If this is the case and control measures are not taken a new increase could be observed every summer, they said.
An increase of S. Chester in September 2015 was seen with 55 cases (at least 16 with travel history to Morocco) in France, 36 in Belgium, seven in Spain and four in Denmark.
In 2016 in France, there was an increase again with 70 cases between April and October, 16 had travel history to Morocco.
There was also a slight increase in S. Chester cases in France during the summer 2013 with 14 during August - September 2013 compared with four in 2012 for the same period.
Between 2009 and 2013, EU and European Economic Area countries reported through The European Surveillance System (TESSy) a mean of 91 S. Chester cases per year.
Multinational outbreak of S. Chester
Findings come from an investigation into a multinational outbreak of travel-related S. Chester infections in Europe in the summers of 2014 and 2015.
The European Centre for Disease Prevention and Control (ECDC) was informed of an increase in S. Chester cases between 2014 and 2015 with travel to Morocco in six European countries.
A total of 162 cases, mostly from France, followed by Belgium, the Netherlands, Spain, Denmark and Sweden were reported and chicken and shrimp in Morocco were possible sources.
The European-wide outbreak was likely multi-source with several food vehicles contaminated by multidrug-resistant S. Chester strains.
The true extent was probably larger than observed with unreported cases in visitors to and residents of the affected area, said the researchers.
No S. Chester outbreak was reported by Moroccan authorities before and during the investigation.
Seafood as infection source?
Researchers obtained travel history for 55 cases and 45 (82%) had recently travelled to Morocco.
Based on cases interviewed in the Netherlands, Belgium, Spain and Denmark the food exposures most frequent were chicken (22 cases of 25 for which this information was available), grapes (17/20) and pasteurised milk (21/25).
In France, food exposures most frequently mentioned were ice cream (14 of 15 cases; 14/15), grapes (10/11), chicken (13/15), pasteurised milk (13/15) and spreadable cheese (13/15). Shrimps were mentioned by five of 15 cases.
“We found significant associations between S. Chester infection and shrimp consumption, visiting the coast and restaurant attendance before symptoms,” said the researchers.
“The OR [odds ratio] associated with squid consumption was high although it did not reach statistical significance. These results suggest that seafood, shrimp in particular, could be one of the sources of this outbreak.”
Multiple other sources were suggested by molecular and WGS analysis of non-human strains.
“The chicken sausage could explain the human cases with isolates carrying the incN-qnrS1 plasmid that appeared in 2014 and the turkey meat, some of human cases of 2015 with the Col-qnrB19 positive isolates,” said the researchers.
“The fishmeal and the decanted water samples, also contaminated by Col-qnrB19 strains, may indicate the possible contamination of the environment by Moroccan poultries.”
In France, the Salmonella surveillance system is based on a voluntary network of labs that send or report isolates to the French National Reference Center for E. coli, Shigella and Salmonella (NRC).
In September 2014, the French NRC notified Santé publique France of an increase in numbers of S. Chester isolates, with 31 received between August and September 2014. Most cases had travelled to Morocco within two weeks prior to symptom onset.
During the same period, Belgium made a similar notification to the European Epidemic Intelligence Information System (EPIS) with 18 S. Chester cases. The Netherlands, Spain and Denmark reported clusters of, respectively eight, six, and four cases and Sweden one case.
Despite the source(s) being in Morocco, making it more difficult to investigate than an outbreak with a source in an EU country, multinational collaboration enabled information sharing as part of epidemiological and microbiological investigations.
Source: Eurosurveillance, Volume 22, Issue 7, 16 February 2017
Authors: L Fonteneau, N Jourdan Da Silva, L Fabre, P Ashton, M Torpdahl, L Müller, B Bouchrif, A El Boulani, E Valkanou, W Mattheus, I Friesema, S Herrera Leon, C Varela Martínez, J Mossong, E Severi, K Grant, F Weill, CM Gossner, S Bertrand, T Dallman, S Le Hello