Listeria hysteria

Foodstuffs Own Brands in New Zealand has this weekend recalled baby spinach and various salad mixes because of possible contamination with Listeria.

The recall was reported in newspapers and picked up by TVNZ news and given a special segment in the evening Close Up current affairs programme.  Unfortunately, it was given the same sort of treatment as might befit a tsunami. 

In my opinion, Foodstuffs has done the right thing - they know that there is a problem with a product from their supplier and they have withdrawn it from the market to protect their consumers' health and their reputation.

But the presence of Listeria in salad vegetables is practically impossible to avoid.  The Microbiological Reference Criteria for Foods note that all foods produced by a process which is capable of achieving a Listeria-free product should test negative for L. monocytogenes in 5 samples of 25g.  (Reference Criteria are guidelines to indicate when food can be considered unacceptable or unsafe).  The criteria do not apply to raw fruits and vegetables, and bagged salad vegetables do not receive a listericidal process.

Where does L. monocytogenes come from?  The bacterium is widely distributed in the environment and can be found in decaying vegetation, soils, animal faeces, sewage, silage and water.  It is not surprising that salad vegetables will sometimes be contaminated from these sources.

Most bagged salad vegetables are given multiple washes, culminating in a chlorine rinse, which can reduce the levels of L. monocytogenes by a factor of about 10 (a 1 log reduction).  Provided that the numbers of L. monocytogenes are less than about 100/g at the point of consumption, they are unlikely to cause disease.  Many producers recommend re-washing at home before consumption.  However, it's difficult to dry the leaves and dressing doesn't stick to wet leaves, so it is likely that this recommendation is often ignored.  Refrigeration is also recommended, but it is probably better to take note of the 3-4 day shelf life, as Listeria can grow in the refrigerator, albeit slowly.  Growth is much less likely on dry, uncut surfaces.

The levels of L. monocytogenes required to cause disease are very difficult to assess and this is reflected in the varying compliance criteria adopted by different countries.  Listeriosis is a rare but serious disease in humans, despite frequent exposure to the causative organism, with an incidence of 2-3 cases per million of the total population in England and Wales.  Those most at risk are, in descending order, organ transplant patients, patients with AIDS or HIV, pregnant women and their unborn babies, cancer patients and the elderly.  New-born infants may also be infected from their mothers or other infants.  Non-pregnant healthy individuals are highly resistant to listeriosis.


What does this all mean?  Basically, if you look for Listeria in vegetables in the marketplace (and vegetables grown in your own garden) you will sometimes find it.  Various surveys suggest that the frequency of detection will be between 1 and 86%.  If a sample does test positive for Listeria, efforts should be made to find and eliminate the source of contamination.  The frequency of sampling should be increased and subsequent samples should test negative. 

There is no room for complacency, even in companies that have never had a positive Listeria test, but a single positive sample should be viewed with a modicum of restraint.