A contaminated meat slicer was behind four Listeria infections in an Italian hospital in late 2020, according to a study.

Researchers said it was the first report of an outbreak of listeriosis caused by sequence type (ST) 451 in Italy.

Infections involved one immunocompromised and three cancer patients in different units of a hospital in the city of Latina, Lazio, in the center of Italy in September and October 2020.

A 53-year-old man died, a 75-year-old man and 55-year-old woman were discharged to go home and a 39-year-old man was referred to hospice care, according to the study published in the journal Pathogens.

Two patients consumed only hospital meals, while another said she also ate parmesan cheese brought from home once and stored in a thermic bag for a few days.

From the available menus of affected patients, it was not possible to identify one food product as the common direct source of infection.

Outbreak strain found in hospital kitchen
Researchers said findings support the hypothesis of a secondary contamination event involving a food treated with the slicer. However, the original source of the kitchen contamination by this strain was not identified.

The only environmental sample positive for Listeria monocytogenes was a kitchen slicer that was dedicated to cutting raw meat and, after a sanitizing step, cooked meat such as roasts.

After identification of the meat slicer as the contaminant source, a temporary suspension was put in place and external service relied on for the supply of packaged meals so the hospital kitchen could be sanitized. Following the absence of Listeria monocytogenes, after five days the cooking center was reopened.

The epidemiological investigation involved traditional microbiological methodology and whole genome sequencing.

Scientists analyzed the sequence data of strains isolated from seven listeriosis cases treated in the same hospital since 2017 but only the latest four were related.

A further 10 cases of Listeria monocytogenes ST451 were found in a database of sequenced Listeria monocytogenes clinical isolates collected in Italy in recent years with one each from Lazio, Tuscany, Umbria, four from Lombardy, and three from Piedmont.

Researchers said healthcare professionals should advise people at increased risk for severe listeriosis to avoid foods that may be more likely to be contaminated, such as lightly cooked or raw ready-to-eat products. Hospitals and healthcare facilities should also be aware of the risk of Listeria monocytogenes contamination of food service equipment and food processing plants, they added.