Listeria cases per year

What is Listeriosis

Listeriosis, is a rare but serious disease caused by the bacteria Listeria monocytogenes (L. monocytogenes) that can survive and grow on certain high risk foods. While it is probably common for people to eat foods contaminated with a small amount of the bacteria, only some people are at risk of becoming sick. The people who do get sick may require hospitalisation and it may lead to death.

What are the symptoms?

Listeriosis can cause different symptoms depending on which part of the body has been affected and the usual health of the person. Symptoms can include fever, muscle aches, and sometimes nausea and diarrhoea. Infection with L. monocytogenes may also cause septicaemia (blood poisoning) and meningitis (inflammation of the outside of the brain), and death can occur because of these complications.

Pregnant women generally experience mild symptoms themselves; however infections during the pregnancy can lead to miscarriage, stillbirth or infection of the newborn baby.

Symptoms usually start between 3 to 70 days (average 21 days) after eating food contaminated with the bacteria.

How is it spread?

L. monocytogenes is commonly found in the environment (such as soil) and some raw foods. Unlike many other bacteria, L. monocytogenes are unusual because they can grow in the refrigerator. Eating foods that contain L. monocytogenes does not cause illness in most people however some can become sick.�Babies can be born with listeriosis if their mothers eat contaminated food during the pregnancy.

Listeriosis does not spread from person-to-person.

Who is at risk?

Eating foods that contain L. monocytogenes does not cause illness in most people. The disease mainly affects the elderly, pregnant women and their unborn and newborn babies, and people with weakened immune systems due to illness or medication (for example, people on cancer treatment or steroids, and people with diabetes, kidney disease, liver disease and HIV infection).

How is it prevented?

If you (or someone in your household) have a weakened immune system or are pregnant, the best way to avoid L. monocytogenes is to eat freshly cooked or freshly prepared food.

Try to avoid foods that have a higher risk of L. monocytogenes contamination such as:

• chilled seafood such as raw oysters, sashimi and sushi, smoked ready-to-eat seafood and cooked ready-to-eat prawns
• cold meats from delicatessen counters and sandwich bars, and packaged, sliced ready-to-eat meats
• cold cooked ready-to-eat chicken (whole, portions, or diced)
• rockmelon
• pre-prepared or pre-packaged fruit or vegetable salads, including those from buffets and salad bars
• soft, semi-soft and surface-ripened cheeses such as brie, camembert, ricotta, blue and feta
• refrigerated pat� or meat spreads
• soft serve ice cream
• unpasteurised dairy products
• raw mushrooms.

You can further reduce your risk by:

• avoiding food that is past its best before or use by date
• refrigerating leftovers promptly and using them within 24 hours, or freezing them
• cooking food thoroughly
• reheating food until it is steaming hot.

The NSW Food Authority provides further information on foods to eat and avoid during pregnancy.

How is it diagnosed?

The diagnosis of listeriosis can be confirmed by blood or other tests requested by a doctor.

How is it treated?

Treatment for listeriosis involves antibiotics and supportive care. When infection occurs during pregnancy, antibiotics can often prevent infection of the unborn baby or newborn.

Listeria in Australia

While listeriosis can be a very severe illness, the number of cases reported in Australia each year is relatively low, with around 80 cases reported each year. Most people infected with listeriosis are not connected to an outbreak, however outbreaks can occur. Outbreaks caused by listeriosis have been associated with rockmelon, delicatessen meats, raw milk, soft cheeses, pre-prepared salads (for example, from salad bars), unwashed raw vegetables, pat�, cold diced chicken and pre-cut fruit and fruit salad.

Preventing the spread of listeria in Australia

Listeriosis is mainly acquired by eating contaminated foods. Food safety standards in Australia are designed to minimise the contamination of food with bacteria including L. monocytogenes. �It is difficult to completely remove the risk as this bacteria is so widespread in the environment. Cases of listeriosis are reported to public health authorities so outbreaks can be identified and managed, and particular causes detected.

Further Information

Talk to your doctor about preventing listeriosis if you are pregnant or if you think you might be at increased risk due to illness or medications.

More information on listeriosis can also be found by contacting your state or territory health department.

Listeria bacteria are found widely in the environment in soil, including in decaying vegetation and water, and may be part of the fecal flora of a large number of mammals, including healthy human adults.

According to the United States Food and Drug Administration (FDA), some healthy adults may be asymptomatic intestinal carriers of the bacteria. Another authority notes that the “organism has been isolated from the stool of approximately 5% of healthy adults.”

