Listeriosis, the “Achilles' heel” of the food sector

Listeriosis is a rare but potentially lethal infection with a mortality rate of 20-30%, comparable to that of salmonellosis, caused by Listeria monocytogenes (Lm) which in recent years has shown an increase in cases.

It thus emerges that 80% of foodborne diseases are attributable to biological agents, viruses and bacteria, parasites and/or their toxins, far more than to chemical contaminants such as organic pesticides (such as residues in vegetables) or heavy metals (as pollutants in the soil and water) or even to additives added to improve preservation and sensory characteristics (preservatives, antioxidants, colorings, flavorings, sweeteners).

In the case of infections and intoxications, responsible for 40% of foodborne diseases, the infection/intoxication is transmitted to humans through the consumption of raw or undercooked foods of animal origin that are contaminated and/or poorly preserved (meat, milk, eggs, fish), or through contact with infected animals (for example: brucellosis, tuberculosis, toxoplasmosis, trichinellosis).

In the case of toxin-mediated foodborne illnesses, responsible for 60% of foodborne diseases, the growth of agents and the production of toxins in chilled cooked foods and prepared gastronomic products are due to incorrect handling techniques, to failures in maintaining the cold or hot chain and/or in preservation, which occur in professional settings (commercial and institutional catering) and in the domestic setting.

Listeriosis is taking on serious implications for food safety and public health, both because of its potential severity and because outbreaks have manifested above all following the serving of contaminated food in large catering chains, frequented every day by millions of consumers of all ages.

In nature there are six species of listeria, of which only LM is the species most implicated in cases of human infection; the probability of contracting listeriosis by consuming foods contaminated with LM is influenced by three main factors: the food matrix, the virulence of the strain and the susceptibility of the consumer.

The form of LM is typically a gram-positive rod (that is, positive to Gram staining), it can also live in the absence of oxygen and develops at temperatures between 3°-45°C, with an ideal growth temperature of 37°-38°C, retaining its viability even at 0°C or at temperatures above 60°C.
It is not very resistant to treatments with chemical and/or physical disinfectants and is destroyed through pasteurization; recent research has shown that it can survive in acidic environments at pH 4.4 and saline concentrations above 20%.

LM is a ubiquitous species that can be found in the soil, in plants, in water and in fertilizers of animal origin, also carried by the feces of humans (2-6% of carriers) and of animals such as calves, poultry, pigs (10-50% of carriers), and one of the main sources of infection is represented by ensiled fodder.

Among its characteristics is that of withstanding low temperatures well, which is why among the food products at greatest risk we find the "cooked and chilled" ones that make up the so-called "ready to eat" category.
Among the products at risk, however, we also find numerous cheeses (soft and semi-hard – fresh or semi-aged) such as Gorgonzola, taleggio, robiole... especially if produced with raw milk or pasteurized at low temperatures for short periods.

Among the risk factors are also raw vegetables, ice cream, pâté, brined tongue, filled sandwiches, raw and cooked poultry, especially as ingredients in gastronomic preparations, and raw cured meats/sausages (in some cases the product was contaminated by unsanitized slicers).
Among fishery products, smoked and marinated fish turn out to be the most vulnerable.

Investigations conducted on at-risk foods have highlighted the following as the main factors of contamination:

• the lack of an effective food safety management system (HACCP)

• the lack of hygiene skills among the staff handling/portioning the food

• the time elapsing between the preparation and consumption of foods and the storage temperature

• the cutting and portioning technologies

• vacuum packaging or packaging in a protective atmosphere

• the sanitization of refrigerators used to store loose semi-finished products

• the sanitization of collection grates and drain traps.

Listeriosis presents in two forms: gastroenteric and invasive.
-The first, with a moderate-to-severe course, is characterized by a high infectious dose, has an incubation time of 6-24 hours and produces symptoms such as diarrhea, fever, headache and abdominal pain.
-The second, with a severe course due to the presence of toxic factors released into the bloodstream (for example: hemolysin, lipolysin, exotoxin), is characterized by a low infectious dose, has an incubation time of 20-80 days and causes meningitis, encephalitis and septicemia.

