Medical practice

New strain blamed for record norovirus outbreak

The emergence of a new Australian strain of norovirus – the 'winter vomiting bug' – has featured in most newspapers today. The Daily Telegraph reports that the new strain is to blame for the record number of cases of the virus seen this winter, which it terms a 'severe norovirus season'.

Experts have been quick to point out that the new strain of the virus does not cause more serious illness than any other strain. The strain is called Sydney 2012, prompting the Daily Mirror to headline its coverage ‘Down chunder’.

Norovirus is contagious and its spread can be prevented by washing hands (particularly after using the toilet and before preparing food) and disinfecting surfaces. Although norovirus is extremely unpleasant, most people make a full recovery in a few days.

What is the norovirus?

Noroviruses are the most common cause of stomach bugs (gastroenteritis) in the UK. Norovirus can affect people of all ages and is more common in winter, although it can be caught at any time of year. It is highly contagious and causes vomiting and diarrhoea.

Between 600,000 and 1 million people in the UK catch norovirus every year, although only a fraction of cases are confirmed by laboratory tests.

Experts believe that during the 2012–13 period there have been more cases than usual. The latest figures show there have been 4,140 laboratory confirmed cases this season, a figure that is 63% higher than the number reported at this point last year. Estimates suggest that this means that, in total, there have been well over a million cases of norovirus in the 2012–13 outbreak.

What have officials said about this new norovirus strain?

As part of its monitoring of norovirus, the Health Protection Agency carries out genetic testing for different strains of the virus. The HPA says that tests carried out when cases started to rise last October revealed a ‘cocktail’ of different strains circulating, including Sydney 2012 and another called New Orleans 2009, but no single strain dominated.

The latest testing of the most recent outbreaks, completed this week, has now shown that Sydney 2012 has overtaken all others to become the dominant strain. Sydney 2012 was first seen in Sydney, Australia (hence its name) and has also been found in France, New Zealand and Japan.

What is the evidence that this Sydney norovirus strain is a new problem?

The HPA’s announcement is based on a scientific paper, published in the medical journal Eurosurveillance this month, which describes the new strain as a variant of the norovirus genotype GII.4.

The paper suggests that the emergence of Sydney 2012 may be responsible for the increase in cases of the virus compared with previous seasons in a number of different countries. This implies that people may have less immunity against this new viral strain. However, the report says more data is needed to confirm this.

The paper also says that in the past decade, strains belonging to genotype GII.4 have been responsible for the majority of cases of acute gastroenteritis. It has been suggested that hospitalisation and deaths occur more frequently during peak seasons associated with new norovirus GII.4 variants. Since 1995, new epidemic variants of GII.4 have emerged every two to three years, says the paper.

It also says the emergence of new variants has been associated with an increase in norovirus cases early in the season.

How do new strains of virus emerge? 

To replicate, noroviruses attach and enter the living cells of an organism. However, over time the host organism builds up immunity to the virus. The key to viruses’ success (in evolutionary terms) is that even though they are quickly eliminated by animal immune systems, they replicate so quickly that genetic mutations soon emerge in subsequent generations of the virus.

Mutations that are beneficial to the viruses (that is, those that help the virus avoid the immune system) will survive among the population while those viruses without these beneficial mutations tend to be killed off by the immune system before they can reproduce. This leads to the emergence of new strains that most immune systems are not prepared to fight.

The best example of this is the different strains of flu viruses that have emerged in recent years, such as swine flu, bird flu and the various types of seasonal flu.

Noroviruses mutate rapidly, says the HPA, and new strains are constantly emerging. At the start of the season it is normal for outbreaks to be caused by a range of different strains. However, as the season progresses particular strains are more successful and become dominant.

Is the new strain more dangerous?

Dr David Brown, director of the Virology Reference Department at the HPA says that the emergence of a new strain does not mean that it causes more serious illness.

He said: “There is no specific treatment for norovirus infection other than to let the illness take its course, with symptoms usually lasting around two days. Keeping hydrated (by drinking water regularly) is very important and you can take over-the-counter medicines to relieve headaches and aches and pains”.

For outbreaks and especially those occurring in hospitals, nursing homes, schools and childcare facilities, careful disinfection and isolation policies are important as the virus can survive for several days on surfaces or objects touched by an infected person.

Read the key ways to stop yourself and other people being infected with norovirus.  

Norovirus FAQs

What is norovirus?

Noroviruses are Britain’s most common cause of infectious gastroenteritis, also known as 'winter vomiting disease' or 'stomach flu'. Although not usually dangerous – the vast majority of sufferers recover after one or two days – the very young and the elderly are at risk of complications, such as dehydration, and may need hospital treatment.

It’s estimated that, typically, between 600,000 and 1 million people suffer from norovirus every year. That makes the infection – caused by one of a number of closely related viruses – the most common stomach infection in the UK.

What are the symptoms?

Symptoms of norovirus typically begin between 24 and 48 hours after infection with the virus. Sudden onset of nausea is usually the first sign of infection, followed by vomiting and watery diarrhoea. Some may also experience a mild fever, aching limbs and headaches. Symptoms typically disappear after a day or two.

How do people catch norovirus?

Through contact with an infected person, as well as contact with surfaces – such as door handles and tables – that are contaminated with the virus. It is also caught by consuming contaminated food or liquid. This means that outbreaks are particularly common within contained environments such as hospitals, schools and offices.

Once you have caught norovirus you are immune to the illness for around 14 weeks. After this time it is possible to be reinfected with the virus and suffer the same symptoms.

What can I do to stop myself catching norovirus?

It is not possible to guarantee that you will not catch norovirus. But good hygiene will lower your risk of catching or spreading norovirus. Wash your hands frequently, particularly after going to the toilet, and before eating or preparing food. Avoiding raw, unwashed foods during a norovirus outbreak can also lower your risk of infection.

What should I do if I have norovirus?

There is no specific treatment for norovirus illness, and you will have to let the illness run its course. Stay at home and drink plenty of fluids to avoid dehydration. That means regular sips of water or fruit juice, even if you are feeling sick. Adults can take rehydration drinks and anti-diarrhoea medicines available from pharmacies. Anti-diarrhoea medicines are not suitable for children.

To avoid infecting other people, wash your hands regularly. Stay at home for 48 hours after the last sign of symptoms, and do not prepare food for others for three days after the last sign of symptoms.

The vast majority of those infected make a full recovery within two days. But particular care must be taken with the very young and older people who catch norovirus, as they are at higher risk of dehydration.

Need help now?

If you’re suffering symptoms of norovirus, you can get advice now from NHS Direct 0845 4647.

NHS Attribution