A number of frozen berry brands have been linked to Hepatitis A cases but warning consumers which ones won't help, the Ministry for Primary Industries (MPI) says.

Four cases of people infected with the rare virus have been identified and linked to imported frozen berries - two in Auckland and two elsewhere in the North Island.

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All cases involved hospital stays for the infected people.

The ministry issued a formal warning yesterday about the potential risks of eating frozen imported strawberries, raspberries, blackberries, blueberries and boysenberries.

But it has not said where it thinks the berries came from.

Labour primary industries spokesman Damien O'Connor said the products should not be kept secret.

"The Ministry for Primary Industries must publicly name the affected products and recall them.

"It is not good enough to just warn people to boil the berries before eating them. Thousands of people every day around the country are drinking smoothies full of berries," he said.

But MPI director of plants, food and environment Peter Thomson told Checkpoint it would not warn people what brands it thought could be contaminated.

"When we interview the cases, we're asking them to recall what they ate up to 50 days prior to them getting ill, so you can imagine that's quite challenging for them.

"There are some brands that have been identified but there's no one brand that is standing out as the answer.

"In terms of our primary aim of consumer safety, we want consumers to be aware that all imported frozen product could present a risk."

Mr Thomson said people might be put at risk if MPI released a list of brands that was too narrow and it would "not be helpful at this time" to say what the brands so far identified were.

He said both the Ministry of Health and MPI had been investigating the four Hepatitis A cases over the past week before it established the link with frozen imported berries.

Mr Thomson said interviews with the infected people, and lab tests to establish a similar strain of the virus, meant there was "sufficient belief" that the berries were the cause of the illnesses.

Berries could cause more Hep A cases

An Auckland medical officer of health, Richard Hoskins, said there could yet be more cases of the illness.

"If there are frozen berries that are out there that are still contaminated - and if people don't follow the advice - then we could get new cases.

"It's got an average incubation period - the time from when you eat it to the time you might start to get sick - of about four weeks.

"So there could well be people out there now who will become sick in the next three or four weeks from something they ate a week or so ago," he said.

Dr Hoskins said there had previously been a fairly large Hepatitis A outbreak involving blueberries in New Zealand, but that was about a decade ago.

The elderly and people with chronic liver damage have been told to avoid frozen berries while the ministry sets up a "high priority" testing programme.

The ABC reported in February that an outbreak in Australia at the time was linked with poor hygiene among workers in China, as well as potentially contaminated water supplies in that country.

About 20 cases a year are normally reported to the Auckland Regional Public Health Service each year.

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Cafes, companies respond

Restaurant Association chief executive Marisa Bidois said some cafes had been calling asking for advice today, and the advisory from MPI meant imported frozen berries should not be used in popular smoothies.

"The people that we've spoken to were certainly concerned this morning hearing that sort of news because often people will start their day with a smoothie... our industry has been on alert.

"It does make it difficult not knowing exactly where the products are from and which ones to isolate, there is a little bit of confusion around that," she said.

Nelson-based frozen berry producer Sujon said it had not been able to get any information from MPI.

Managing director John Gibb said the ministry had taken "drastic" measures in making the announcement the way it had, with limited information.

"I'd be horrified if they had information about a type [of berry] or a country that it's likely to be coming from and they weren't letting those of us know. They've just thrown the blanket out wide over everybody.

"If the evidence is limited, why have they scared everybody by saying you should treat all imported berries all of a sudden as if they should be harmful?"

Mr Gibb said he had tried contacting MPI today without success.

Supermarket chain Countdown has not been implicated but the formal warning from MPI has made it seek assurances from its frozen berry suppliers.

It said it had been in contact with three suppliers, and those suppliers had given assurances that they undertook regular chemical and microbiological testing.

The chain's suppliers said they had not received any recent complaints or reports of illness relating to any of the products sold at Countdown.

Countdown's Select and Signature Range berries are sourced from New Zealand, Chile, Canada, the US, Argentina and parts of the European Union.

What is Hepatitis A?

Elderly persons and those with chronic liver damage should avoid frozen berries that have not been heat treated.

MPI said bringing frozen berries briefly to the boil would deal with any potential risk, as would cooking for 85 degrees Celsius for at least one minute.

It said washing frozen berries would not remove the risk.

Hepatitis A, an infectious liver disease caused by a virus, was rare in this country and was normally spread by contact with the waste of an infected person.

Symptoms to watch for are: