Human norovirus has been detected in mussels on two reefs off Waitara, prompting a public advisory on shellfish collection.
Of four samples taken from shellfish on the Tuaranga and Airedale reefs, the virus was found in low concentrations in three and moderate concentrations in one. A further eight samples were taken from the nearby Epiha Reef and on water in the Waitara River. On these occasions, human norovirus was not found.
The testing was conducted by the New Plymouth District Council (NPDC) as part of its application for a resource consent to discharge treated effluent from the Waitara Wastewater Treatment Plant. It was requested by the Taranaki Regional Council (TRC) to ensure it has comprehensive baseline information on water quality.
TDHB Medical Officer of Health Dr Greg Simmons says there is a potential risk of illness including nausea, vomiting and diarrhoea if contaminated shellfish are eaten. On his advice, beach signs will be erected advising the public of the hazard.
"We are not aware of any outbreaks of gastric illness related to this virus so this advisory is a precautionary measure," says Dr Simmons.
Dr Simmons adds that testing for viruses in shellfish on reefs is rare in New Zealand. "A small number of councils have conducted this testing. Often, when there has been testing, the human norovirus has been found, so these results are not surprising and should not be a cause for great alarm."
Gary Bedford, TRC Director-Environment Quality says the virus has been found in predominantly low concentrations.
"Not only is viral monitoring of reefs rare, but interpretation is complex, with no national standards to refer to," says Mr Bedford.
The TDHB advisory applies to the consumption of shellfish only. There are no concerns regarding bathing water quality.
"Faecal bacteria monitoring is regularly conducted at Waitara and analysed according to long-established national protocols and criteria," says Mr Bedford. "The results consistently confirm that Waitara is a generally safe place for water-based recreation, as safe as most coastal settlement areas in New Zealand."
He says the source of the virus is unknown. "We don't know if it's from malfunctioning septic tanks, illegal discharges from leisure craft, discharges from further up the Waitara River, the Waitara Wastewater Treatment Plant outfall, or an overflow from the town's sewers, as occurred on 24 June this year.
"We also don't know if this is a periodic contamination or a continual one, as these samples were taken two weeks apart."
In its environmental health role, NPDC will erect the signs and conduct further testing of the affected reefs along with a sanitary survey of residential septic tanks.