"Contamination of tomatoes can happen at various points from the farm-to-fork, such as farm, packinghouses, or fresh-cut processing facilities [26]. They can potentially become contaminated with faeces from wild animals [29] or migratory birds [30,31]. Since water used for irrigation does not need to be potable, this may be a further source of contamination. Additionally, during water restrictions, different types of water can be used. Surface waters, such as rivers, streams or lakes, are more susceptible to contamination than protected sources like wells. The sewage sludge isolates from Germany and Austria support the hypothesis that sewage water can carry harmful pathogens and could be transferred to crops when used for irrigation or applied as a fertiliser [21]."
Eurosurveillance Volume 30, Issue 41, 16/Oct/2025
https://www.eurosurveillance.org/content/10.2807/1560-7917.ES.2025.30.41.2500224
Insights into recurring multi-country outbreaks of Salmonella Strathcona associated with tomatoes, Europe, 2011 to 2024
Introduction
In 2023, non-typhoidal salmonellosis was the second most frequently reported gastrointestinal infection among humans in the European Union and European Economic Area (EU/EEA) countries after campylobacteriosis, accounting for 78,307 cases reported by 30 EU/EEA countries with an incidence of 18.15 cases per 100,000 population [1]. Notification of non-typhoidal salmonellosis is mandatory in 27 EU/EEA countries [1], while in three countries (Belgium, France and the Netherlands), it is voluntary [2]. Non-typhoidal salmonellosis is characterised by diarrhoea, abdominal pain and fever. In some cases, infections may become invasive causing bacteraemia and sepsis, requiring hospitalisation and antimicrobial treatment [3]. The most frequently notified Salmonella enterica subspecies enterica (S.) serovars in humans in the EU/EEA countries are Enteritidis and Typhimurium, including monophasic Typhimurium [2,4].
Prior to 2010, no cases with S. Strathcona (antigenic formula: 6,7:l,z13,z28:1,7) were notified in the EU countries [1]. In early autumn 2011, for the first time, this rare serovar — previously documented only three times globally — was identified in an outbreak affecting multiple European countries, with most cases reported in Denmark. Through consumer purchase data analysis, a case-control study and traceback investigations, the vehicle was found to be small tomatoes originating from Sicily [5,6].
On 27 October 2023, Germany posted a notification on the European surveillance portal for infectious diseases (EpiPulse, maintained by European Centre for Disease Prevention and Control (ECDC)) reporting on 46 cases of S. Strathcona occurring since August 2023. Several other EU/EEA countries also reported cases of S. Strathcona infections during the same period and subsequently, the outbreak was investigated by the affected countries. A first report on the outbreak was issued in the ECDC Communicable disease threats report on 17 November 2023 [7]. Additionally, retrospective genomic analysis linked the 2011 event with the 2023–2024 outbreak period [4,7].
Given the rarity of the occurrence of S. Strathcona before 2011, we suspected that the more frequent occurrence as of 2011 was related to a common source. Therefore, the aims of this study were to: (i) comprehensively describe the epidemiology of S. Strathcona infections in the EU/EEA countries, Switzerland, England and Scotland, from 2011 to 2024; (ii) explore the genomic relatedness of strains throughout this period and (iii) to identify the food vehicle for recent S. Strathcona outbreaks.
Discussion
The results of our multi-country, epidemiological and genomic investigation of S. Strathcona infections in Europe from 2011 to 2024 suggest that the vast majority belonged to yearly recurring outbreaks since 2011, caused by a shared common source. Overall, we identified 643 S. Strathcona outbreak cases (469 confirmed, 161 probable and 13 possible) across 17 European countries. As not all affected countries joined this collaborative investigation coupled with the fact that case identification was largely based on mandatory notification, the true number of infections is likely substantially higher. Furthermore, the outbreak has continued in 2025 with 29 confirmed outbreak cases from nine countries reported by 4 September 2025 [19]. Given the observed seasonality, more cases are expected to occur this year.
Epidemiological and traceback investigation evidence from Austria identified small cherry tomatoes from Sicily as the suspected food vehicle in 2023. We consider the evidence is strong for several reasons. Firstly, in Austria, case interviews in 2023 (also in 2024, data not shown) revealed a remarkably consistent common consumption pattern of small tomatoes on the vine, including similar preferences for product packaging types and purchasing habits. Our binomial probability analysis suggested that the high consumption frequency was unlikely to be due to chance, and we considered it unlikely that a case-control study would produce a different result. Secondly, in France, cherry tomatoes were among the most reported food items and in Germany, general tomato consumption was the highest single food category reported among cases. Thirdly, in 2024, the Italian Food Safety Authority issued a RASFF notification regarding a national food-borne outbreak of S. Strathcona in several schools in Italy, where S. Strathcona was detected in a meal consisting of cherry tomatoes and pesto that had been consumed by the patients [4]. Subsequent traceback also pointed towards cherry tomatoes from Sicily as the common suspected food vehicle.