The dangerousness of LM is due in part to the low infectious dose compared with other bacteria: in fact, just 1,000 germs per gram of product are enough to trigger the disease in healthy adult subjects, and only 100 germs per gram of product in pediatric or weak or immunocompromised subjects, who together with children and the elderly represent the subjects most at risk; but also pregnant women, who have a 20 times greater probability of contracting the infection, with the risk of premature death of the fetus.

The disease presents with the symptoms of pneumonia and meningitis, which are difficult to distinguish at the symptomatic level, and only through analysis of the blood and spinal fluid can the presence of the bacterium be ascertained, which then requires antibiotic treatment.

According to data from the WHO (World Health Organization), the annual incidence of human listeriosis in the world, per million people, ranges from 0.1 to 11.3 cases, 3 cases in Australia and 0.3-7.8 cases in Europe, where in 2004 1,267 cases were documented with 107 deaths. In Finland, where 30-50 cases are recorded each year, an outbreak between December 1998 and February 1999, caused by the consumption of contaminated butter, produced 18 illnesses with 4 deaths. In Germany, LM infections are estimated at between 60-80 cases annually, with 11-16 cases reported each year. In France between 1996 and 2002, there were between 215-230 cases per year. In December 1999, 26 cases of listeriosis with 7 deaths were recorded, associated with the consumption of pork tongue in jelly, and in 2002, 211 cases were recorded, many of them linked to the consumption of cured meats and pâté.

In Italy, a major outbreak of gastroenteric listeriosis occurred in May 1997, an outbreak deriving from the consumption of corn salad and contaminated tuna used in salads, which involved over 1,500 people, of whom 300 were hospitalized. In most cases these were children and the staff of two elementary schools in Turin, while other cases occurred among students of the university in the same city; all the people affected by the infection had eaten at two cafeterias served by the same catering chain. Among the ill, almost. The sad record for listeriosis goes to Sweden in 2007, with 18 deaths out of 44 cases of the disease, while fortunately in Italy there were only 25 cases with no deaths.

From an institutional standpoint, listeriosis is part of the group of diseases for which food safety protocols with mandatory notification have also been established in Europe. The RASFF (Rapid Alert System for Food and Feed), in addition to identifying outbreaks of infection and determining their cause, makes it possible to act by withdrawing products from the market, adopting the necessary measures with regard to production facilities and informing the population of the related risks. The attention paid to LM by the food industry is also due to the possible trade restrictions that can be established as a consequence of any detection of contamination in "convenience foods" or "ready to eat" products.

In 1995 the WTO (World Trade Organization) established that a country may apply restrictions on imports of animals and products of animal origin to protect the health of its own citizens. To reduce the risk of listeriosis, the cold chain and the application of the HACCP system in artisanal and industrial production must be improved, as well as carrying out information campaigns on the correct cooking and preservation of food in the domestic setting.

Prevention rules

The best strategy always passes through efficient prevention, achievable by implementing general hygiene rules

• complete and correct cooking of foods derived from animals

• sanitizing raw vegetables before consumption

• thorough washing of knives and cutting boards after handling raw foods

• scrupulous hand hygiene between one preparation and another

• consumption of perishable foods well within the expiry date

• separation of raw meats from vegetables and from cooked and ready-to-eat foods

• consumption of pasteurized dairy products

And further:

• avoid cross-contamination between cooked foods and raw foods and/or foods coming from supermarkets and delicatessens

• do not eat soft cheeses made with raw milk and remove the rind of soft cheeses

• do not eat fresh artisanal meat pâtés

• avoid smoked fish, unless it is canned with a long expiry date

• reheat foods to high temperatures (>75°C) before they are consumed.

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