Two New Zealand outbreaks of norovirus gastroenteritis linked to commercially farmed oysters.

AIM

We report on the investigations of two gastroenteritis outbreaks, which were linked to a common source.

METHODS

Retrospective cohort studies were conducted for two gastroenteritis outbreaks which occurred in Auckland and in Waikato. Faecal samples and samples of oyster meat were analysed. Environmental surveys of implicated areas were conducted.

RESULTS

10 out of 16 people who had eaten at a catered event in Auckland, and 3 out of 15 people who had eaten at a Waikato restaurant, experienced gastroenteritis. The symptoms, duration of illness and incubation periods were consistent with norovirus gastroenteritis in both outbreaks. The consumption of oysters was strongly associated with an increased risk of illness. Faecal samples were positive for norovirus. Oysters from both outbreaks were traced back to the same growing area. Samples of oyster meat from one of the restaurants and from the growing area were positive for norovirus. The growing area was closed for further investigation. A pipe from a waste water treatment plant was later found to be leaking partially treated effluent into a stream discharging near the implicated growing area.

CONCLUSION

Investigation of these two outbreaks led to the discovery of a common source of norovirus at a commercial oyster growing area.