Since 2011, S. Strathcona infections in Europe have occurred in recurring seasonal pattern every year, from midsummer to year-end. We deem it likely that tomatoes may have been the vehicle in most if not all the years. Notably, a S. Strathcona outbreak investigation conducted in Denmark in 2011 identified small (Datterino) tomatoes from Sicily as the vehicle of the infection. At the time, these tomatoes were not widely distributed in Europe, but had been sold in the other European countries, where cases were reported [6]. This is supported by the fact that Datterino tomatoes are typically in season from early summer to early autumn, with a peak harvest period between July and October [20]. Thus, both the Austrian (2023) and Danish investigations pointed to small tomatoes from Sicily as the suspected vehicle of infection. Furthermore, epidemiological investigations from seven countries in 2024 showed that 90% of interviewed cases had consumed any type of tomato. The consistent seasonal case occurrence and the stable pattern with respect to age and sex throughout the years suggest both a seasonal food product and a consistent population segment with similar food consumption habits over the years. The recurring seasonal pattern suggests that the tomato-growing environment could be a persistent source of contamination across harvest seasons [21].
The genomic analysis revealed high clonality among most clinical isolates from 15 countries from 2011 to 2024, which is compatible with a shared, common source. The selected cgMLST threshold of 7 AD to the reference strain for confirmed cases, while relatively high compared with other Salmonella outbreaks [22,23], was chosen to account for the expected genetic drift over the extended time frame (2011–2024).
This threshold may represent a limitation, as it increases the risk of including unrelated cases. However, it was considered appropriate given the observed SNP distances [5-11] between outbreak clades and the hypothesised scenario of a single introduction in or before 2011 followed by persistence and diversification within food production or processing environment. In addition, the comparison between using a threshold of 7 AD vs 5 AD showed that only 40 fewer cases would have been classified as confirmed, suggesting that a more conservative threshold would have led to similar results.
The annual and seasonal reappearance of this serovar in Europe further supports this theory, indicating a possible environmental or agricultural reservoir, such as contaminated water, which facilitates its persistence. Findings of isolates with > 200 AD divergence in Germany and England between 2011 and 2023 suggest that S. Strathcona in general is genetically more diverse than previously assumed and this points to the possibility that this serovar has persisted for a considerable period in a specialised niche. It may have potentially gone undetected due to its association with environments or reservoirs that do not commonly result in human disease. These findings support the validity of cgMLST-based approaches for outbreak investigation, while highlighting the importance of interpreting thresholds within the specific epidemiological context.
Fresh tomatoes are an established vehicle for Salmonella outbreaks, as exemplified by outbreaks in Sweden [24], Finland [25] and the US [26,27]. This may be related to consumption of tomatoes without cooking [21]. Pathogens may not only adhere to the surface but also internalise within the plant tissues [28]. This indicates washing of tomatoes may not necessarily be an effective preventive measure. Nevertheless, following proper hygiene practices at home, including handwashing, rinsing fresh produce and avoiding cross-contamination remains of importance to prevent illness. Contamination of tomatoes can happen at various points from the farm-to-fork, such as farm, packinghouses, or fresh-cut processing facilities [26]. They can potentially become contaminated with faeces from wild animals [29] or migratory birds [30,31]. Since water used for irrigation does not need to be potable, this may be a further source of contamination. Additionally, during water restrictions, different types of water can be used. Surface waters, such as rivers, streams or lakes, are more susceptible to contamination than protected sources like wells. The sewage sludge isolates from Germany and Austria support the hypothesis that sewage water can carry harmful pathogens and could be transferred to crops when used for irrigation or applied as a fertiliser [21].
Our study has several limitations. Firstly, tomatoes are a frequently consumed food item [32], making it challenging to identify their disproportionately frequent consumption. This is further complicated by the fact that they are often mixed with other ingredients in ready-to-eat foods, and therefore not always remembered by consumers when recalling their food history. A high proportion of cases reported eating in restaurants and other food service establishments, which also makes the recollection of exact ingredients harder to remember. In addition, not all information about food exposure and purchasing behaviour was collected consistently across the collaborating countries and the exposure period before illness onset varied between different investigation teams. Notably, despite thorough interviews, patients did not recall consuming certain food items, highlighting the limitations of self-reported food histories and the value of objective data sources for identifying contaminated food items. These factors may explain why tomato exposure was not consistently identified in national outbreak investigations in the collaborating countries and explain why the comparability across countries is limited. Furthermore, it is plausible that small tomatoes were the vehicle throughout this period. However, since no intensive epidemiological investigations were conducted between 2011 and 2023, this remains speculative. The observed change in the food vehicle from Datterino to cherry tomatoes likely may be due to changes in crop cultivation rather than a change in the underlying source. Using consumer purchase data should be emphasised as a useful tool; it was key to solving the original Danish outbreak in 2011. It was only when consumer purchase data pointed out the specific type of tomatoes that the Datterino tomatoes hypothesis emerged. Furthermore, consumer purchase data were also crucial in the traceback and trace-forward investigation to link to other affected countries. However, not all countries would be able to use consumer data due to data protection issues.
Conclusion.
Our investigations suggest recurring outbreaks in Europe since 2011 with a seasonal pattern of S. Strathcona cases that are linked to small tomatoes originating from a common source. The investigation exemplifies the added value but also the need of extensive collaborative cross-border and cross-sectoral investigations, ideally supported by ECDC and EFSA, to address complex food-borne outbreaks. To prevent future S. Strathcona infections in Europe, further investigations into the exact source(s) of the contamination with subsequent implementation of targeted measures are necessary.
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