Multi-country outbreak of Listeria monocytogenes serogroup IVb, multi-locus sequence type 6, infections linked to frozen corn and possibly to other frozen vegetables – first update
An outbreak of invasive Listeria monocytogenes (L. monocytogenes) infections confirmed by whole-genome sequencing (WGS) and linked to frozen corn and possibly to other frozen vegetables has been ongoing in five EU Member States (Austria, Denmark, Finland, Sweden and the United Kingdom) since 2015. As of 15 June 2018, 47 cases have been reported and nine patients have died due to or with the infection (case fatality rate 19%). WGS analysis of 29 non-human L. monocytogenes isolates found them to be closely related to the multi-country human cluster of L. monocytogenes serogroup IVb, multi-locus sequence type 6 (ST6). The majority of the non-human isolates were obtained from 2017 season products: mainly frozen corn (13 samples), followed by frozen vegetable mixes including corn (8 samples), frozen spinach (1) and frozen green beans (1). Only one isolate was reported from a frozen vegetable mix produced in 2016, while three isolates were obtained from spinach products produced in 2018. In addition, two isolates were also obtained from two environmental samples collected at two different plants which were freezing and handling frozen vegetables in France and Hungary during the 2017 and the 2018 production seasons, respectively. The WGS analysis provides a strong microbiological link between the human and the non-human isolates and this is indicative of a common source related to frozen corn and other frozen vegetable mixes, including corn, persisting in the food chain. Traceability information for the contaminated products pointed to the source of contamination in a freezing plant in Hungary (company A). As L. monocytogenes IVb ST6 matching the outbreak strain has been isolated from frozen spinach and frozen green beans sampled at the Hungarian plant, it is possible that frozen vegetables other than corn which have been processed in this plant, could also be implicated as a vehicle of human infection. The finding of L. monocytogenes IVb, ST6 matching the outbreak strain in frozen corn and other frozen vegetables produced during the 2016, 2017 and 2018 production seasons at the plant of Hungarian company A suggests that this strain could be persisting in the environment of the processing plant after standard cleaning and disinfection procedures carried out during periods of no production activity and the rotation of the processed products. Moreover, the use of the contaminated production lines for several food products may represent an additional risk for potential cross-contamination of the various final products processed at the plant. The information available confirms contamination within the Hungarian processing plant, but does not yet enable identification of the exact point(s) and/or stage in production at which L. monocytogenes contamination has occurred. Further investigations, including thorough sampling and testing, are needed to identify the source of contamination at the Hungarian processing plant concerned. Consumption of frozen or non-frozen corn has been confirmed by eleven out of 26 patients interviewed from Denmark, Finland, Sweden and the United Kingdom. Of the 15 cases that did not report corn consumption, two replied that they had consumed non-frozen mixed vegetables, three cases reported no consumption of corn or mixed vegetables, six cases did not know if they consumed corn or mixed vegetables, four cases had possibly not consumed corn and one of these four had possibly consumed frozen mixed vegetables. Food business operators in Estonia, Finland, Poland and Sweden have withdrawn and recalled the implicated frozen corn products from the market. Since March 2018, the implicated Hungarian plant has been under increased official control and no frozen vegetable products from the 2018 production season have been distributed to the market yet. Following the positive findings from food and environmental samples collected during the 2018 production, freezing activities at the affected Hungarian plant have been halted since June 2018. On 29 June 2018, the Hungarian Food Chain Safety Office banned the marketing of all frozen vegetable and frozen mixed vegetable products produced by the plant between August 2016 and June 2018, and ordered their immediate withdrawal and recall. This restrictive measure is likely to significantly reduce the risk of human infections and contain the outbreak. As the outbreak is still continuing or at least has been ongoing until very recently, there are indications that contaminated products may still be on the market or that contaminated products purchased before the recalls are still being consumed. Any potentially contaminated frozen vegetables (e.g. frozen corn, frozen vegetable mixes including corn, frozen spinach and frozen green beans) from the 2017 and 2016 production seasons could still represent a possible risk to consumers until completely withdrawn and recalled. This risk may exist even at a low level of contamination if the products are not properly cooked before consumption. In addition, new invasive listeriosis cases may be identified due to the long incubation period (1–70 days), the long shelf-lives of frozen corn products, and potential consumption of frozen vegetable products bought by consumers before the recalls and eaten without being properly cooked.