According to the Centers for Disease Control and Prevention’s (CDC) National Center for Zoonotic, Vector-Borne, and Enteric Diseases, listeriosis was added to the list of nationally notifiable diseases in 2001:

To improve surveillance, the Council of State and Territorial Epidemiologists has recommended that all L. monocytogenes isolates be forwarded to state public health laboratories for subtyping through the National Molecular Subtyping Network for Foodborne Disease Surveillance (PulseNet). All states have regulations requiring health care providers to report listeriosis cases, and public health officials should try to interview all persons with listeriosis promptly using a standard questionnaire about high risk foods. To reach this goal, FoodNet conducts active laboratory- and population-based surveillance.

The following table shows selected data from the CDC’s Annual Surveillance Summaries of reported Listeria cases. Increases over time could be explained in part by increased surveillance and reporting.

Table 1. Selected data from the CDC’s Annual Surveillance Summaries of reported Listeria cases

Despite these numbers, CDC’s Technical Information website estimates that there were more than 1,600 cases of Listeria infection annually in the United States based on data through 2008. Although the nature and degree of underreporting is subject to dispute, all sources agree that the confirmed cases represent just the tip of the iceberg.

The Listeria Initiative and PulseNet conduct nationwide surveillance to rapidly detect and respond to outbreaks; the Foodborne Diseases Active Surveillance Network (FoodNet) conducts active, sentinel population-based surveillance to track incidence trends; and the Foodborne Disease Outbreak Surveillance System (FDOSS) receives reports of investigated outbreaks to track foods and settings associated with outbreaks. In a CDC report summarizing data on 2009-2011 listeriosis cases and outbreaks reported to U.S. surveillance systems:

Nationwide, 1,651 cases of listeriosis occurring during 2009–2011 were reported. The case-fatality rate was 21%. Most cases occurred among adults aged ≥65 years (950 [58%]), and 14% (227) were pregnancy-associated. At least 74% of nonpregnant patients aged <65 years had an immunocompromising condition, most commonly immunosuppressive therapy or malignancy. The average annual incidence was 0.29 cases per 100,000 population. Compared with the overall population, incidence was markedly higher among adults aged ≥65 years (1.3; relative rate [RR]: 4.4) and pregnant women (3.0; RR: 10.1). Twelve reported outbreaks affected 224 patients in 38 states. Five outbreak investigations implicated soft cheeses made from pasteurized milk that were likely contaminated during cheese-making (four implicated Mexican-style cheese, and one implicated two other types of cheese). Two outbreaks were linked to raw produce.

Foodborne listeriosis is one of the most serious and severe foodborne diseases. It is caused by the bacteria Listeria monocytogenes. It is a relatively rare disease with 0.1 to 10 cases per 1 million people per year depending on the countries and regions of the world. Although the number of cases of listeriosis is small, the high rate of death associated with this infection makes it a significant public health concern.

Unlike many other common foodborne diseases causing bacteria, L. monocytogenescan survive and multiply at low temperatures usually found in refrigerators. Eating contaminated food with high numbers of L. monocytogenes is the main route of infection. Infection can also be transmitted between humans, notably from pregnant women to unborn babies.

L. monocytogenes are ubiquitous in nature and found in soil, water and animal digestive tracts. Vegetables may be contaminated through soil or the use of manure as fertilizer. Ready-to-eat food can also become contaminated during processing and the bacteria can multiply to dangerous levels during distribution and storage.

Food most often associated with listeriosis include:

• foods with a long shelf-life under refrigeration (L. monocytogenes can grow to significant numbers in food at refrigeration temperatures when given sufficient time); and
• foods that are consumed without further treatment, such as cooking, which would otherwise kill L. monocytogenes.

In past outbreaks, foods involved included ready-to-eat meat products, such as frankfurters, meat spread (paté), smoked salmon and fermented raw meat sausages, as well as dairy products (including soft cheeses, unpasteurized milk and ice cream) and prepared salads (including coleslaw and bean sprouts) as well as fresh vegetables and fruits.

The disease

Listeriosis is a series of diseases caused by the bacteria L. monocytogenes, outbreaks of which occur in all countries. There are two main types of listeriosis: a non-invasive form and an invasive form.

Noninvasive listeriosis (febrile listerial gastroenteritis) is a mild form of the disease affecting mainly otherwise healthy people. Symptoms include diarrhoea, fever, headache and myalgia (muscle pain). The incubation period is short (a few days). Outbreaks of this disease have generally involved the ingestion of foods containing high doses of L. monocytogenes.

