Wednesday, January 19, 2011

Poor quality of reporting

I have just read a couple of articles in a national newspaper.  These report on the recall of bagged salads because of possible Listeria contamination and the death from avian botulism of wild birds in Auckland.

Listeria monocytogenes is a bacterium that causes a rare, but serious disease in humans.  (See Listeria hysteria).  The first article includes an information box, in which the symptoms are described as "a mild viral infection".  Bacteria are not viruses and the two entities are as different as lobsters and soft cheese!

The second article describes the increase in numbers of wild birds dying because the very warm water temperatures (up to 26C) have allowed rapid proliferation of Clostridium botulinum, which produces a neuroparalytic toxin.  The information box in this article claims that "the toxin thrives in still, shallow, warm water".  The toxin does not "grow or increase in bulk", though the bacterial cells that produce it may do.

Neither of these errors is likely to result in injury if the reader follows the advice in the articles, but it is just plain, sloppy reporting.  Google lists 1.16 million articles on L. monocytogenes and 541,000 on C. botulinum, so there is no excuse for the reporters' not having done a bit of background reading.  We all make mistakes in our writing, but if we set out to educate, then we really ought to check the simple details. 

I stand ready to get hammered if this blog contains stupid errors!

Avian botulism is caused by Type C toxin, which does not affect humans.

Tuesday, January 18, 2011

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has done the right thing - they know that there is a problem with a product from their supplier and they have withdrawn it from the market to protect their consumers' health and their reputation.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