subsequently discovered in the carpet.)

Wash all fruits and vegetables with potable (drinking) water.

Collect shellfish from unpolluted water.

See the fact sheet Collecting shellfish and keeping it safe.

Improved detection and reporting

NZFSA has commissioned various research projects to improve detection and reporting of norovirus

ESR has developed better detection techniques for norovirus, and food samples implicated in an outbreak can now be tested and confirmed within 8 hours. Routine contamination tests in shellfish beds conducted by councils and public health boards can now include Norovirus.

Correlations between pollution sources and microbiological risks from recreational shellfish collection sites in Tauranga harbour are being investigated in a project co-funded by NZFSA. Information from this project should improve the ability to predict the risk of illness occurring in these areas.

Other research will look at food-virus combinations to see if particular foods have a high risk of viral contamination. This will provide guidance for future risk assessment work on foodborne viruses in New Zealand.

A new standard was recently introduced requiring imported shellfish to meet similar sanitary conditions to the New Zealand product. These tighter rules are to help reduce outbreaks of foodborne illness from shellfish.