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Food Safety Focus (8th Issue, March 2007) – Incident in Focus

Listeria monocytogenes in food

Reported by Ms. Janny MA, Research Officer
Risk Assessment Section, Centre for Food Safety

Recent Recall on Listeria monocytogenes contaminated food

On 18 February 2007, the Food Safety and Inspection Service of the US Department of Agriculture announced that the food manufacturer, Carolina Culinary Foods, was recalling a fully cooked ready-to-eat (RTE) chicken breast strips product that might be contaminated with Listeria monocytogenes. The company further expanded the recall on 28 February 2007. The Centre for Food Safety (CFS) promptly contacted the US authority and made an appeal to retailers and the public to stop selling or consuming the affected batches of products. The CFS would continue to monitor the situation and take actions as necessary.

Features of Listeria monocytogenes

Listeria monocytogenes is a pathogenic bacterium. It is universally found in the environment, particularly in soil, vegetation, animal feed, and in human and animal faeces. Such bacterium can survive and multiply at temperature as low as 0°C, but can be easily destroyed under normal cooking temperature. Other conditions supporting the growth of Listeria monocytogenes are listed in Table 1. Consuming Listeria contaminated food may lead to the development of a disease called listeriosis.

Table 1: Conditions supporting the growth of Listeria monocytogenes

Temperature *

0°C - 45°C

pH

pH 4.4 - 9.4

Water Activity

≥0.92

Foodborne listeriosis is a relatively rare but serious disease with high fatality rates (20%–30%). Listeria predominantly affects newborns, elderly and immunocompromised individuals such as patients with AIDS, diabetes mellitus, cancer or kidney disease. Symptoms usually appear within 3 to 70 days (estimated median incubation period is three weeks) after consuming contaminated food. The disease symptoms include flu-like symptoms, nausea, vomiting, abdominal cramps, diarrhoea, headache, constipation and persistent fever. Asymptomatic infection probably occurs in most people. However, serious infections of Listeria monocytogenes are manifested by septicaemia and meningitis.

Prevention of Listeria infections is of major concern in pregnant women. Even though symptoms may be relatively mild in mothers, the passage of Listeria monocytogenes through the placenta may cause miscarriage, stillbirth or perinatal septicaemia and meningitis in the newborns.

Foods at Higher Risk

Listeria monocytogenes is widely dispersed in the environment and foods. RTE foods with long shelf lives under refrigeration such as soft cheeses and RTE poultry and meat pose the greatest risk as Listeria monocytogenes may grow to significant numbers at refrigeration temperatures when given sufficient time (Figure 1). However, the growth of Listeria monocytogenes is not supported under freezing condition.

 Figure 1: Decision tree for identification of relatively high risk foods associated with listeriosis

Occurrence of Listeriosis

Listeriosis is an uncommonly diagnosed infection that occurs worldwide. The foods implicated in major outbreaks of listeriosis worldwide are all products in which Listeria monocytogenes can grow to levels that could present a risk to consumers. Human listeriosis have been linked to the consumption of foods like soft cheeses, pate, cooked cold chicken, processed deli turkey meat, cold-smoked fish and unacidified jellied pork tongue.

In Hong Kong, there have been relatively few confirmed cases of listeriosis. According to the Centre for Health Protection, there were 21 listeriosis cases during June 2004 to February 2007. Almost half of the cases were elderly and two-thirds had underlying chronic illnesses (e.g. diabetes mellitus and ischaemic heart disease) or immunocompromised conditions (e.g. cancer and systemic lupus erythematosus).

Advice to the Trade

  1. Maintain good hygienic and food handling practices in food manufacturing plants, food service and retail establishments.
  2. Avoid post-cooking contamination of RTE foods particularly those with long shelf lives.

Advice to the Consumers

  1. Maintain good food and personal hygiene.
  2. Check whether the food package is intact and the expiry date on the label is not overdue especially RTE foods under refrigeration with long shelf lives.
  3. Consume perishable foods that are precooked and ready-to-eat as soon as possible.
  4. Reheat leftovers thoroughly before consumption.

Additional Advice to the Groups at Risk

Avoid high risk foods, such as RTE items with shelf lives longer than 10 days under refrigeration or items that have been implicated in human listeriosis, unless cooked thoroughly.

Further Information

 Further information about the recent recall on Listeria contaminated food and the bacterium can be obtained from the following webpages:

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