An outbreak of Listeria monocytogenes in five European countries has infected at least 47 people, killing nine of them.

Eighteen of the cases have been reported this year, with the most recent person becoming sick in May. The outbreak is believed to have begun in 2015.

Frozen corn and other frozen vegetables are the likely source of the outbreak that has hit Finland with 23 cases, the United Kingdom with 11, Sweden with seven, Denmark with four and Austria with two. For the 16 patients for whom the information is available, all were hospitalized.

The European Centre for Disease Prevention and Control (ECDC) and the European Food Safety Authority (EFSA) reported that whole genome sequencing was used to identify the food source, which was thought to be limited to just frozen corn.

“It is also likely that the extent of this outbreak has been underestimated since the outbreak was identified through sequencing and only a subset of the EU/EEA countries routinely use this advanced technique to characterize L. monocytogenes isolates,” said the agencies.

Listeria monocytogenes IVb sequence type (ST) 6 that matches the outbreak strain was isolated from frozen spinach and frozen green beans sampled at a Hungarian plant.

The Hungary based facility in Baja is owned by Greenyard, a producer of fresh, frozen and prepared fruit and vegetables, which claims to have Europe’s leading retailers amongst its customer base.

The company said it has stopped production at the plant while it conducts a review of production lines and is looking into alternative sourcing possibilities to supply customers.

The same strains of Listeria monocytogenes have been detected in frozen vegetables produced by the company in 2016, 2017 and 2018. EFSA said this suggests the bacteria has persisted in the processing plant despite cleaning and disinfection.

The agency reported further investigations, including sampling and testing, is needed to identify exact points of environmental contamination.

Since March this year, the Hungarian plant has been under increased official control and no frozen vegetable products from the 2018 production season have been distributed.

Following positive findings from food and environmental samples collected during 2018 production, freezing activities at the site have been halted since June.

In late June, the Hungarian Food Chain Safety Office banned the marketing of frozen corn, peas, beans, spinach and sorrel made by the plant between Aug. 13, 2016, and June 20, 2018. The authorities also ordered a product withdrawal and recall.

Greenyard said it is not yet in a position to estimate the financial impact of the recall. The turnover of Greenyard Frozen Hungary for 2017-2018 was €24 million.

Frozen corn and vegetable mixes have been distributed to plants belonging to the company in other EU Member States, including Belgium, UK, Germany, France and Poland. Final products were also sent to Romania, Italy, Slovenia, Slovakia, Germany, Finland, Czech Republic, Croatia and Austria.

The plant of a Polish company, initially considered as a possible point of contamination, was excluded after environmental sampling and testing showed the points of contamination by a Listeria monocytogenes strain not related to the outbreak.

Consumption of frozen or non-frozen corn has been confirmed by 11 of 26 infected patients interviewed from Denmark, Finland, Sweden and the UK.

Of the other 15 cases, six consumed or possibly consumed frozen mixed vegetables, six did not know if they ate corn or mixed vegetables, and three reported not having eaten corn or mixed vegetables.

The UK has reported two deaths in relation to the outbreak.

The Food Standards Agency, Food Standards Scotland, Public Health England and Health Protection Scotland said people should follow manufacturers’ instructions when preparing frozen sweet corn.

If the product is not labelled as “ready to eat,” cooking instructions should be followed before eating the food hot or cold.

Dr. Kathie Grant, head of Gastrointestinal Bacteria Reference Unit at PHE, said the best way to prevent listeriosis is good food hygiene.

“We have been working with partners to identify the cause of 11 cases of listeriosis dating back to 2015, which are part of a larger outbreak across Europe,” she said.

“Most people won’t have any symptoms of the infection or will only experience mild symptoms such as vomiting and diarrhoea, which usually pass within a few days without the need for treatment. More serious infection can develop in those with weakened immune systems or in vulnerable groups including babies, the elderly or pregnant women.”

Listeriosis is an infection caused by Listeria monocytogenes bacteria. People usually become ill after eating contaminated food.

New cases may be identified due to the long incubation period of listeriosis, which is up to 70 days, the long shelf life of frozen vegetables, and possible future consumption of implicated product bought before the recall.

(To sign up for a free subscription to Food Safety News, click here.)