The exact source of a listeria outbreak that led to the deaths of two New Zealand women will be hard to pinpoint, a food safety expert said.
Four cases of the notifiable disease have occurred in hospital patients here since May, two of whom - both women aged in their 60s and 80s with compromised immunity - died.
Traces of listeria were found in ready-to-eat meat supplied to Hawke's Bay Hospital. The supplier of the products, Napier company Bay Cuisine, issued a voluntary recall notice for products they said may contain traces of the bacteria - salami, pepperoni, and ham products were among them.
The Ministry of Primary Industries was investigating the outbreak, and a spokesman said investigations were continuing to identify the source.
University of Canterbury toxicology, Prof Ian Shaw said the source of contamination would be hard to identify.
"There have been a few outbreaks of listeria in New Zealand over the last ten years that I have been here. It doesn't mean that the regulatory authorities are bad, it means that listeria is a clever bacteria that gets around regulations very easily."
Listeria can grow in both high and low temperatures, so even refrigeration does not kill the bacteria.
Shaw said: "The only way you can really stop listeria from spreading is from stopping the initial contamination."
• University of Canterbury
• Bay Cuisine
• Ministry of Primary Industries