Research Imported spring onions related to the first recorded outbreak of enteroinvasive Escherichia coli in Denmark, November to December 2021


Enteroinvasive  (EIEC) is a Gram-negative bacteria causing diarrhoeal disease. EIEC is transmitted via the faecal–oral route, with a usual incubation period of 1–3 days; infections are frequently related to contaminated food and water [1,2]. In Denmark, disease caused by EIEC is mostly observed in returning travellers, but secondary transmission from person-to-person may occur [3]. In Europe, outbreaks of EIEC in 2012, 2014 and 2017 have been reported and, for all of these, contaminated vegetables were suspected as the source [46].

Clinically, EIEC infections present either with watery diarrhoea or dysentery. EIEC invade the epithelial cells of the large intestine in the same manner as  and symptoms resulting from EIEC infection are clinically indistinguishable from shigellosis [7]. Studies have shown that  and  species have high genomic and phenotypic similarity, leading to propose that  species should be reclassified as a subspecies of  [8,9].

Diagnostics of EIEC in Denmark are done locally through 10 different clinical microbiology laboratories situated at hospitals in the five Danish regions. The criteria for carrying out an EIEC diagnostic vary. Some laboratories test all faecal samples for diarrhoeagenic  including EIEC, while others test only faecal samples from suspected patients, based on their age, travel history and presence of bloody diarrhoea. The PCR diagnostic assays target the invasive plasmid gene () shared by both  spp. and EIEC [10]. Culture is required to differentiate the two species, and if culture is not possible or unsuccessful, faecal specimens are considered positive for the combination /EIEC. Detection of /EIEC is voluntarily notified as part of the Danish laboratory surveillance, where episodes are irregularly reported by the clinical microbiology laboratories to Statens Serum Institut (SSI), the national public health institute. All isolates from successfully cultured samples are furthermore routinely sent on a voluntary basis to SSI for further characterisation.

Outbreak detectionOn 10 December 2021, the clinical microbiology laboratory at Slagelse hospital in Region Zealand reported observing an increase between 5 and 8 December of patients diagnosed with domestically-acquired EIEC. During this period, this laboratory had detected a total of five such patients. On the same day (10 December), SSI identified one EIEC isolate with serotype O96:H- and three with serotype O136:H7 (two of which originated from Slagelse hospital), the latter a type never found in human samples from Denmark before. In total, between 6 and 10 December, SSI had received six EIEC isolates from three different regions of Denmark, exceeding the total number of EIEC isolates that were received in the months of December of the 2 previous years. A national outbreak was therefore declared on 13 December.

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