Introduction
Recently there have been several Listeria monocytogenes outbreaks around the world. In Europe, the most notable occurrence – in a traditional surface-ripened cheese called ‘Quargel’ - caused the death of several people in Austria. Therefore the MoniQA Emerging Issues and Microbiological Contaminants Working Groups have produced the following overview of the key issues in connection with L. monocytogenes and recent outbreaks.
What is Listeria monocytogenes?
L. monocytogenes is a bacterium responsible for listeriosis, a rare but potentially lethal food-borne disease. L. monocytogenes occurs in soil, water and sewage, and has been found in uncooked meats and vegetables, and unpasteurized milk including some varieties of cheese (see below) and occasionally in processed foods. Listeria spp. are killed by pasteurization and cooking.
Although the incidence of listeriosis is low compared with outbreaks of other food-borne diseases (such as Salmonella spp.), its mortality rates are high (20% and more), which makes L. monocytogenes one of the most important food pathogens. In humans, listeriosis primarily affects so-called at-risk groups including pregnant women, newborns, elderly people and those with a compromised immune system (e.g. people with HIV/ AIDS, cancer, diabetes and transplant patients). Symptoms range from flu-like aches and pains to meningitis, encephalitis and pneumonia.
Foods associated with L. monocytogenes contamination are often ready-to-eat products such as deli meats, seafood, surface-ripened cheeses and other dairy products. Sources of contamination are variation in processing, preparation and storage conditions. L. monocytogenes can easily spread, and can be introduced into the food chain for instance through vermin or plant material. This means contamination can appear independent of the type of food. Listeria spp. can exist manufacturing environments for long periods, and are capable of surviving in biofilms on surfaces.
How (un)safe is cheese?
The prevalence of L. monocytogenes in cheese making across Europe and North America is about 8% (but 12 %, for fishery processing and about 20% in households). Certain conditions (e.g. depending on packaging and atmosphere) encourage the growth of L. monocytogenes. Such products are classified as ‘high risk’ and covered by the European regulation No. 2073/ 2005 on microbiological criteria for foodstuffs.
L. monocytogenes can occur both in cheeses made from raw and pasteurized milk. While L. monocytogenes is not able to survive pasteurization it can enter a pasteurized product from other sources and ingredients. Evidence suggests prevalence is higher in pasteurized products than raw-milk based foods. Regardless raw-milk quality must be satisfactory with respect to animal health and nutrient profile (i.e. protein content and enzyme activity) for use in foodstuff manufacture.
Contamination can also arise through fermentation and ripening of cheese, which is strongly dependent on the type of starter cultures (mainly naturally isolated from raw milk) which must be capable of “outcompeting” potential pathogens (such as L. monocytogenes) under normal predefined conditions (e.g. moisture, acidity, temperature). In the case of surface-ripened cheeses, accidental use of contaminated ripening culture can contaminate the entire product. If hygiene control measures are not followed in all production areas, the risk of cross-contamination also increases dramatically.
These examples provide a good overview of the factors influencing the safety of cheese and dairy products, which may be applicable to other foods. Several national and international codices, which outline good and safe manufacturing of cheese, have been established. These cover issues such as the quality of milk (including microbial levels), maintaining proper cheese making and ageing environments, personal hygiene, sanitation measures, and proper recording from production to point-of-sale.
The “Quargel” case in Austria
Cases of listeriosis have increased in Austria in recent years but the case fatality rate remained fairly stable over the period 1997-2007 (150 cases). This steady increase was also noted in other EU member states. On 22nd January 2010 authorities were notified, via the EU-wide RASFF (Rapid Alert System for Food and Feed) system, that acid curd cheese produced by the company Prolactal had been contaminated by L. monocytogenes. A day later, these products were removed from the market and the public was informed. In Austria, 24 people contracted listeriosis, five of them fatally, resulting in detailed media coverage.
At the end of February, the cause of the contamination became clear. The media reported that a beetle (Aphodius fimetarius) had been identified as the likely carrier. The production plant was cleaned and disinfected. However, further investigations (as pertains to some earlier outbreaks in 2009) are still ongoing. Authorities have charged Prolactal with negligence and endangering public safety with fatal consequences. The company states that the contamination resulted from a failure in the manufacturing process and quality control system. The incident has called into question the adequacy of current regimens for food business operators to protect products from cross-contamination and the Austrian health minister, Alois Stöger, has been widely criticised.
Other recent cases around the world (different foods)
24 Aug 2009 - A listeria (stat) outbreak that hit five Virgin Blue airline passengers in Australia was traced to a poultry supplier in New South Wales (NSW). GMI Food Wholesalers Pty Ltd., trading as Australian Poultry and Food Wholesalers, was named last week in connection with a spate of illnesses earlier this year from the potentially deadly bug. Media reports in the country said two pregnant women affected with Listeria food poisoning subsequently gave birth prematurely.
18 Dec 2009 Superior Sausage and Meat Products Ltd recalled two sausage products suspected of being contaminated with Listeria monocytogenes. The company, which raised the alarm through the Canadian Food Inspection Agency (CFIA), said its Village Superior Sausage and Ham Sausage were suspected of being contaminated with the potentially deadly bug.
17 Feb 2010 – Kellogg warned by US food regulators over a raft of food hygiene violations after the discovery of widespread Listeria contamination at a waffle plant. Kellogg was guilty of “significant deviations from the current Good Manufacturing Practice (CGMP) regulations” at its Eggo Buttermilk Waffle plant in Georgia, said the Food and Drug Administration (FDA) in it warning letter sent to the company on 27 January. The agency’s district controller in Atlanta also said that a reply received from Kellogg in November had failed to address the CGMP shortcomings.
26 Feb 2010 – In the UK, Pro-Pak Foods Ltd. recalls various batches of sliced chicken roll due to contamination with L. monocytogenes.
16 Mar 2010 - Consumers in Canada warned not to eat Siena Foods cooked ham on fears of L. monocytogenes contamination.
Some national summaries – recalls due to L. monocytogenes contamination
Food Standards Australia New Zealand – at least one recall almost every month over last 12 months (eight in meat, one in cheese: soft cheese)
Canadian Food Inspection Agency – at least one recall almost every month over last 12 months (17 in meat, three in fish, four in sandwich products/egg, three in leeks, one in mushrooms, one in cheese: described as “fresh cheese”)
US Food Safety and Inspection Service – seven recalls in 2009 (six in meat, one in ready-to-eat meal)
UK Food Standards Agency – six recalls in 2008 (five in meat, one in cheese: Unpasteurised Soft Mould Ripened Cheese)
created: April 2010
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