The facts about...

Listeria

The organism and its natural habitat

The bacterium which is the cause of listeriosis, Listeria monocytogenes, is widespread in the environment. It can be found in soil, dust, mud, vegetation, silage, sewage and most of the animals that have been tested. It has been found in up to five per cent or more of normal healthy people usually in the gut. For this reason exposure to this bacterium is unavoidable.

It has the unusual characteristic of being able to grow, albeit slowly, at temperatures as low as 0oC. There are several different subtypes which can be distinguished by laboratory tests. There are listerias other than L. monocytogenes, for example L. innocua, which are very similar to it but apparently do not cause disease in man.

The Diseases

L. monocytogenes can cause a variety of diseases, including infections in pregnancy ranging from a mild chill to a severe illness which may precipitate premature birth or miscarriage, and meningitis in newborn children. Septicaemia and meningitis occurs in adults whose immunity to infection is impaired, such as those suffering from cancer or leukaemia or transplant patients. Infection does occur in otherwise healthy adults and children although this is extremely rare. The infection may be treated with antibiotics but in about one third of cases the disease is fatal. There is no vaccine available to help prevent this infection.

Listeria infection is not statutorily notifiable, but microbiology laboratories which grow L. monocytogenes from infected patients notify either the PHLS Communicable Disease Surveillance Centre (CDSC) or, in Scotland, the Scottish Centre for Infection and Environmental Health. It is unlikely that many serious cases of diagnosed infection go unreported. Although the organism is common in the environment it is a relatively rare cause of infection and commonly only affects 1-3 cases per million of the population per year.

The number of cases in England and Wales between 1983 and 1986 ranged from 115 to 149 cases per year. However in 1987 there was a sharp increase in cases to 259, in 1988 there were 291 cases and in 1989, 250 cases. Since then the numbers have dropped to between 102 and 130 cases per year during the years 1990 and 1994. The relatively sharp increase between 1987 and 1989 has now been associated with a newly recognised subtype of L. monocytogenes which was also isolated at the same time from a large number of retail samples of meat pate.

Listeria in food

Although infection via contact with animals and by neonatal spread occurs, the consumption of contaminated food is believed to be the principal route of infection. A wide range of food types have been implicated with transmission including meat, dairy, fish, shellfish and vegetable products. The laboratory methods used to detect L. monocytogenes in food have improved in recent years and the organism has been found, but usually in small numbers, in many foods including raw fish, shellfish and fish products; raw meat, poultry and their products including hot dogs and pate; raw and processed vegetables; ripened soft cheeses; ice cream; retail cook-chill meals; salads including coleslaw; raw and inadequately pasteurised milk as well as raw and liquid egg. The food industry has been active in investigating Listeria in food with food manufacturing environments implementing hazard analysis and codes of practice so that during the 1990s the number of foods contaminated with L. monocytogenes types has been reduced.

Although the organism is killed by proper cooking and pasteurisation, it has been found occasionally in some cheeses made from pasteurised as well as unpasteurised cows and goats milk and the presence of the organism in cooked foods indicated inadequate cooking or contamination after cooking. This is clearly undesirable and the food industry and trade are reported to be taking steps to eliminate it.

Retail chilled meals carry instructions to reheat before consumption and if this is done correctly L. monocytogenes should be killed. Similarly, it should be eliminated from food taken from cook-chill catering units when it is properly reheated. Conventional ovens should cause no problems, but microwave cookers do not always reheat food uniformly and bacteria may survive in cold spots.

In spite of these facts the number of occasions on which listeriosis has been definitely linked to consumption of food in the UK is small. However these associations may be difficult to establish.

On the basis of the number of reported cases in England and Wales the Chief Medical Officer has estimated the incidence of listeriosis in pregnancy to be approximately 1 in 30,000 live and stillbirths. Because of the risks of listeriosis during pregnancy he has recommended that pregnant women should avoid eating certain ripened soft cheeses such as Brie, Camembert and the blue veined types such as Danish Blue, Stilton or Gorgonzola as these may contain high levels of the listeria organism. However, hard cheeses such as the Cheddar and Cheshire types; soft fresh cheeses such as cottage cheese, cream cheese and Fromage Frais as well as processed cheese products in sealed packages (such as cheese spreads) have not given cause for concern when eaten fresh. In addition pregnant women should avoid eating any type of meat based pate as these may also contain high levels of the listeria organism. Although the number of organisms found in other foods, such as cooked chilled meals and ready-to-eat poultry, is usually low the Chief Medical Officer has also advised pregnant women to reheat these types of foods thoroughly until they are 'piping hot' rather than eat them cold. Pregnant women should not help with lambing, milk ewes that have recently given birth, touch the after-birth or come into contact with newborn lambs, as these are also a potential source of the listeria organism. The Department of Health's booklet 'While you are pregnant; safe eating and how to avoid infection from food and animals' issued in September 1991 contains useful advice about avoiding listeriosis and other infections during pregnancy, and copies should be available to all pregnant women at antenatal clinics or general practitioner surgeries.

People with deficient immunity are advised to take the same precautions as pregnant women but the risk of infection in healthy people over the age of four weeks is regarded as being too low to warrant any dietary changes.