Listeria and pregnancy

Pregnant women are routinely warned about Listeria but what is the real risk of this bacteria? By Kirsten Braun

Pregnant woman looking undecided into an open fridge

What is Listeria and listeriosis?

Listeriosis is an infection caused by the bacterium Listeria monocytogenes. Listeria bacteria occur naturally in the environment (soil, marine environment). Plants grown in soil with Listeria can become contaminated as can fish and seafood products. Animals can also be carriers of the bacteria, often from being fed contaminated feed. Unlike some other bacteria, Listeria can survive in low temperatures. This means that it can be difficult to control and eliminate in food processing environments.

Why are pregnant women more susceptible?

People who have a lowered immune system are most at risk of listeriosis. This includes pregnant women, the elderly and those that are immunosuppressed. During pregnancy, hormonal changes suppress a woman’s immune system making her more susceptible to infections.

How common is listeriosis?

Listeriosis is relatively rare with approximately 65 cases reported each year in Australia. Around one in ten of these cases occur in pregnant women. Therefore, on average there are 7 cases of listeriosis reported in Australian pregnant women each year, compared to 300,000 births. The risk of listeriosis infection appears to be highest in the third trimester.

What are the symptoms?

Listeriosis is one of the most severe foodborne infections often causing hospitalisation, even death. The initial symptoms are flu-like such as headaches and fever and sometimes nausea, vomiting and diarrhoea. In severe cases a person can develop meningitis and septicaemia.

What impacts can it have on the unborn baby?

Although the risk is rare, a listeriosis infection can have serious consequences for the health of an unborn baby. One in five cases (20%) of listeriosis in pregnant women will result in the death of the unborn baby. It increases the risk of a premature labour, the delivery of a low-birth weight baby and a range of health problems in the new baby.

How can you avoid being infected?

Pregnant women are advised to avoid foods that are considered high risk. The list of high risk foods is predominantly compiled by examining Listeria outbreaks worldwide and understanding the foods that are potentially most at risk because of the way they are processed and/or stored. The Australian Government recommends pregnant women avoid the following:

  • processed, ready-to-eat deli meats, chicken and poultry. These products are at high risk due to the way they are processed (i.e., sliced) as it is more difficult to clean the equipment used for this purpose. It is estimated that ready-to-eat processed luncheon meats may be responsible for up to 40% of cases of listeriosis each year.

  • soft cheeses (Brie, blue, Camembert, fetta, ricotta). These cheeses are more risky as the pH level of the cheese is quite high, allowing the bacteria to grow during the shelf life. While cheeses made in Australia from pasteurised milk were previously considered quite low risk, a Listeria outbreak occurred in 2012 in Australian produced soft cheeses.

  • chilled seafood and cold-smoked seafood (e.g., chilled prawns, smoked salmon). In the case of chilled prawns the cooking process will initially kill Listeria but they can become contaminated following cooking. The cold smoking process does not adequately kill Listeria in cold-smoked seafood, which is why they are still at risk.

  • raw seafood (oysters, tuna).

  • sushi made with raw seafood and pre-made sushi. Pre-made sushi can become contaminated with Listeria during the handling process and it is not always stored at correct temperatures.

  • pâté and meat spreads that require refrigeration.

  • pre-cut fruit. There have been several Listeria outbreaks related to cut rockmelon so it is best to avoid pre-cut fruit as these carry an additional risk due to the extra handling.

  • pre-packaged salads. These foods are included due to the way they are produced (shredding, slicing of ingredients) and because they have an extended shelf life. In reality, many pre-packaged salads have dressings that are acidic reducing the risk of Listeria growth. However, as there are so many different varieties of salad the advice is to avoid them.

  • seed sprouts. Sprouts are at high risk of being contaminated with a number of bacteria including Listeria but also Salmonella and E.coli. The bacteria can enter the actual seed and once established is difficult to eliminate.

  • unpasteurised juices. These are considered risky due to the risks of pre-cut fruit and the fact that juicing machines can be difficult to clean. Juices sold in the cold section of the supermarket are generally pasteurised (refer to the labelling if not sure). However, freshly squeezed juices from juice bars and/or cafes are unpasteurised.

  • unpasteurised milk. It is illegal to sell unpasteurised milk in Australia for human consumption but it is sometimes sold as bath or cosmetic milk. Milk can become contaminated during the milking, packaging or storing process.

  • soft serve ice-cream and drinks made with it such as thickshakes. Soft serve ice-cream can become contaminated with Listeria due to the machinery used not being cleaned adequately. New models of machinery self-pasteurise, meaning they heat the ice-cream to a temperature that kills bacteria regularly. However, as consumers are not always aware of what particular machinery is used, it is best to avoid soft serve ice-cream.

What is still okay to eat?

  • Ready-to-eat deli meats, chicken or poultry if it is reheated until steaming hot (75°C) or cooked in a product.

  • Hard cheeses (e.g., cheddar, edam, parmesan, gouda, jarslberg) and products made with soft cheeses that are fully cooked. For example, it is okay to eat a pizza with fetta cheese as a topping as any Listeria the cheese may have harboured is killed during the cooking process of the pizza.

  • Pasteurised juices.

  • Seafood (including smoked) if it is included in fully cooked dishes.

  • Freshly prepared sushi containing freshly cooked ingredients.

  • Hard frozen ice-cream and gelato.

All about balance

While pregnant women should be careful about what they eat during pregnancy they also need to maintain a healthy diet. By understanding the level of risk that different foods pose, women can still maintain a varied and nutritious diet.

Please note this is a list of foods that should be avoided to decrease the risk of Listeria infection. There are other foods that pregnant woman should also avoid including:

  • raw or partially cooked eggs, including freshly made mayonnaise dressings (Salmonella)

  • raw or undercooked meat (toxoplasmosis)

  • liver products (Vitamin A)

  • shark, broadbill, marlin, swordfish, orange roughy and oily fish intake – (mercury level).

For more information on what to eat during pregnancy, see our Preconception and pregnancy fact sheet.

Last updated: June 2015

© Women's Health Queensland Wide Inc. This article was written by Kirsten Braun and reviewed by the Women's Health Queensland Wide editorial committee in June 2015. It was first published in Health Journey 2015 Issue 2.

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