The Health Protection Agency (HPA) can confirm that findings of an investigation into an outbreak of Cryptosporidium infection that affected around 300 people in England and Scotland in May 2012 showed strong evidence of an association with eating pre-cut bagged salad products which are likely to have been labelled as ‘ready-to-eat’.

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The outbreak was short lived and the numbers of cases returned to expected seasonal levels within a month of the first cases being reported. Most of those affected had a mild to moderate form of illness and there were no deaths associated with the outbreak.

The HPA conducted an extensive investigation which involved interviewing people who became unwell about their food history and shopping habits and comparing these with a similar number of people who were not unwell.

In the analysis of the exposure to different salad vegetables (irrespective of retailer) a significant statistical association was found between infection and the consumption of pre-cut spinach. When specific retailers were included in the analysis, the strongest association with infection was found to be with consumption of ready to eat pre-cut mixed salad leaves from a major supermarket chain. In this analysis, exposure to pre-cut spinach only reached conventional levels of significance for one retailer – a second major supermarket chain. A link to spinach from a number of other retailers was also suggested but these were not statistically significant. Together these findings suggest that one or more types of salad vegetables could have been contaminated.

The Food Standards Agency (FSA), who were part of the outbreak control team led by the HPA, gathered information on the production and distribution of salad vegetables to try to identify the likely source of the outbreak. Investigation of the food chain including practice and procedures throughout each stage of growing, processing, packing and distribution of salad vegetables has not identified a source of contamination. Bagged salad on sale in supermarkets is often sourced from the same suppliers for most leaf types, often with common production lines packing product for several retailers at the same time.  This was the situation in this case.

Dr Stephen Morton, regional director of the HPA’s Yorkshire and the Humber region and head of the multi-agency Outbreak Control Team, said: “This outbreak was fortunately short lived but it was important to see if we could find the source. Our findings suggest that eating mixed leaf bagged salad was the most likely cause of illness.

“It is however often difficult to identify the source of short lived outbreaks of this type as by the time that the outbreak can be investigated, the affected food and much of the microbiological evidence may no longer be available.

“As this was an isolated and short lived outbreak there is no specific action for the public to take but we hope the investigations between the FSA and the food industry will help to prevent further outbreaks of this type from happening again.”

Dr Alison Gleadle, director of food safety at the FSA, said: “We’d like to remind everyone of our usual advice to wash all fruits and vegetables, including salad, before you eat them, unless they are labelled ‘ready-to-eat’. It’s also important to wash hands thoroughly as well as clean chopping boards, knives and other utensils between preparing raw and ready-to-eat foods to prevent cross contamination.”

The Cryptosporidium parasite causes the disease cryptosporidiosis. The most common symptom is diarrhoea, which can range from mild to severe. There are a number of potential sources, including consumption of contaminated water or food, swimming in contaminated water or through contact with contaminated food or affected animals.

Further Information

  1. In total, 362 laboratory confirmed cases of Cryptosporidium were recorded in England in people aged 16 and over during the outbreak period of 14 May – 3 June. Around 300 of these cases were thought to relate to the outbreak – the remaining relate to the normal number of background cases you would expect to see at this time of year. 210 cases reported across four regions – North East, Yorkshire and Humber, East Midlands and West Midlands with 190 of these cases thought to be outbreak related. By way of comparison, during the same period in the previous seven years (14 May – 3 Jun), an average of 54 cases were reported across England including an average of 20 cases across the four regions affected.
  2. A summary of the investigation’s findings has been published here.
  3. More information on cryptosporidiosis can be found on the HPA website here.