Listeria monocytogenes

There are several Listeria species but only Listeria monocytogenes is harmful to humans. This bacterium can cause a serious illness called listeriosis. Find out about how Listeria monocytogenes can make you ill and how to avoid infection.


Symptoms of listeriosis

There are 2 forms of foodborne listeriosis – non-invasive and invasive.

Non-invasive listeriosis symptoms include:

  • diarrhoea
  • fever
  • muscle pain
  • headache
  • occasional abdominal cramps
  • vomiting.

Most people recover fully.

Invasive listeriosis symptoms include:

  • fever
  • headache
  • diarrhoea
  • vomiting
  • meningitis and blood poisoning in serious forms of the illness.

In pregnant women, symptoms may be mild but can result in miscarriage, premature birth, or in rare cases, stillbirth.

Because this is a serious illness for pregnant women or people with low immunity, you should consult a doctor immediately if you think you have listeriosis. Symptoms usually appear within 2 to 30 days of eating contaminated food, but it can take up to 70 days before people experience symptoms.

Listeriosis is a notifiable disease and doctors will report cases to their local public health unit for further investigation. Listeriosis is uncommon, with only 20 to 40 notified cases a year in New Zealand.

Sources

Listeria monocytogenes occurs widely in nature and can be transmitted through food, especially ready-to-eat products such as deli meats and salads, cooked poultry products, smoked seafood, soft cheeses and refrigerated foods with a long shelf life. Once food is contaminated, the bacteria can multiply quickly, even at recommended refrigeration temperatures (2 degrees Celsius to 4 degrees Celsius).

What makes listeria monocytogenes different to other harmful bacteria is its ability to grow in the fridge.

Infection more dangerous for at-risk groups

People who are at risk of developing listeriosis include:

  • pregnant women and their unborn children
  • newborn babies
  • the frail elderly
  • anyone with an illness that weakens the immune system, such as cancer, AIDS, diabetes or kidney disease
  • anyone on medication that suppresses the immune system, such as transplant patients.

How to reduce the risk of infection

If you are in a group at higher risk of listeriosis, you can reduce the risk of infection by:

  • eating only foods that are freshly prepared and well washed
  • following good food hygiene practices, including washing and drying hands
  • cooking foods thoroughly to kill any listeria bacteria
  • refrigerating leftovers immediately and not keeping them for more than 2 days
  • reheat food to steaming hot (over 70 degree Celsius) before eating
  • avoiding foods that are at high risk of listeria.

High-risk foods

Some foods have a higher risk of being contaminated with Listeria monocytogenes than others, and should be avoided by people in at-risk groups.

Download a list of high-risk foods for Listeria and safer substitutes [PDF, 784 KB]

Find out more

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