Listeriosistrends 2014-2018 report on disease occurrence

Listeriosis trends 2014-2018

In the 2014-2018 period, a total of 280 cases of invasive infection with L. monocytogenes, listeriosis, were recorded. Apart from 2014, when several outbreaks and in particular one very large outbreak occurred, nearly 50 cases have been recorded annually in the period from 2015-2018 [range 39-59], corresponding to an annual incidence of 0.98/100,000 inhabitants in Denmark.


During 2014-2018, a total of 15 confirmed outbreaks with L. monocytogenes were registered in the national Food- and waterborne Outbreak Database (FUD). The larger listeriosis outbreaks were caused by:

  • Spiced meat roll (rullepølse): 2014 brought the largest outbreak of L. monocytogenes recorded to date, counting a total of 41 registered cases. In this outbreak, the source was spiced meat roll. Subsequently, a prolonged outbreak was recorded with another type of L. monocytogenes, in which the source was also cold cuts, and the same type was found in spiced meat roll. In this outbreak, a total of seven people fell ill, including one Sweden.
  • Smoked fish products: During 2014-2018 there has been four outbreaks counting from 6 to 11 cases each, in which smoked fish products have been established as the cause and the same type of L. monocytogenes was found in various products. These outbreaks have been described previously (1), (2), (3) and (4).
  • Other products: In 2013-15, an outbreak counting six cases was recorded. The L. monocytogenes causing this outbreak was also found in an asparagus soup from a hospital kitchen and in frozen meatballs that were used in the soup.


The majority of the invasive infections with L. monocytogenes in 2014-2018 manifested as bacteraemia/sepsis, a total of 199 cases, corresponding to 71%, Table 1. In all, 61 (22%) patients had meningitis. From one to three cases of listeriosis were recorded each year in pregnant women from 2014-2018, a total of 12 cases (corresponding to 4.1/100,000 births). Among the 12 registered cases in pregnant women, eight led to abortion, foetal death or neonatal death shortly after birth.


Distribution by age, sex and area

Figure 2 presents the number of listeriosis cases in 2014-2018 by sex and age groups. A total of 75% of the listeriosis cases were more than 65 years old and 52% of the cases were women. Apart from the cases recorded among pregnant women, the female cases were, on average, four years older than the male cases, whereas the mean number of cases above 70 years of age was identical for women and men (4.6/100,000 per year). In the period from 2014-2018, cases were recorded in all Danish areas (landsdele), and the incidence ranged from 0.74 to 1.3 annual cases per 100,000 inhabitants in the area.


Underlying diseases

Information was collected about underlying conditions predisposing for listeriosis (e.g, cancer, blood disorders, renal disorders and diabetes), immunosuppressant therapy and pregnancy for 263/280 (94%) of the listeriosis cases. Of these, 94% (246/263) had underlying, predisposing conditions, were undergoing immunosuppressant therapy or were pregnant. In all, 44% had one underlying predisposing condition, 25% had two underlying predisposing conditions, whereas 10% had three or more underlying, predisposing conditions.


Case fatality

Of the 280 persons who were registered with listeriosis in the period from 2014 to 2018, 73 died (26%) within 30 days.

Outbreaks and genetically related bacteria

Since 2014, all isolates have been whole-genome sequenced at the SSI, whereby clusters of genetically related bacteria and potential outbreaks may be identified and studied. For all isolates, the multi-locus sequence type (MLST) was established and named according to the internationally established nomenclature. Subsequently, the shared part of the genetic material was analysed by core genome MLST (cgMLST, Moura et al. 2016). When comparing the sequences (using single linkage analysis), it is possible to establish the number of differences between the sequences. Genetically related listeria strains typically have ≤ 7 different alleles.

In the period from 2014-2018, 52% of the listeriosis cases had become infected by a listeria strain that was genetically related to a strain seen in other cases. Among these, 76% were defined as part of an outbreak (Figure 1). The remaining 47% of the listeriosis cases had fallen ill due to a strain that was different from other L. monocytogenes found in patients in 2014-2018 and for the final 1% (n = 2) of the cases, it was not possible to compare with other cases as no isolate was available for sequencing.

Epidemiological follow-up

Since 2014, the Department of Infectious Disease Epidemiology and Prevention has intensified the surveillance of listeriosis. Patients or relatives to patients with listeriosis have routinely been interviewed in an effort to collect information about what they had eaten in the month before their listeriosis symptoms started. These interviews have continuously provided valuable information about possible sources of infection and importantly, they have showed if findings of L. monocytogenes in foods correspond with food items that the patients may have eaten.

In the period from 2014-2018, interviews were conducted with approx. 60% of the patients or their relatives.

Collaboration with the Danish Veterinary and Food Authority/DTU Food

Concurrently with the interviews with all listeriosis cases and whole-genome sequencing of all L. monocytogenes-isolates from patients, whole-genome sequencing of food isolates was also initiated in 2014. Thus, it has become possible to compare sequences from human and food isolates, respectively, which - as described above - has allowed for solving more outbreaks.

This report is also described in EPI-NEWS 18/19.