What is listeriosis?

Listeriosis is a condition caused by Listeria monocytogenes bacteria. These bacteria are widespread in nature and can be found in the soil, water and in many animals. People most often get infected with listeria after eating food that is contaminated with the bacteria.

Listeriosis is normally a mild condition that passes after a few days and has no lasting effect. However, it can lead to complications. In pregnant women, listeriosis can lead to premature birth, miscarriage or stillbirth. In addition, a pregnant woman can transfer the bacteria to her unborn baby.

Stillbirth

The death of a fetus after 20 weeks of pregnancy.

Causes

Most listeriosis cases occur as a result of eating contaminated food. Listeria monocytogenes bacteria can contaminate raw or processed food, including:

  • Raw vegetables and fruit;
  • Raw or smoked fish and other seafood;
  • Sandwiches;
  • Unpasteurised milk and dairy products such as soft cheeses and butter;
  • Pâtés;
  • Deli meats such as ham or salami, and;
  • Raw or undercooked meat.

Foods that can be a source of the Listeria monocytogenes bacteria.Foods that can harbour Listeria bacteria include salami, raw fish, soft cheeses and pre-packaged foods. 

Listeria monocytogenes bacteria are widespread in nature and are often found when testing food. They can contaminate food at any point of the food supply chain, from the soil in farms, through to all stages of food storage and processing, and in kitchens where food is prepared. Listeria are also unusual among bacteria, in that they can grow in refrigerated food.

Listeriosis often appears as outbreaks, due to an infected food source shared by multiple people. Listeriosis outbreaks occur more often in developed countries. They are rare events, appearing in 1 to 10 people per million, but severe listeriosis has a high death rate of one in five people. [1]

The incubation period of listeriosis, which is the time between infection and the appearance of illness, is highly variable and can be very long. Thus, it is sometimes hard to trace back the infection to its source. Many animal species also harbour the bacteria, which can then develop into listeriosis, in some animals. 

1. Claiborn K. (2011) Update on the listeriosis outbreak. The Journal of Clinical Investigation 121:4569.

Risk factors

Anyone can be infected with listeria, but not all people experience problems. People at risk of developing listeriosis include:

  • Pregnant women;
  • Babies;
  • Elderly people, and;
  • People with weak immune systems, due to illness or medication.

Listeriosis in pregnancy

In pregnant women infected with Listeria, the unborn baby can also become infected, when the bacteria move across the placenta and into the unborn baby's bloodstream. About one in five pregnant women who have listeriosis are in real danger of experiencing premature birth, birth complications, miscarriage or stillbirth.

Unborn babies and infants who contract listeriosis are at increased risk of developing complications of listeriosis, especially meningitis (see below).

Immune systems

The organs and cells involved in protecting the body against infection.

Placenta

The organ that forms within the uterus of a pregnant woman to provide the fetus with nourishment from the blood supply of the mother.

Stillbirth

The death of a fetus after 20 weeks of pregnancy.

1. Claiborn K. (2011) Update on the listeriosis outbreak. The Journal of Clinical Investigation 121:4569.

Signs and symptoms

Most people who are infected with listeria will not get ill, or will experience only mild symptoms. Symptoms can resemble those of the flu and may include:

Severe listeriosis

If the bacteria infect the nervous system or the blood, severe listeriosis can set in. Severe listeriosis is a serious condition and requires hospitalisation.

Symptoms of severe listeriosis can include:

  • Stiff neck muscles;
  • Twitching and seizures;
  • Loss of physical coordination, and;
  • Confusion.

Listeriosis in infants

In infants, symptoms can include:

  • Loss of appetite and food refusal;
  • Jaundice;
  • Trouble breathing;
  • Shock;
  • Irritability;
  • Lack of energy;
  • Skin rash, and;
  • Upset stomach or vomiting.

Jaundice

A yellowing of the skin, the whites of the eyes and the mucous membranes, due to an accumulation of bilirubin in the blood. Often a symptom of liver problems.

Nervous system

The extensive network of cells and structures that is responsible for activating and coordinating the body's functions, sensory input and cognition.

Seizures

A sudden, involuntary contraction of muscle groups caused by abnormal electrical activity in the brain.

1. Claiborn K. (2011) Update on the listeriosis outbreak. The Journal of Clinical Investigation 121:4569.

Methods for diagnosis

If listeriosis is suspected, your doctor may send off tissue samples (or often blood samples) to test for the presence of Listeria.

In cases of suspected listeriosis in pregnancy, samples from the amniotic fluid and the placenta may be taken.

If your doctor suspects that the bacteria may have spread to the nervous system, they may wish to sample the fluid around the spinal cord and brain, by performing a lumbar puncture. In some cases, a magnetic resonance imaging (MRI) scan of the brain is performed.

For people infected with the bacteria, who do not feel ill, further testing or treatment is usually not necessary.

Amniotic fluid

The fluid surrounding and protecting the fetus inside the uterus during pregnancy.

Magnetic resonance imaging

A type of imaging that uses a magnetic field and low-energy radio waves, instead of X-rays, to obtain images of organs.

Nervous system

The extensive network of cells and structures that is responsible for activating and coordinating the body's functions, sensory input and cognition.

Placenta

The organ that forms within the uterus of a pregnant woman to provide the fetus with nourishment from the blood supply of the mother.

Lumbar puncture

A procedure that uses a needle to collect a sample of cerebrospinal fluid, which is the clear fluid surrounding the brain and spine, from the lower back (lumbar region) for analysis. It can also be performed to remove any excess fluid or to deliver medications.

1. Claiborn K. (2011) Update on the listeriosis outbreak. The Journal of Clinical Investigation 121:4569.

Types of treatment

If treatment is necessary, your doctor will prescribe a course of antibiotic medication, such as amoxycillin or ampicillin.

1. Claiborn K. (2011) Update on the listeriosis outbreak. The Journal of Clinical Investigation 121:4569.

Potential complications

Listeriosis can cause the following complications:

  • Inflammation of the meninges (meningitis) and/or the brain (encephalitis);
  • Blood poisoning (sepsis), and;
  • Inflammation of the heart.

Meninges

The three thin layers of tissue surrounding and protecting the brain and spinal cord.

1. Claiborn K. (2011) Update on the listeriosis outbreak. The Journal of Clinical Investigation 121:4569.

Prognosis

Complications of listeriosis can result in serious, long-lasting harm and even death, particularly among infants who develop meningitis.

In healthy older children and adults, listeriosis usually passes after a few days with no lasting harm.

1. Claiborn K. (2011) Update on the listeriosis outbreak. The Journal of Clinical Investigation 121:4569.

Prevention

The best way to prevent listeriosis is to practise good food-handling practices, including:

  • Washing, scrubbing, drying and peeling fruits and vegetables;
  • Handling and storing raw meat separately from vegetables and prepared food;
  • Washing hands and food preparation utensils and surfaces;
  • Keeping your refrigerator clean from food spills and old food;
  • Paying attention to use-by dates of food products, and;
  • Thoroughly cooking food, especially meat.

People at risk, such as pregnant women, may choose to avoid the risky foods listed above, including soft cheeses, unpasteurised dairy products, pâtés, deli meats, raw fish and other seafood. 

1. Claiborn K. (2011) Update on the listeriosis outbreak. The Journal of Clinical Investigation 121:4569.