Listeriosis represents a serious invasive illness that predominantly affects pregnant women, newborns and immunocompromised individuals. Contaminated foods are vehicle used by the causative organism, Listeria monocytogenes, for its transmission to humans. Bigger and smaller outbreaks of listeriosis have been reported throughout the globe.

The public health significance of listeriosis is not adequately recognized, especially since it is considered rare in comparison to other foodborne diseases. Nonetheless, due to its severity and high mortality, listeriosis is considered as one of the most frequent causes of death when foodborne illnesses are concerned.

Prevalence and incidence of the disease

Listeriosis has a low incidence rate, and is reported almost exclusively in industrialized countries (data from Asia, Africa and South America is scarce). In Europe, the incidence of this disease is approximately three cases per million inhabitants, but increased rates have been reported in countries such as Germany, Estonia, Latvia, Lithuania, Italy, Netherlands and the UK. Furthermore, in Austria the incidence of listeriosis doubled between 1997 and 2007.

In Canada and the USA, the reported incidence is approximately four and three cases per million inhabitants, respectively. The standout outbreak of listeriosis in the United States occurred in 2011, which was traced back to contaminated cantaloupes that resulted in 142 hospitalizations and 33 deaths. From August 2008 to December 2008, 22 people died in the 2008 Canadian listeriosis outbreak.

A wide range of food products have been associated with the transmission of listeriosis, primarily milk and dairy products, meat products (cooked chicken, pâté, rillettes, sausages, salami), vegetables (coleslaw and rice salads, celery, alfalfa) and, in a lesser degree, fish products (smoked fish, shellfish, shrimp). Generally, these foods have extended shelf life, do not require further cooking and support the growth of the bacterium.

Listeria monocytogenes most frequently associated with listeriosis cases belong to serotypes 1/2a, 1/2b, and 4b. Epidemiological monitoring studies employ different genotyping techniques, with pulse field gel electrophoresis (PFGE) as the golden standard due to its discrimination and reproducibility for molecular sub-typing of this pathogen.

Risk groups

Medical advances and demographic changes have resulted in an increase in the size of certain at-risk groups for listeriosis, namely the elderly and immunocompromised patients. It must be noted that, even though listeriosis can cause disease in all populations, more than 50% of all cases occur in those older than 65 years of age.

In addition, certain groups within the general population that have an underlying condition which can lead to the impairment of T-cell-mediated immunity are particularly susceptible to this infection – namely immunocompromised individuals on certain medications (such as organ transplant patients) and HIV-infected individuals.

Listeriosis is approximately 18 times more common in pregnant women than in the general population. Furthermore, pregnant women account for 16-27% of all listerial infections, which can cause mild illness in mothers, but can be devastating to the fetus, in some cases leading to severe disease or fetal death. The incidence of listeriosis in the newborn is estimated at a rate of 8.6 per 100,000 live births.