As mentioned in section 2.2, each year in Australia there are an estimated 1.8 million cases of norovirus infection, making it the most common cause of gastroenteritis .
4.1 Reported outbreaksIn Australia, norovirus outbreaks caused by person-to-person transmission have been reported in a variety of settings. Institutional outbreaks are by far the most commonly reported and have been described in childcare centres , youth refuges, school outings and camps , restaurants , ACF and hospitals [69-73]. Outbreaks have also been reported in other settings. From 1999–2003, three confirmed norovirus outbreaks were reported on cruise ships visiting Sydney .
The number of norovirus outbreaks reported to OzFoodNet varied from 19–772 per year from 2000–08. In total, there were 2923 norovirus outbreaks affecting 86570 people during this nine year period. States and territories reported that 75% (2196/2923) of outbreaks of norovirus were spread from person-to-person and 22% (649/2923) were of unknown mode of transmission. Only 2.7% (78/2923) were foodborne or suspected foodborne transmission. Norovirus was confirmed as the cause of 57.4% (2196/3824) of outbreaks spread from person-to-person. In total, most suspected person-to-person outbreaks occurred in ACF (69%), hospitals (19%), childcare centres (3%) and other institutions (3%). Due to the mild self-limiting nature of norovirus gastroenteritis, these figures are a significant underestimation of the true burden of norovirus infections in Australia.
The first reported norovirus foodborne outbreak associated with the consumption of locally produced oysters was in 1978 [13, 14]. Consumption of raw oysters as the likely vehicle of transmission in subsequent norovirus outbreaks has been reported in the Northern Territory (NT) , Queensland (Qld) and New South Wales (NSW) . Orange juice was reported as the vehicle for norovirus transmission affecting approximately 25,000 passengers on domestic airline flights in 1991 (Pers. Comm. T Stewart, 2007). Norovirus was detected in the orange juice well after the conclusion of the outbreak. The outbreak ceased when the juice was withdrawn from sale. Public health investigation identified some potential sources of contamination at the processing plant where contamination could have occurred .
Between 2002 and 2004, consumption of raw and cooked imported oyster meat from Japan was associated with norovirus illness in outbreaks in the NT, Western Australia (WA) and Qld . In the 2003 gastroenteritis outbreaks that occurred in WA and the NT, norovirus was detected in clinical specimens in two of the outbreaks and suspected as the cause of the third. At the time of the investigations, epidemiological and trace-back evidence implicated the oysters as the likely source of infection even though virological testing did not detect norovirus. However, oyster meat from the NT outbreak was tested the following year and was positive for norovirus. At the same time as the outbreaks in 2003, New Zealand (NZ) reported three outbreaks of confirmed norovirus associated with imported oyster meat from Korea, which was followed by a further large outbreak in 2006 . The outbreaks resulted in the Australian Quarantine and Inspection Service restricting imports of oysters from certain growing areas in Japan and Korea . Oyster-associated outbreaks of norovirus from oysters grown in Australia, particularly NSW, continue to occur, although infrequently [79, 80].
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A large outbreak of norovirus in 2003 was associated with ready-made foods prepared by a catering company. The investigation suggested that contamination of food had occurred by ill food handlers . Similarly, salmon- and egg-filled sandwiches and bakery items were implicated in two norovirus outbreaks that were related to preparation of food that required considerable handling. Food handlers with gastroenteritis worked while symptomatic in both outbreaks . Norovirus as the causative agent in waterborne outbreaks is uncommon in Australia. There has been one report described that suggests norovirus was the cause of an outbreak that was linked to drinking contaminated water at a caravan park in NSW .
Figure 2: Number of outbreaks due to norovirus spread by different modes of transmission in Australia, by month of onset of first case, 2000–08
Source: OzFoodNet (unpublished) March 2010
4.2 Strains implicated in recent outbreaksThe predominant strain circulating in Australia has been GII. Outbreaks in south-eastern Australia from 1980–96 reported the GII strain. Conversely, a study of norovirus in central Australia from 1995–97 identified a predominance of GI strains .
A new variant of the GII strain, GII.4, was associated with a sudden increase in norovirus outbreaks in Europe and the US, which has since spread globally. A GII.4 variant designated US95/96, which caused global epidemics in 1995 and 1996 was later identified in Australia. This strain was associated with a large number of outbreaks reported in NSW in 1997 and 2000. Similarly, in 2002 an increase in norovirus outbreaks was reported across Europe, the US and Australia. The predominant strain was Farmington Hills and the majority of strains collected in NSW between 2001 and 2002 were related to the Farmington Hills cluster. A norovirus GII strain termed ‘Hunter’ was first identified in NSW in 2004 and caused significant outbreaks in 2004 and 2005 .
The variant GII.4 has been responsible for large outbreaks, usually person-to-person spread, in Australia. The sudden increase in outbreaks in 2004 and 2006–7, mainly spread from person-to-person, occurred in ACF and hospital settings [4, 6]. Genotyping showed that the outbreaks were associated with the emergence of new variant strains, in particular GII.4 strain that had spread worldwide [6, 7, 83]. A study by Bull et al. demonstrated that GII strains isolated in Australia mirror the strains identified in the Northern Hemisphere during similar time periods .