High rate and changing molecular epidemiology pattern of norovirus infections in sporadic cases and outbreaks of gastroenteritis in Hong Kong.

Noroviruses (Norwalk-like viruses (NLV)) are recognised as major causes of acute gastroenteritis worldwide. Numerous studies had been carried out on the molecular epidemiology of norovirus in outbreaks but relatively few on sporadic cases. In this study, the molecular epidemiology of noroviruses in sporadic and outbreak cases of acute gastroenteritis in Hong Kong was examined over a 12-month period from July 2001 to June 2002. Specimens from three groups of patients were used in this study. Nine hundred ninety-five specimens from patients enrolled in the Acute Diarrhoeal Diseases Surveillance Programme of the Department of Health, Hong Kong Government; 735 clinical specimens from hospital patients with acute gastroenteritis, and 122 specimens from 44 norovirus outbreaks. Ninety-two (9.2%) surveillance specimens were positive for norovirus RNA by reverse transcription-polymerase chain reaction (RT-PCR), compared to 123 (16.7%) clinical and 101 (82.8%) outbreak specimens. For the first 6 months of the study period, the predominant strain was the Bristol strain that belongs to genogroup II (GII). In the latter 6 months of the study, genogroup I (GI) and strains belonging to other clusters of GII were seen more commonly. The vast majority of strains belonging to the Bristol virus cluster were closely related to the 95/96-US subset that was associated with pandemic infection from 1995 onwards. This study clearly establishes the importance of norovirus as a cause of sporadic cases of acute gastroenteritis in all age groups in Hong Kong.