Large Nationwide Outbreak of Invasive Listeriosis Associated with Blood Sausage, Germany, 2018-2019

Abstract

Invasive listeriosis is a severe foodborne infection in humans and is difficult to control. Listeriosis incidence is increasing worldwide, but some countries have implemented molecular surveillance programs to improve recognition and management of listeriosis outbreaks. In Germany, routine whole-genome sequencing, core genome multilocus sequence typing, and single nucleotide polymorphism calling are used for subtyping of Listeria monocytogenes isolates from listeriosis cases and suspected foods. During 2018-2019, an unusually large cluster of L. monocytogenes isolates was identified, including 134 highly clonal, benzalkonium-resistant sequence type 6 isolates collected from 112 notified listeriosis cases. The outbreak was one of the largest reported in Europe during the past 25 years. Epidemiologic investigations identified blood sausage contaminated with L. monocytogenes highly related to clinical isolates; withdrawal of the product from the market ended the outbreak. We describe how epidemiologic investigations and complementary molecular typing of food isolates helped identify the outbreak vehicle.

Keywords: Germany; Listeriosis; food safety; foodborne diseases; molecular surveillance; outbreak; public health; whole-genome sequencing.

Figure 1

Phylogenic tree constructed by using unweighted pair group method with arithmetic mean and core genome multilocus sequence typing data of Listeria monocytogenes isolates from a large listeriosis outbreak, Germany. Green indicates clinical isolates of Epsilon1a subcluster; blue indicates food isolates of Epsilon1a subcluster; pink indicates isolates from the Epsilon1 cluster; violet indicates 2 complex type 4465 isolates not belonging to Epsilon1a from earlier listeriosis cases in July 2017 and June 2018; yellow indicates isolates from a listeriosis outbreak in South Africa (11); black indicates reference strain 10-092876-0769 LM12 used for SNP calling (Appendix Figure 2). Scale bar indicates allelic substitutions per site. SNP, single-nucleotide polymorphism.

Figure 2

Spatial and temporal distribution of cases during a large listeriosis outbreak, Germany. A) Number of Listeria monocytogenes isolates from subcluster Epsilon1a received by the consulting laboratory per week during the outbreak. B) Geographic distribution of laboratory-confirmed Epsilon1a cases in Germany during the outbreak. CW, calendar week.

Figure 3

Tolerance of isolates of Listeria monocytogenes from subcluster Epsilon1a in Germany to benzalkonium chloride. Three representative isolates from human listeriosis clusters Epsilon1a, Epsilon1, and distinct listeriosis clusters Lambda2 (ST2, CT2402), Pi3 (ST217, CT5744), or Theta3 (ST249, CT4449) were tested for resistance to benzalkonium chloride by disc diffusion, along with 3 representative ST6 isolates, not belonging to Epsilon1. Epsilon1a and Epsilon1 isolates showed increased resistance to benzalkonium chloride. Circles, squares, and diamonds represent results of 3 independent replicates for 3 isolates per group. Asterisk indicates statistically significant differences to Epsilon1a (p<0.01) calculated by using the Student t-test. CT, complex type; Eps1, Epsilon1; Eps1a, Epsilon1a; Lam2, Lambda2; nonEps1, nonEpsilon1; ST, sequence type.