Listeriosis patient isolates have shown a new identical pulsed-field gel electrophoresis (PFGE) pattern. Almost all isolates (Listeria monocytogenes serotype 1/2a) belong to cases living in southern Germany.
Next generation sequencing (NGS) revealed the unique cluster type CT1248 and confirmed the outbreak.
It is the largest outbreak of listeriosis described in Germany to date.
German states affected
Geographical distribution is largely confined to states of Baden-Wuerttemberg, Bavaria and Hesse.
“The geographical limitation to southern Germany and the size of the outbreak area with a population of 27 million inhabitants suggest Listeria-contaminated food in a supra-regional supermarket grocery chain as the vehicle of infection,” said a report in Eurosurveillance.
Last year 28 cases were reported and in total four were pregnancy-associated and six people have died with three of these confirmed to be due to listeriosis as the major cause.
There was a first peak in the second half of 2013, but most cases have occurred since June 2014 (compared with a total of 609 invasive listeriosis cases in Germany in 2014).
Although the number of new cases has decreased since August 2015, they are still being reported.
“We must therefore assume that the source of infection is still active and further cases are possible. Further epidemiological studies, laboratory investigations and trace-back of food items are needed and ongoing to narrow down the source of infection,” said the report.
The outbreak was communicated via the European Epidemic Intelligence Information System (EPIS) platform on 17 July 2015 and updated on 5 November 2015.
No other participating countries reported cases with the outbreak PFGE pattern or NGS cluster types.
Finding the source
Preliminary results largely exclude fish and cheese products as a possible source but this has to be complemented by systematic screening of Listeria isolates collected from food, said the report.
Investigations of listeriosis outbreaks are difficult due to the multitude of possible food vehicles including a range of ready-to-eat foods.
Initial screening of food-related Listeria isolates in the strain collection of Robert Koch Institute (RKI) and Austrian Agency for Health and Food Safety (AGES) found six isolates (five from Austria and one from Germany) which had indistinguishable PFGE patterns but different NGS cluster types.
Among 62 not pregnancy-associated outbreak patients 32 were men. The outbreak affected 38 senior citizens (≥ 70 years), 23 younger adults (18–69 years) and one two-year-old child.
Source: Eurosurveillance, Volume 20, Issue 50, 17 December 2015
Authors: W Ruppitsch, R Prager, S Halbedel, P Hyden, A Pietzka, S Huhulescu, D Lohr, K Schönberger, E Aichinger, A Hauri, K Stark, S Vygen, E Tietze, F Allerberger, H Wilking