Food Safety Focus Banner

To the main page Next Article

Food Safety Focus (75th Issue, October 2012) – Incident in Focus

Listeriosis and Pregnancy

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

Last month, the Centre for Health Protection (CHP) reported that, by end of August this year, there have already been 21 reported cases of listeriosis, much higher than the annual reported figures of 3 to 17 between 2004 and 2011. This marks the highest number recorded since listeriosis became a notifiable disease in 2008. Although listeriosis is an uncommon disease in healthy individuals, it may pose significant health risk to certain subpopulations including pregnant women. This article discusses foodborne listeriosis in regnant women.

Listeriosis in Pregnant Women

Listeriosis is an infectious disease usually caused by eating food contaminated with Listeria monocytogenes. This bacterium is commonly found in the natural environment, for example, soil, water, decaying plants and animal intestines.

During pregnancy, the immune system is weakened and thus pregnant women are more susceptible to listeriosis. While pregnant women, when infected, may just experience mild, flu like symptoms or even with no symptoms, L. monocytogenes can pass to the baby through the placenta. During the first trimester of pregnancy, listeriosis may cause miscarriage. As the pregnancy progresses to third trimester, listeriosis can result in preterm birth, delivery of a low-birth-weight infant or even infant death. About half of the infants infected at or near term may die.

Foetuses who suffer a late infection may develop a wide range of health problems, including mental retardation, paralysis, blindness, impairments of the brain, heart and kidney.

Local Situation

Among the 106 listeriosis cases recorded by the CHP between June 2004 and August 2012, 14 (13%) cases were related to pregnant women.

Four out of the 14 pregnant cases had spontaneous abortion or stillbirth. One mother had a preterm delivery but the baby died one day after birth. The other nine pregnant women delivered their babies in stable condition with two of the babies infected and required antibiotic treatment. All pregnant women were clinically stable and recovered after treatment.

High Risk Foods

Due to its ubiquity in the environment, low concentration of L. monocytogenes may be present in food. In general, foods containing low levels of L. monocytogenes (e.g. < 100 colony-forming unit (cfu)/g) pose very little risk to consumers. However, as the number of bacteria increase, consumers will be at risk of listeriosis.

Although L. monocytogenes can be killed under normal cooking temperature and that it cannot grow at frozen temperature, unlike other food poisoning bacteria, it may continue to grow slowly at refrigerated temperature as low as 0°C. This characteristic makes long shelf life (greater than five days) refrigerated ready-to-eat food a potential high risk item for listeriosis. Prolonged storage in refrigerator may allow L. monocytogenes to have sufficient time to grow gradually to exceed 100 cfu/g throughout the shelf life. This may lead to infection when the food is consumed without further cooking.

From 2008 to July 2012, the Centre for Food Safety tested around 1 800 food samples for L. monocytogenes, in which seven of them, including six smoked salmon and one smoked ham were found unsatisfactory.

Pregnant women should choose foods carefully and avoid high risk foods e.g. smoked salmon and smoked ham
Pregnant women should choose foods carefully and avoid high risk foods e.g. smoked salmon and smoked ham


In general, proper food handling by applying 5 keys to food safety can help prevent listeriosis. Since L. monocytogenes can easily be destroyed under normal cooking temperature, immediate consumption of freshly and thoroughly cooked food should be safe from listeriosis. Since L. monocytogenes can still grow slowly at low temperatures (e.g. at 4 °C ), refrigerated perishable foods including leftover food should be consumed as soon as possible and reheated thoroughly before consumption. While eating out, only eat cooked and hot served food but not those which are raw/ undercooked and served lukewarm.

In addition, avoiding high risk foods and/or choosing safer alternatives is an effective way to prevent listeriosis (see Table).

Table: Examples of high risk foods and their safer alternatives

Key Points to Note:

  1. Listeriosis can cause serious implications for susceptible subpopulations like pregnant women.
  2. Even food stored in a refrigerator at lower temperatures (e.g. at 4°C ) may allow L. monocytogenes to grow slowly.
  3. Pregnant women should avoid eating high risk foods especially refrigerated (excluding frozen) ready-to-eat foods with long shelf life.

Advice to Pregnant Women

  1. Maintain good food and personal hygiene.
  2. Avoid high risk foods especially refrigerated ready-to-eat foods with long shelf life.
  3. Keep perishable food at refrigerator operates at or below 4 °C and avoid cross-contamination.
  4. Cook all food including raw, processed and leftovers thoroughly and use ready-to-eat food as soon as possible.

Advice to Trade

  1. Estimate the demand of each food carefully to avoid over-production.
  2. Keep perishable foods at refrigerator operates at or below 4°C and avoid cross-contamination.
  3. Cook food thoroughly before serve.