Invasive listeriosis is a more severe form of the disease and affects certain high risk groups of the population. These include pregnant women, patients undergoing treatment for cancer, AIDS and organ transplants, elderly people and infants. This form of disease is characterized by severe symptoms and a high mortality rate (20%–30%). The symptoms include fever, myalgia (muscle pain), septicemia, meningitis. The incubation period is usually one to two weeks but can vary between a few days and up to 90 days.

The initial diagnosis of listeriosis is made based on clinical symptoms and detection of the bacteria in a smear from blood, cerebrospinal fluid (CSF), meconium of newborns (or the fetus in abortion cases), as well as from faeces, vomitus, foods or animal feed. Various detection methods, including polymerase chain reaction (PCR), are available for diagnosis of listeriosis in humans. During pregnancy, blood and placenta cultures are the most reliable ways to discover if symptoms are due to listeriosis.

Pregnant women are about 20 times more likely to contract listeriosis than other healthy adults. It can result in miscarriage or stillbirth. Newborn may also have low birth weight, septicaemia and meningitis. People with HIV/AIDS are at least 300 times more likely to get ill than those with a normally functioning immune system.

Due to the long incubation period, it is challenging to identify the food which was the actual source of the infection.

Treatment

Listeriosis can be treated if diagnosed early. Antibiotics are used to treat severe symptoms such as meningitis. When infection occurs during pregnancy, prompt administration of antibiotics prevents infection of the foetus or newborn.

Control methods

The control of L. monocytogenes is required at all stages in the food chain and an integrated approach is needed to prevent the multiplication of this bacteria in the final food product. The challenges for controlling L. monocytogenes are considerable given its ubiquitous nature, high resistance to common preservative methods, such as the use of salt, smoke or acidic condition in the food, and its ability to survive and grow at refrigeration temperatures (around 5 °C). All sectors of the food chain should Implement Good Hygienic Practices (GHP) and Good Manufacturing Practices (GMP) as well as implement a food safety management system based on the principles of Hazard Analysis Critical Control Points (HACCP).

Food manufacturers should also test against microbiological criteria, as appropriate, when validating and verifying the correct functioning of their HACCP based procedures and other hygiene control measures. In addition, producers manufacturing food associated with risks of Listeria must conduct environmental monitoring to identify and eliminate niche environments, including areas that favor the establishment and proliferation of L. monocytogenes.

Modern technologies using genetic fingerprint - Whole Genome Sequencing (WGS) - allow for more rapid identification of the food source of listeriosis outbreaks by linking L. monocytogens isolated from patients with those isolated from foods.

Prevention

L. monocytogenes in food are killed by pasteurization and cooking.

In general, guidance on the prevention of listeriosis is similar to guidance used to help prevent other foodborne illnesses. This includes practicing safe food handling and following the WHO Five Keys to Safer Food (1. Keep clean. 2. Separate raw and cooked. 3. Cook thoroughly. 4 Keep food at safe temperatures. 5. Use safe water and raw materials.)

• Poster: Five Keys to Safer Food

Persons in high risk groups should:

• Avoid consuming dairy products made of unpasteurized milk; deli meats and ready-to-eat meat products such as sausages, hams, patés and meat spreads, as well as cold-smoked seafood (such as smoked salmon);
• Read and carefully follow the shelf life period and storage temperatures indicated on the product label.

It is important to respect the shelf-life and storage temperature written on labels of ready-to-eat foods to ensure that bacteria potentially present in these foods does not multiply to dangerously high numbers. Cooking before eating is another very effective way to kill the bacteria.

WHO response

WHO promotes the strengthening of food safety systems, good manufacturing practices and educating retailers and consumers on appropriate food handling and avoiding contamination. Educating consumers, especially those in high risk groups, and training of food handlers in safe food handling are among the most critical means to prevent foodborne illnesses including listeriosis.

WHO and FAO have published an international quantitative risk assessment of Listeria in ready-to-eat foods This has formed the scientific basis for the Codex Alimentarius Commission Guidelines on the Application of General Principles of Food Hygiene to the Control of Listeria Monocytogenes in Foods. This guidance includes microbiological criteria (i.e. maximum limits for the presence of L. monocytogenes in foods)

WHO’s main tool to assist Member States in surveillance, coordination and response to outbreaks is the International Network of Food Safety Authorities (INFOSAN) which links national authorities in Member States in charge of managing food safety events. This network is managed jointly by WHO and FAO.