Sewage spill linked to bad oysters

The norovirus outbreak has been linked to oysters contaminated by a sewage spill in the Coromandel area. Photo / Thinkstock

Thirteen people fell ill with norovirus stomach infection after eating raw or cooked oysters linked to a sewage spill in the Coromandel area.

Norovirus can cause nausea, vomiting, diarrhoea and fever.

In 2009 public health authorities traced the source of two outbreaks, in Auckland and Waikato, back to the Coromandel, according to today's New Zealand Medical Journal.

Ten people were infected at a catered event in Auckland and three at a Cambridge restaurant. Four more at the Auckland event ate oysters but did not fall ill. Neither venue nor the oyster farm are named in the journal report.

In Cambridge, two of the unlucky diners ate their oysters raw while the third consumed cooked oyster Kilpatrick but complained the shellfish was undercooked and sent it back for re-cooking.

The Food Safety Authority closed the growing area where the oysters came from in late July 2009 following the Auckland outbreak but eight days before the Cambridge diners had their contaminated meal.

The journal report says the leaking sewer was found only by chance. In early August 2009 the Thames Coromandel District Council reported the sewer had been disturbed during maintenance of the wastewater treatment plant near the oyster growing area.

"The pipe had been leaking partially treated effluent into the stream that flowed into the affected growing area," says the report by public health doctor Richard Wall and colleagues.

"It was thought that over a five-week period, including the harvesting period of the implicated oysters, a substantial amount of partially treated effluent had contaminated the growing area."

No other likely sources of contamination were found along the shoreline.

Stools from people with norovirus can contain more than a million viral particles per millilitre so one stool is enough to contaminate an oyster bed.

All oysters harvested during the five-week period were traced and all frozen product recalled, including a frozen shipment bound for China.

Further non-frozen oysters were "assumed to have been consumed ... We are not aware of any other cases relating to this same source".

Later water samples from the marine farm were negative for norovirus, allowing the area to be re-opened for harvesting.

Dr Wall and colleagues say temperatures above 60C deactivate norovirus, although cooking oysters has not been shown to reliably inactivate viruses.

In 2006 imported Korean oysters were blamed for five outbreaks of the disease. One of these was at Eden Park in which it was estimated more than 300 corporate guests at an All Blacks-Ireland test were poisoned after eating the raw oysters.