Focus on Listeriosis

What is Listeriosis

Listeriosis is an infectious disease with a worldwide distribution caused by Listeria monocytogenes (L. monocytogenes). Until 1960, listeriosis was rarely reported; starting in 1980, thousands of cases began to be reported annually around the world, demonstrating the great attention created after the infection was associated with the consumption of contaminated food. The main mode of infection is represented, in 98% of cases, by the consumption of foods contaminated with L. monocytogenes, while other less frequent forms of spread are represented by vertical mother-to-child, zoonotic, nosocomial and aerosol transmission. Generally the minimum infectious dose is around 10⁶ CFU (Colony Forming Units) per gram of food, while for the risk categories, it is around 10² - 10⁴ CFU / g. It is important to consider that the infectious dose capable of causing infection is influenced not only by the virulence and pathogenicity of the bacterial strain and by the amount of food consumed in relation to its level of contamination, but also by the state of health of the host. The clinical manifestations, in fact, take on particular gravity in some categories of individuals who generally have a compromise in the functionality of the immune system.

The categories of subjects considered at risk are the following:

• newborns, in the first 30 days of life
• elderly, over the age of 65 in whom the activity of the immune system is physiologically reduced due to an immunosenescence phenomenon
• pregnant women
• cancer patients
• subjects undergoing organ transplantation
• patients with autoimmune diseases often treated with immunosuppressive therapies and anti-inflammatory drugs, which predispose the subject to infection
• subjects infected with HIV (Human Immunodeficiency Virus)
• diabetic subjects

Among foodborne diseases, listeriosis has the highest percentage of hospitalization and lethality, recorded mainly in elderly subjects.

Listeriosis can manifest in two different clinical forms:

• non-invasive listeriosis: affects subjects without predisposing factors and manifests itself as a self-limiting febrile gastroenteritis, typical of food poisoning
• invasive listeriosis: mainly affects subjects belonging to risk categories, in which the infection of the primary target organs (liver and spleen) is not effectively contained and consequently the microorganism can reach other target organs, including the brain. Clinical manifestations of invasive listeriosis are: 

   

  • central nervous system diseases such as meningitis, meningoencephalitis, abscesses; (L.  monocytogenes appears to be responsible for about 10% of the forms of meningitis developed at the population level)
  • sepsis
  • endocarditis and other localized infections such as hepatitis, peritonitis, myocarditis, etc. The incubation period is variable, but can last up to 3 months if the infecting bacterial load is very low

High risk food
The foods most frequently associated with cases of listeriosis are the Ready-To-Eat foods (RTE). These are ready-to-eat foods which, before being consumed, do not require any heat treatment, capable of reducing and/or eliminating any initial contamination by L. monocytogenes. This category includes dairy products, such as soft cheeses, blue cheeses and slightly seasoned cheeses; processed meats such as short-aged cured meats; fresh vegetables and processed fishery products, such as smoked salmon.

Prevention
The best strategy to fight listeriosis passes through an efficient prevention, which can be easily implemented by applying the general rules of hygiene and the precautions required for all other foodborne infections (see the infographic).

Diagnosis
The correct diagnosis of listeriosis is carried out using certain clinical, laboratory and epidemiological criteria (Decision 2018/945/EU)
The diagnosis of invasive listeriosis and forms associated with pregnancy generally occurs through the isolation of L. monocytogenes from a normally sterile site. The samples, mainly blood, cerebrospinal fluid, amniotic fluid, meconium, vaginal swab, ocular, ear, nasal, umbilical swab of newborns, are directly sown on selective and enriched culture media. Once the microorganism has been isolated, identification is carried out by: Gram stain, haemolysis test, biochemical tests. For faster diagnosis of all forms of listeriosis, molecular techniques can be used.

Therapy
The primary therapy for human listeriosis is represented by the combination of antibiotics belonging to the beta-lactam family, such as ampicillin, penicillin and amoxicillin, with an aminoglycoside, for example. gentamicin. Almost all strains of L. monocytogenes are susceptible to most of the antibiotics commonly used in the treatment of listeriosis; however, some multidrug-resistant strains were observed. 

Notification
Notification of human listeriosis cases is mandatory in most EU member states. In Italy, confirmed and sporadic human cases of listeriosis must be reported with the notification methods provided by D.M. 15 dicembre 1990 (Ministerial Decree) for Class II diseases, while epidemic outbreaks must be reported in the manner prescribed for Class IV diseases which include foodborne outbreaks. Furthermore, in compliance with the provisions of Article 7, paragraph 4 of D.L (Legislative Decree) no. 191 of 04 April 2006, foodborne outbreaks must be registered in the “Man section” of the Zoonoses National Information System (SINZOO). Since 2017, according to the provisions of the Circular of the Ministry of Health of 13.03.2017, the reports, both of sporadic cases and of outbreaks of human listeriosis, must be sent to the Ministry of Health and the Istituto Superiore di Sanità; outbreak reports should also be sent to the rapid alert system. Clinical isolates, on the other hand, must be sent to the OCP-ECDC for listeriosis, at the Istituto Superiore di Sanità, for molecular characterization. In the EU, L. monocytogenes infections are subject to public health surveillance under the Foodborne and Waterborne Diseases (FWD) program - https://www.ecdc.europa.eu/en/food-and-waterborne-diseases-and-zoonoses of the European Center for Diseases Prevention and Control (ECDC) through the European surveillance system TESSy case-based (surveillance of clinical cases) and TESSy isolate-based (molecular surveillance on L. monocytogenes isolates); they are also included in the event-based surveillance through the EPIS platform.