IMPLICATION OF FRESH PRODUCE IN MULTIPLE LISTERIA OUTBREAKS A POTENTIALLY WORRYING DEVELOPMENT
by John McCarthy | October 9th, 2011
Listeria monocytogenes is no stranger to food safety professionals. It is one of the more common pathogens that can cause serious food poisoning in consumers.
The fact that it is not usually heat resistant means that, apart from the risk of contaminating other food which it is in contact with, any products which are cooked before they are consumed do not pose a serious danger. The real risk is therefore associated with ready-to-eat (RTE) foods. Listeria also has a very nasty habit of growing at temperatures at which foods are typically refrigerated meaning that things that we are inclined to leave in our fridge for long periods, like certain cheeses, can be ideal breeding grounds for the bug.
It is usually associated with deli meats, raw milk, cheeses, raw poultry and meats, and seafood and, as far as some of the more common sources of listeria infection go, thankfully Ireland fares relatively well: in March of this year the Food Safety Authority of Ireland (FSAI) published the findings of a national survey on establishing baseline data on the presence of Listeria monocytogenes on cooked meat slicers in retail and catering premises.
The results showed that the presence of Listeria on cooked meat slicers was quite low, with only 0.71% of samples testing positive for Listeria species and only 0.23% positive for Listeria monocytogenes. Meat slicers are typically used continuously and often at room temperature and can be difficult to clean, which means that they can become susceptible to contamination.
If anyone needs a reason for ensuring that hygiene standards in the processing of cooked meats are maintained they need look no further than the 2008 Maple Leaf case in Canada. A widespread outbreak linked to cold cuts from a meat processing plant in Toronto gave rise to 57 confirmed cases of listeriosis, from which 22 people died.
Given the pathogen’s ubiquity in the environment, it is not surprising that contamination of fresh produce is occasionally observed with between 10 and 20% of prepared vegetable samples possibly being contaminated with Listeria monocytogenes. The bacterium has been found on a wide variety of vegetables including bean spouts, potatoes, lettuce, radishes, cabbage, cauliflower, tomatoes, onions, cucumbers and mushrooms.
The real danger lies with tainted RTE fruits and vegetables with several outbreaks having been associated with consumption of products such as coleslaw and prepared salad mixes.
The Jensen Farms cantaloupe outbreak has at this stage been comprehensively reported on. The death toll so far stands at 21 and this may well rise having regard to the particularly lengthy incubation period associated with listeriosis, which can be anywhere from 3 to 70 days, with the median incubation period being three weeks.
It has been suggested that the grazing of sheep on the fields used by Jensen Farms to grow cantaloupes may have been the source of infection in that case but this has not been confirmed and investigators at the Centers for Disease Control and Prevention (CDC) in the US are continuing to search for the causes of the outbreaks in each case by examining possibilities of water contamination and improper farming practices.
There has also been a recent entirely unrelated outbreak involving romaine lettuce grown in California. 2,500 cartons of greens were implicated but only 90 or so cartons had been distributed to retailers, and all have been withdrawn. The rest of the cartons went to food service distributors, all of which have been notified of the recall. There have not been any reports of illness so far.
These two cases concerning RTE fresh produce are entirely unrelated and happened within the same month. Is this a pure fluke or are we witnessing an increasing trend of the pathogen tainting fresh fruit and vegetables? Only time will tell.
