An on-going outbreak of listeriosis in the USA and Canada has hospitalised 19 people since last July and may also have contributed to two deaths. The evidence points to packaged salad products produced at a single facility in Ohio as the likely source of infection. That facility, operated by Dole Fresh Vegetables Inc., has suspended operations and a large number of salads have been withdrawn from the market while an investigation continues.
This is a serious foodborne disease outbreak by any standards, but is just the latest in a long line related to salad vegetables going back many years. For example, in 2000, two Salmonella Typhimurium outbreaks associated with iceberg lettuce caused more than 500 cases of illness in five European countries. In 2006, more than 200 people were infected by E. coli O157:H7 after eating contaminated packaged baby spinach, and a large outbreak of Cryptosporidium infection in the UK in 2012 was traced to bagged salads. Even so, fresh produce remains a relatively uncommon vehicle for foodborne disease, although evidence from the USA in particular suggests that the number of outbreaks linked to fruit and vegetables is growing. Unfortunately, salad-related outbreaks can have a disproportionately large public health impact, despite their comparative rarity. They often occur over a wider geographical area and are more prolonged and more difficult to identify than those caused by other food groups. Recent figures from CDC in the USA reveal that among 34 multistate Shiga toxin-producing E. coli (STEC) outbreaks between 2010 and 2014, more than 40% were associated with leafy greens and similar vegetable products.
But why should salads be even an occasional vehicle of foodborne disease? The problem is that leafy greens are vulnerable to contamination all along the production and supply chain. For example, the 2006 STEC outbreak in the USA was probably caused by contamination in the field from complex interactions between animals and irrigation water. Contamination can also happen during harvest and transport and during washing and packing. Listeria contamination is most likely to occur at packing facilities, where the cool wet environment is ideal for the bacteria to grow and become established in biofilms on surfaces. Once contamination of plants has occurred it is very hard to get rid of. Washing – even with chlorinated water and other chemicals – has been shown to have a limited effect and there are few other practical options. Safety is best assured by Good Agricultural Practice at the growing stage and by good hygiene during transport and packing to prevent contamination. This works very well most of the time, but cannot guarantee safety all of the time. Any crop growing out in the field is at some risk of contamination however well the environment is managed. The overall public health risk from fresh field-grown salads is small – and can surely be made smaller as we learn more about how microbes and plants interact – but I doubt it can be eliminated altogether.
If you would like to know more about foodborne pathogens in salad vegetables, take at look at parts I and II of our feature on the topic.