Outbreak of Listeria monocytogenes in hospital linked to a fava bean product, Finland, 2015 to 2019

Abstract

Listeria monocytogenes (Lm) is a bacterium widely distributed in the environment. Listeriosis is a severe disease associated with high hospitalisation and mortality rates. In April 2019, listeriosis was diagnosed in two hospital patients in Finland. We conducted a descriptive study to identify the source of the infection and defined a case as a person with a laboratory-confirmed Lm serogroup IIa sequence type (ST) 37. Six cases with Lm ST 37 were notified to the Finnish Infectious Diseases Registry between 2015 and 2019. Patient interviews and hospital menus were used to target traceback investigation of the implicated foods. In 2021 and 2022, similar Lm ST 37 was detected from samples of a ready-to-eat plant-based food product including fava beans. Inspections by the manufacturer and the local food control authority indicated that the food products were contaminated with Lm after pasteurisation. Our investigation highlights the importance that companies producing plant-based food are subject to similar controls as those producing food of animal origin. Hospital menus can be a useful source of information that is not dependent on patient recall.