Enteroinvasive Escherichia coli (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 . 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 [4–6].
Clinically, EIEC infections present either with watery diarrhoea or dysentery. EIEC invade the epithelial cells of the large intestine in the same manner as Shigella and symptoms resulting from EIEC infection are clinically indistinguishable from shigellosis . Studies have shown that E. coli and Shigella species have high genomic and phenotypic similarity, leading to propose that Shigella species should be reclassified as a subspecies of E. coli [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 E. coli 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 (IpaH) shared by both Shigella spp. and EIEC . Culture is required to differentiate the two species, and if culture is not possible or unsuccessful, faecal specimens are considered positive for the combination Shigella/EIEC. Detection of Shigella/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.