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WELLINGTON - An outbreak of a rare strain of salmonella is causing concern for health authorities.

The Ministry of Health is working with local and national authorities to determine what has caused the outbreak of salmonella mbandaka, a strain rarely seen in New Zealand.

Nationally there have been 28 cases, 10 of which were in the Nelson Marlborough district.

An elderly woman who had been infected with salmonella mbandaka recently died in Nelson Hospital.

Director of public health, Dr Mark Jacobs, said the ministry was working with the New Zealand Food Safety Authority (NZFSA), the country's District Health Boards and ESR in an effort to determine the cause of the outbreak.

He said there had been a marked increase in notifications of the infection since the beginning of the year.

"However, it's worth noting at this stage, no food premises have been implicated and all food samples tested have returned negative results," Dr Jacobs said.

"While there's not a great difference in the number of cases reported, we don't usually see the mbandaka strain in New Zealand and this is one of the things causing concern. We're working with other authorities to find out where it's come from and to stop its spread as quickly as possible."

Salmonella are bacteria which typically live in the gut of domestic and wild animals, including poultry, pigs, cattle, rodents and pets (such as cats, dogs, turtles and chicks).

Symptoms usually included diarrhoea, stomach cramps, fever, nausea, vomiting and headache.

Common risk factors in New Zealand include contact with farm animals and pets, drinking untreated water and overseas travel during the incubation period.

Contact with an infected person or with recreational water is less commonly reported.

An outbreak with a different strain of salmonella around 2000 was associated with sparrows.

Ways to prevent salmonella transmission and infection include washing hands after using the toilet or before eating or preparing food, keeping raw and prepared foods separate, cooking poultry and meat thoroughly and cleaning food preparation areas immediately.