30 of the cases, caused by salmonella typhimurium, have been recorded in England, with clusters around the Thames Valley, Leeds, Cumbria and Stafford. The remainder have been reported in Conwy and Gwynedd in Wales.
Patients include a seven month old baby and an 87 year-old pensioner and nine of the sick have received hospital treatment. Other potential cases are being investigated.
Experts from Public Health Wales (PHW) have been working with Public Health England (PHE), the Food Standards Agency and environmental health officers to find possible links between the cases.
The outbreak began last month, but since then incident reports have steadily increased. Cooked ham sold in a small number of independent butchers is common to some of the cases, but the suppliers have not been traced.
“The strain of salmonella we are investigating is very unusual so it is highly unlikely that the cases in England and Wales are coincidence,” said Dr Judy Hart, consultant in communicable disease control for PHW, in a statement.
“Testing has been carried out on ham supplied to a number of butchers identified as part of the investigation.”
Hart told FoodManufacture.co.uk the situation was unique because the type of salmonella typhimurium involved was a sub-strain not previously seen in the UK.
“That makes it easy to link cases and be sure they have all got the same infection,” she added.
However, she said other issues made the case complex. “You have got two countries involved, so that’s quite a complicated investigation.”
No longer in the food chain
Another problem, she stressed, was that the outbreak had been identified some time after the initial infection had occurred. That meant contaminated products were probably no longer in the food chain and all investigators had to go on were the experiences and behaviour of those affected.
“The difficulty is that the food has been eaten, finished and gone, so the investigation is looking for epidemiological, rather than microbiological links.”
In a statement, Dr Bob Adak, head of the gastrointestinal diseases department at PHE, said: “Investigating outbreaks of foodborne illness is a complex process as people have to try and remember what they ate some weeks prior to becoming unwell. In this instance many reported eating cooked meats.”
No trace of salmonella has been found, but other hygiene issues were identified that led to one supplier voluntarily withdrawing certain batches of ham.
Given the spread of the outbreak, Hart said it could involve a company capable of UK-wide meat distribution. However, the involvement of a major retail chain was unlikely, as this would have affected many more people, she said.
‘Haven’t ruled out anything’
“It may be someone who has been supplying the meat across the UK. We are not thinking in terms of massive supermarket outlets, although we haven’t ruled out anything,” she told FoodManufacture.co.uk.
In her statement, she said: "A number of lines of enquiry are still being investigated and we continue to monitor the situation.
“Although it is an unpleasant illness, people who become ill with salmonella generally make a full recovery."
The outbreak began in mid July, with public health officials confirming it was not concentrated on one particular town or locality. Salmonella typhimurium is not generally fatal, but can be for the extremely young, old or infirm.
The latest advice on preventing a food poisoning outbreak and how to manage a product recall will be just two of the subjects under discussion at Food Manufacture's Food Safety Conference to be staged at the National Motorcycle Museum near Birmingham on Thursday October 17 2013.
More details of the conference programme and ticket reservations are available here.