The Ministry for Primary Industries has released reports that show a high proportion of people suffering a painful food poisoning illness were exposed to lettuce and carrots.
Environmental and Scientific Research (ESR) provided two reports to the Ministry for Primary Industries (MPI) last week updating the possible source of the stomach bug.
The study included 96 cases and 112 controls.
"We found multiple food exposures were associated with disease. Three of these food exposures occurred in over 50 per cent of cases," one of the reports said.
Those foods were:
• lettuce - 91 per cent of cases;
• lettuce from supermarket or shop - 90 per cent of cases; and
• carrots from supermarket or shop - 72 per cent of cases.
There have been 127 confirmed cases of yersinia pseudotuberculosis and 38 people have been taken to hospital because of the bug.
The report said the magnitude of the association with disease in people who ate carrots and lettuce was greater than in those who that ate only lettuce.
However, the report said the information was not definitive and should lead to investigation of possible contamination of both lettuce and carrots at some point between growth and consumption.
There were a number of possible explanation that could relate to the high infection rate, it said.
"Both products may be contaminated, however the greater surface area of lettuce may make it a more effective vehicle of infection given the high infectious dose.
"Or, if people that eat both lettuce and carrots eat more lettuce than people that eat only lettuce then they may receive a higher dose and explain the higher magnitude of association."
Some specific brands, types of lettuce and carrots were significantly more common in cases than controls, the report said.
For lettuce in descending order of magnitude of association:
• Pam's Fresh Express mesclun salad lettuce ready to serve (Auckland);
• Pam's Fresh Express;
• Lettuce from supermarket or shop;
• Whole bagged;
• source New World;
• source Pak n Save; and
• loose leaf bagged.
And for carrots:
• bagged whole;
• from supermarket or shop;
• source New World; and
• free flow whole.
The ministry had earlier refused to release the list to the public, but on Monday Foodstuffs confirmed two of its products - Pams Fresh Mesclun Salad Lettuce and Pams Fresh Express Lettuce - were named in the ESR reports.
Neither of those products were still on the shelves.
MPI deputy director general regulation and assurance Scott Gallacher said the ministry was continuing to seek to identify the origins of the outbreak, drawing upon the best available expertise and information, both domestically and internationally.
"The health data is telling us we are dealing with a contamination event that happened in August. This was identified as an outbreak of illness on 23 September.
"According to the latest health data, there have been no further confirmed cases since the first week of October," he said.
Mr Gallacher said the incomplete and preliminary information that was released by Foodstuffs to the media last week, fuelled speculation.
Produce industry United Fresh New Zealand said it was providing advice and support to its 84 members as authorities worked to identify the source of this illness.
"We are also working closely with MPI to understand the level of risk associated with the spread of the bug," the organisation's knowledge officer Hans Maurer said.
"It is our understanding the source may never be confirmed. It is important to note the MPI's investigation is over a range of foods, and not isolated to fresh fruit and vegetables and the early speculation has been somewhat unhelpful."
Dr Maurer questioned the effectiveness of the reports given they did not conclusively answer where the bug had originated from.
"Major food suppliers are being checked by MPI to ensure the food handling practices are meeting the required standard. We are confident the systems and processes used by our members meet these standards," Dr Maurer said.
"While the investigation has not yet identified the source, it is important to stop the speculation as to what foods are being investigated, which is why we are releasing the case-control studies from ESR today," Mr Gallacher said.
Mr Gallacher said the case-control studies from ESR identified a range of foods that needed further investigation to establish if they were the source of the illness.
Horticulture New Zealand said it was not surprised the ESR reports did not come to a conclusion over the origins of the illness.
Only one outbreak of yersinia either in New Zealand or overseas had ever been definitively traced back to fresh produce - in Finland in 1984, chief executive Peter Silcock said.
The vast majority of previous cases had been linked to contact with animals, he said.
"Now that the outbreak is over, it is becoming even more obvious the source may never be identified."
Foodstuffs said it had been working with MPI and running a parallel investigation to determine the possible source of the Yersinia contamination, and whether there was any link to its products.
"It is clear from all of the reports to date, that a definitive answer to these questions is still very uncertain," corporate public relations director Antoinette Shallue said.
"We do however now believe, based on the timeline of events and the scientific reports provided by the ESR, that the most likely source of the contamination is farm based.
"In exploring our own supply chain, should the contamination have come from Pam's Lettuce, we can confirm we have been able to narrow our investigation down to a single paddock from the supplier, Living Foods."
The paddock had been quarantined at Foodstuffs request and further independent testing had been commissioned alongside the MPI investigation, Ms Shallue said.
The results of the company's initial supplier audit review were expected early next week and sample analysis results from a certified independent laboratory were expected towards the end of the month.
Foodstuffs' investigation excluded Pam's bagged carrots as a likely source of contamination because no one supplier of Pam's was responsible for distribution to both the North and South islands where sickness had been reported, she said.