Listeriosis – Why the panic?

Some of our best-known supermarkets have recalled packets of frozen vegetables containing sweetcorn, which may be contaminated with the deadly bacteria, Listeria. 43 products have currently been withdrawn from sale.

Included in the withdrawal of products are Aldi, Lidl, Sainsbury’s, Tesco, Iceland, and Waitrose. The products originate from a food factory in Hungary and were distributed by Belgium-based Greenyard Frozen.

One of the most common sources of food poisoning, listeria is a widely distributed bacteria found naturally in soil and water. Most animals, including approximately 5% of humans, carry the bacteria in their gut, where it causes no harm. When it falls onto the ground or water, in the form of faeces, it can then transfer to foods either by direct contamination or by the plant taking up the infected water. Listeria has also been found in seawater where it can contaminate fish and shellfish.

The first identifiable outbreak of L. monocytogenes was linked to coleslaw production in Canada in 1981. At least 41 cases were identified with 7 deaths and in 1987-89 a large outbreak of listeriosis in the UK was linked to the consumption of pate with more than 350 cases and 48 deaths.

Why does Listeriosis worry us so much?

Although cases are relatively rare, listeria infections are very dangerous for individuals who are categorised as high risk. These will include older people, those with weakened immune systems, perhaps through illness or cancer treatment, and babies, infants and pregnant women.

Infection with listeriosis is usually divided into invasive and non-invasive categories. Invasive listeriosis, when the bacteria has spread beyond the gastro intestinal tract, can result in septicaemia, encephalitis or meningitis and can cause death. Canadian research has highlighted that the risk of developing invasive listeriosis increases 4 times for those aged 65 – 69 and 9 times for those over 75.

People at highest risk from listeriosis

Young healthy people usually show mild symptoms (non-invasive listeriosis), and this can also be the case for young pregnant women, but the transmission of the bacteria to the developing foetus, either via the placenta or during childbirth, can cause miscarriage, stillbirth, preterm labour and the illness and death of the baby. About 14% of listeria infections occur during pregnancy and death rates of infected infants can be as high as 30%.

According to the European Food Safety Authority (EFSA), there have been 47 cases of listeriosis identified in Europe up to June 8th, and nine have resulted in death.


What foods are more likely to be contaminated with Listeria?

In this outbreak, sweetcorn has been identified as the most likely source, probably from the uptake of contaminated water but sweet corn is far from being the only food which could contain the listeria background. Listeria is a psychotropic bacteria, that is, it is capable of multiplication and survival in the cold temperature,. For this reason it is a threat to RTE (Ready to Eat) foods that we would find in the chiller section of the refrigerator and which sell well particularly for salads and fresh summer food in the hot weather.

Some of the foods previously linked to the outbreaks of listeria are –

  • RTE deli meats and hotdogs
  • Pates and meat spreads
  • Unpasteurised dairy products
  • Soft cheeses made with unpasteurised milk (eg Feta, Brie, Camembert)
  • Smoked seafood
  • Raw vegetables and unwashed fruits

Further Reading Precautions to be taken by caterers and food producers