SYDNEY • A virus that caused a spate of vomiting and diarrhoea cases in Asia last year appears to be spreading globally, threatening bigger outbreaks of gastro infections that are a bane of luxury cruises.

The new strain of norovirus, known as GII.17, that emerged in southern China has the potential to spread widely because people probably lack immunity to it, researchers in Japan said last week. That means the highly contagious bug, which is transmitted by infected food and people, and kills about 800 a year in the United States, could sicken hundreds of millions of people worldwide.

Cases of GII.17 were detected in South America, Europe and Africa. There is no specific medicine to treat the infection.

"We know that noroviruses are able to rapidly spread around the globe," scientists from 16 countries wrote in a paper accompanying the Japanese research. "The public health community and surveillance systems need to be prepared in case of a potential increase of norovirus activity in the next seasons."

The virus made headlines last month when gastroenteritis on the cruise ship Balmoral sickened "hundreds" of people in Scandinavia, causing a three-night cruise from England to be cancelled.

Norovirus infections, sometimes called "stomach flu", usually occur in winter. An increase in the frequency of outbreaks on cruise ships over summer can lead to a greater incidence in the community during the next winter, said Professor Marion Koopmans at Erasmus University Medical Centre in the Netherlands last Friday.

Outbreaks on cruise ships are especially noticeable because the contained environment and shared dining areas allow the disease to spread quickly - more than 80 per cent of passengers can be affected, according to the World Health Organisation. But the vast majority of epidemics actually occur on land.

Like flu, new strains emerge as the virus mutates. But while flu typically mutates quickly, new norovirus bugs tend to emerge only every two to four years, often leading to gastroenteritis pandemics. This could mean that a new genotype is emerging, said Prof Koopmans.

Infected food workers are frequently the source of outbreaks, often by touching ready-to-eat food served in restaurants, according to the US Centres for Disease Control and Prevention (CDC).

As few as 18 viral particles on food or hands can make someone sick, which means the amount of norovirus on the head of a pin could infect more than 1,000 people.

The CDC has reported nine outbreaks on cruises already this year among vessels it monitors, compared with eight last year.

"The cruise ships have gone to long lengths to make sure that gastroenteritis outbreaks are contained," said Professor Martyn Kirk at the Australian National University in Canberra.

"They have been quite slick at making sure people stay in their rooms; that ships are disinfected between cruises."

BLOOMBERG