A severe outbreak of listeriosis in central Italy with a rare pulsotype associated with processed pork products.

PURPOSE: From May 2015 to March 2016, an outbreak due to Listeria monocytogenes serotype 1/2a and clinical pulsotype never previously isolated in Europe occurred in central Italy, involving 24 confirmed clinical cases. The article provides a description of the outbreak and the investigation carried out by a multidisciplinary network.

METHODOLOGY: Epidemiological and microbiological surveillance was conducted to confirm the outbreak and to detect the food vehicle of infection. The origin and destination of the implicated food and its ingredients were investigated by tracing-back and -forward investigation.

RESULTS

Next-generation sequencing confirmed the unique outbreak strain. On 4 January 2016, a L. monocytogenes strain with pulsotype indistinguishable from that isolated from clinical cases in the outbreak was detected in a sample of hog head cheese purchased from a retail supermarket by one of the patients. The hog head cheese was produced by a small meat processing plant in the Marche region, where microbiological investigation confirmed environmental and food contamination by the outbreak strain. Plant production was suspended and all contaminated batches of the hog head cheese were withdrawn from the market by 19 February by local health authority. We subsequently observed a sharp decline in clinical cases, the last being reported on 11 March 2016.

CONCLUSION

The key factor in the timely conclusion of this investigation was intersectoral collaboration among epidemiologists, microbiologists, veterinarians, statisticians and health and food safety authorities at national, regional and local levels.