What Is Listeriosis?

Overview

Food-poisoning does not just happen when you consume foods that have visibly turned bad. Sometimes, what causes food poisoning does it without leaving a hint - no foul smell, no bad taste, and no noticeable presentation. Listeria bacteria is one of them.

Listeriosis is a dangerous bacterial infection that is caused by Listeria bacteria, more specifically the Listeria monocytogenes species. Listeriosis is a food-borne disease, meaning the bacteria are normally transmitted through food. 

According to the World Health Organisation (WHO), listeriosis affects about 0.1-10 individuals per 1 million people annually. It is considered a rare disease, but according to the Centers for Disease Control and Prevention (CDC), almost all infected patients get hospitalised, and the fatality rate can reach up to 20%.  

There are broadly 2 categories of listeriosis: 1) non-invasive and 2) invasive listeriosis.

Non-invasive listeriosis is a milder condition and mainly affects healthy individuals who are infected with listeriosis. Invasive listeriosis, on the other hand, is a more severe condition that affects mainly high-risk groups such as pregnant women, the elderly, and immunocompromised people.

Causes of listeriosis

As mentioned above, listeriosis is an infection caused by Listeria monocytogenes bacteria. Normally, people contract this disease through consuming food contaminated by listeria. However, this bacterial infection can also be passed down from pregnant mother to fetus. 

After the listeria bacteria enter the body, they are capable of infecting cells and evade immune system’s surveillance. They then pass on from one cell to another, and the infection can spead from one organ to other organs.⁵ 

Signs and symptoms of listeriosis

As mentioned above, listeriosis can be categorised as non-invasive (mild) and invasive (severe) listeriosis. The signs and symptoms of listeriosis differ according to the type of listeriosis. 

For milder non-invasive listeriosis, the listeria bacteria’s location is usually limited to the intestines and causes inflammation of the digestive tract lining. This leads to symptoms such as

• watery diarrhoea
• nausea and vomiting
• stomach pain
• fever
• muscle pain 

For severe invasive listeriosis, the listeria bacteria have multiplied and invaded other organs. If the bacteria invade the brain through the blood-brain barrier, they can cause meningitis, meningoencephalitis, and brain abscesses. These patients may present with symptoms such as

• headaches
• fever
• rigid neck
• altered state of consciousness
• seizures. 

Pregnant women who are infected with invasive listeriosis may be at risk of abortion, miscarriages, stillbirth and premature birth. This is because listeria bacteria are capable of crossing the placental barrier and causing great harm to the fetus. Newborn babies with listeriosis can show these symptoms: 

• fever
• listlessness
• vomiting
• not interested in eating

Invasive listeriosis can also cause sepsis.

Management and treatment for listeriosis

Depending on the severity of the disease, your doctor may suggest different treatments. 

Normally, individuals with milder symptoms get better over time without needing treatments. For people with more severe listeriosis, antibiotics may be prescribed. The commonly used antibiotics for listeriosis treatment include ampicillin, amoxicillin and penicillin.⁶ Sometimes, doctors may also use a combination of different antibiotics. Treatment and management of listeriosis are more effective when carried out as soon as possible.

FAQs

How is listeriosis diagnosed

The main way to diagnose listeriosis is to test the blood sample to detect the presence of listeria bacteria. Depending on the symptoms, other samples may also be collected. For example, if the patient has symptoms that hint at infection of the central nervous system, cerebrospinal fluid may be collected for tests. Other samples include amniotic, joint and peritoneal fluids.⁵ 

Can listeriosis be prevented

Listeriosis is preventable, and it all comes down to food and personal hygiene. Here are some steps you can take to keep yourself and your family safe from listeriosis:

1. Rinse your salad greens under running tap water before consuming  
2. Make sure your meat products are heated and cooked thoroughly before consuming
3. Wash your hands before and after handling food, or after gardening
4. Keep your fridge clean and cool
5. Avoid consuming unpasteurised milk and juices

Who are at higher risk of listeriosis

Listeria infection primarily affects people with a weaker immune system. Examples of high-risk groups include:

• Elderly (age 65 years and above)
• Pregnant women 
• Newborns 
• People with compromised immune systems - e.g. people with AIDS, and cancer patients who take immunosuppressive drugs

In what foods is listeria found

The modern lifestyle that emphasises convenience has encouraged the creation of various ready-to-eat foods that need no further handling, from pre-washed salad greens to on-the-go sandwiches. But, did you know that these are often the foods that can cause listeriosis? The following foods are some of the most frequent sources of listeria bacteria:²

• Deli meat
• Dairy products - e.g. unpasteurised milk, cheese, and ice cream
• Pre-washed salads
• Raw or smoked fish and seafood

Listeria bacteria are found everywhere, particularly in damp environments, such as in the soil, water, decaying vegetation, and animal faeces. Because of this, listeria can end up in pre-washed vegetables, raw meat and meat products through contaminated farming environments; in ready-to-eat foods and meats through contaminated food-processing equipment.³

Moreover, listeria can survive harsh conditions such as high salt concentration, high acidity, and refrigeration temperatures. This makes the fridge a suitable place for listeria bacteria to survive and multiply in your foods.¹ What started off as a small number of bacteria may grow into a large population over the course that the food is stored in the fridge. When the number of bacteria gets large enough (reaches ‘infectious dose’), it causes listeriosis infection, particularly among people with a weaker immune system. 

Is listeriosis contagious

Despite having caused several outbreaks around the world in the past years, listeriosis is generally not contagious as it is not passed from person to person (except transmission during pregnancy to the foetus). These outbreaks were generally caused by the widespread distribution and consumption of contaminated foods. 

How common is listeriosis

Listeriosis is actually relatively rare compared to other bacteria-caused food-borne diseases, such as those caused by salmonella and campylobacter bacteria. To put this into perspective, salmonella infection contributes about 1.35 million cases while campylobacter infection contributes approximately 1.5 million cases per year in the US, while listeriosis causes about 1,600 cases per year in the US. These numbers do vary in different countries. 

However, listeriosis still remains a threatening public health problem because it causes high hospitalisation and fatality rates. For example, salmonella may cause approximately 0.2% mortality rate for gastroenteritis infection, and up to 7% mortality rate for enteric fever caused by salmonella typhi.⁴ Listeriosis, on the other hand, has a fatality rate that goes up to as high as 20%. 

When should I see a doctor

You should visit a doctor as soon as possible if you observed the above-mentioned symptoms, especially symptoms that are linked to sepsis and meningitis such as high fever, change in consciousness, muscle pains, dizziness, and fast breathing.

Summary

Listeriosis is a relatively rare food-borne illness caused by listeria monocytogenes bacteria, yet has high fatality and hospitalisation rates. Listeriosis is especially common among people who have a weaker immune system, such as pregnant mothers and immunocompromised individuals. Since listeriosis is mainly contracted through contaminated food, it can be prevented by maintaining food and personal hygiene. If you suspect you have consumed contaminated food, seek medical advice as soon as you can.