Pathogenic Potential of Shiga Toxin-producing Escherichia coli Isolated from Red Deer (Cervus elaphus)

In 2023, human infections by Shiga toxin-producing Escherichia coli (STEC) were reported as the third zoonosis in EU/EEA countries. Symptoms in humans vary from non-bloody diarrhoea to haemorrhagic colitis and the life-threatening haemolytic uremic syndrome (HUS). STEC can produce two types of Shiga toxin (Stx1 and Stx2) comprising four subtypes for Stx1 (a, c, d and e) and 15 for Stx2 (a–o). Adhesion to the intestinal cells is mediated by the protein intimin, encoded by the eae gene, or by different mechanisms. In Europe, five serogroups named “top-5” (O26, O103, O111, O145 and O157) have been associated with most human cases for years. Nevertheless, other human pathogenic serogroups are emerging. The major reservoir of STEC are ruminants, including wild species. The occurrence of STEC in red deer (Cervus elaphus) has been demonstrated in many European countries, with proportions ranging from 9.5 % (Portugal) to 19.9 % (Italy) in faeces and 7 % in carcasses (Spain). Among the top-5, stx1 and/or stx2- and eae-harbouring STEC O157:H7 was found in red deer faeces with variable prevalences (12 %–0.35 %) among countries. Nevertheless, most STEC belongs to O27:H80 and O146:H28 serotypes and are positive for stx2b and negative for eae genes. Although they do not belong to the top-5, they have the potential to cause diarrhoea and HUS in humans, as reported in Belgium in 2023. Currently, we are testing faecal and diaphragm samples from red deer hunted in Italy (Central-Western Alps) according to the ISO/TS 13136:2012 method. The study is ongoing and we only have preliminary results. However, whole genome sequencing of 11 isolates identified only the two stx2b-positive and eae-negative O27:H80 and O146:H28 serotypes. Since a recent EFSA classification of the most frequent serogroups of STEC from human infections includes O146, the wide distribution of STEC O146:H28 in red deer should alert the entire game meat chain to the possible risks for both hunters and consumers.