Invasive Listeriosis in Southern Switzerland: A Local Problem That Is Actually Global

To the Editor—Recently, Conrad et al [1] reported 10 cases of Listeria monocytogenes invasive disease with 3 deaths in a 6-year time frame (2010–2015). Whole-genome sequencing (WGS) demonstrated a link to a widely distributed brand of ice cream, which enabled the source to be narrowed to 2 production facilities, indicating long-standing contamination.

We describe here an outbreak of 6 cases of invasive L. monocytogenes infections in Ticino region (a canton of approximately 350 000 inhabitants in the south of Switzerland, close to the Italian border) from July to September 2022. Table 1 provides the demographic and clinical characteristics of the patients; note that 4 patients had L. monocytogenes serotype 4b, and 2 had serotype 1/2b. Some weeks before this outbreak, a similar event occurred in Italy [2]; 2 patients, both infected with serotype 4b, had bought fresh cheese in Italy from the same grocery store. However, WGS of the isolates was not performed, so we could not establish whether the strains were related or identify a common source. As for the remaining patients, they denied assumption of cheese or pork derivate in the weeks before hospitalization, and cohabitants did not refer symptoms correlated with L. monocytogenes infection.

Table 1.

Demographic and Clinical Characteristics of Patients With Invasive Listeriosis Diagnosed July–September 2022

Characteristic	Patient 1	Patient 2	Patient 3	Patient 4	Patient 5	Patient 6
Sex	M	M	M	F	M	M
Age, y	73	73	69	68	78	82
Comorbid conditions	Heart failure, DM, anemia, liver disease, chronic kidney failure	Ischemic heart disease, AF, chronic kidney failure with dialysis	Heart failure, AF, COPD	AF, kidney transplantation, COPD	Ischemic heart disease, lung cancer	Heart amyloidosis
Clinical diagnosis	Bacteremia	Meningitis	Subdural empyema	Meningitis and cholecystitis	Meningitis	Meningitis
Serotype	4b	1/2b	4b	4b	4b	1/2b
Cause of contagion	Unknown	Unknown	Contaminated cheese from Italy	Contaminated cheese from Italy	Unknown	Unknown
Treatment duration	Amoxicillin + gentamicin for 4 d, followed by amoxicillin alone for 17 d (gentamicin stopped after 4 d for acute kidney failure)	Amoxicillin + gentamicin for 2 wk	Amoxicillin + gentamicin for 2 wk, then amoxicillin + TMP/SMX for 4 wk, then amoxicillin for 6 wk	Amoxicillin + gentamicin for 2 wk, then amoxicillin for 4 wk	Amoxicillin + gentamicin for 2 wk, then amoxicillin for 4 wk	Amoxicillin + gentamicin for 2 wk, then amoxicillin for 4 wk
Outcomea	Alive	Alive	Alive	Alive	Alive	Alive

Abbreviations: AF, atrial fibrillation; COPD, chronic obstructive pulmonary disease; DM, diabetes mellitus; F, female; M, male; TMP/SMX, trimethoprim-sulfamethoxazole.

Outcome defined as survival at 6 months after hospital discharge.

Invasive listeriosis is associated with a high mortality rate and usually affects immune-compromised individuals. In the present series, antibiotic treatment with amoxicillin and gentamicin was effective, and no deaths were observed 6 months after discharge. We speculate that the isolation of 2 different serotypes in such a short time frame might be due to 2 concomitant outbreaks; nevertheless, we could not identify any confirmed source, nor any correlation among the patients’ habits or environment. Although not demonstrated with WGS or other genotypic analysis, it remains plausible that the cases due to serotype 4b have a common source. It would be of interest to compare WGS results from isolates in Ticino with other isolates from recent cases in surrounding areas of Switzerland and Italy. In contrast to the findings by Conrad et al [1], who found serotypes 1/2a, 1/2b, and 3b to be predominant, serotype 4b is commonly associated with outbreaks of invasive disease in Switzerland. As reported with WGS, clinical isolates were traced to environmental contamination of a cheese dairy with L. monocytogenes serotype 4b, sequence type 6 [3].

Sporadic outbreaks of L. monocytogenes are periodically reported in different countries [4–6], despite control measures and warnings. To identify the source of infection early, we suggest that more effective tracking of the products, thorough analysis of each patient's environment, and a microbiological assessment of the likely involved food should be scheduled as soon as a single case is reported. Given that the Ticino region is so close to the Italian border, it is difficult to draw the supply map, but epidemiological studies should be carried out as most accurately as possible, together with WGS typing of the isolates to enable early recognition of the source. Dealing with outbreaks of L. monocytogenes should be a joint effort between public health authorities, local microbiological laboratories, and hospitals.

Author notes

Potential conflicts of interest. All authors: No reported conflicts. All authors have submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest.