In May 2016, two separate clusters of gastroenteritis caused by Listeria monocytogenes were detected by the local health authority in Piedmont, northern Italy.
Those affected were students and staff from two different schools in two different villages in the Province of Turin and five people were hospitalised.
An epidemiological investigation identified cooked beef ham served at the school canteens as the outbreak source.
Short incubation period
Researchers said gastrointestinal symptoms occurred rapidly after consumption of the incriminated meals, probably because a large dose of L. monocytogenes was ingested.
The incubation period is usually one to two weeks but can vary between a few and 90 days, according to the World Health Organization.
Since 1993, listeriosis has been a reportable disease in Italy.
L. monocytogenes was isolated from food, stools of hospitalised pupils and environment of the factory producing the cooked beef ham. All isolates except one were serotype 1/2a, shared an indistinguishable PFGE pattern and were 100% identical by whole genome sequencing (WGS).
On 5 May 2016, five children from a school were taken to the paediatric emergency room for symptoms of acute febrile gastroenteritis.
On the same day, 26 of 162 pupils from the nursery and 25 of 400 pupils from the primary school returned home early because of fever, vomiting and abdominal pain.
Inspectors from the local health authority went to the mass catering food service firm that provides meals to the school and collected retention samples from meals served on 3, 4 and 5 May.
From 25 to 29 May, 20 days after the first cluster was identified, children from another school in the same province, reported gastrointestinal symptoms similar to those in the first cluster.
Local health authority inspectors again collected retention samples from the mass catering food service firm (different from the previous one) that provided meals.
Regarding the first cluster, two weeks after the peak, an unopened package from a different batch, was collected from the producer (a cured meat factory) who had supplied the suspected food to the mass catering food service company.
Regarding the second cluster, only retention samples from the meal served on 26 May were available at the mass catering food service firm and an unopened package of the suspected food item from the same batch as the one served was retrieved from the producer.
The same producer supplied the two mass catering food service firms involved in the two clusters.
In the first cluster, symptoms were reported by 174 people and 162 matched the probable case definition. In the second cluster, 43 people matched the probable case definition.
Nine of 23 samples tested positive for L. monocytogenes.
Five stool samples (four from the first cluster and one from second cluster), two food samples (one unopened pack from a different batch obtained during investigation of the first cluster and one retention sample from the second cluster, both with a count exceeding 15,000 cfu/g and two environmental samples from the producer’s site (one each from the cutter machine and the fridge handle).
Isolation of a matching strain from the cutter machine suggests that food contamination occurred in the production plant, said researchers.
“Unfortunately, it was not possible to determinate the exact number of L. monocytogenes in the beef ham sampled during the investigation of the first cluster because the laboratory in charge of official control had not expected a very high level of contamination and therefore only performed the dilutions needed in compliance with the Commission Regulation (EC) no. 2073/2005 to determine the threshold of L. monocytogenes < 100 cfu/g. It was not possible to repeat the analysis because it is mandatory to discard the samples after microbiological investigation.”
They also highlighted the persistence of contamination.
“In our investigation, the time span was 20 days from the first isolation of Listeria in the beef ham (unfortunately the date of production of the contaminated batch was not available) and the isolation from the environment of the cured meat factory where the beef ham was produced. This further confirms that L. monocytogenes can persist in the environment.”
“Outbreak of febrile gastroenteritis caused by Listeria monocytogenes 1/2a in sliced cold beef ham, Italy, May 2016”
Authors: Maurella Cristiana, Gallina Silvia, Ru Giuseppe, Adriano Daniela, Bellio Alberto, Bianchi Daniela Manila, Chiavacci Laura, Crescio Maria Ines, Croce Margherita, D'Errico Valeria, Dupont Maria Franca, Marra Alessandro, Natangelo Ubaldo, Pomilio Francesco, Romano Angelo, Stanzione Stefano, Zaccaria Teresa, Zuccon Fabio, Caramelli Maria, Decastelli Lucia